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What is this species of angiosperm from Morocco?

What is this species of angiosperm from Morocco?



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The flowers are about 1 cm long. The plant is about 1 metre high. Photographed in spring. The plant was found in a ruderal environment.


Look into the Symphytum genus, or related genera within Boraginaceae (the forget-me-not family). It is loosely similar to Symphytum officinale, but clearly not that species. The overall layout, size, leaves and hairyness of the stalks is similar to species in Symphytum.

Here is Symphytum officinale, as an example of the overall appearance:

Also similar from the Anchusa genus in the same family (Anchusa azurea):


Angiosperm

Angiosperms are a major division of plant life, which make up the majority of all plants on Earth.

Angiosperm plants produce seeds encased in “fruits,” which include the fruits that you eat, but which also includes plants you might not think of as fruits, such as maple seeds, acorns, beans, wheat, rice, and corn.

Angiosperms are also known as “flowering plants” because flowers are a characteristic part of their reproductive structure – though again, you may not always recognize their flowers as the pretty, colorful petaled things you think of when you hear the word.

Angiosperms evolved between 250-200 million years ago. They quickly gained an advantage over the previously dominant plant type – gymnosperms – for two reasons.

Angiosperms’ use of flowers to reproduce made them more reproductively successful. While gymnosperms relied primarily on the wind to achieve sexual reproduction by transferring pollen – which contain the male reproductive cells for plants – into the ovaries of female plants, angiosperms used sweet-smelling, brightly-colored flowers and sugary nectar to attract insets and other animals.

This process of cooperation, whereby animals like bees pollinate flowers in exchange for nectar, made angiosperms more reproductively successful.

Angiosperms also began to encase their seeds in fruits, which both provided extra nourishment and protection for their offspring plants, and created new ways to cooperate with animals. Many angiosperm’s fruits, like their flowers, were designed to attract animals to eat them.

Gymnosperms, which include pines, redwoods, gingko trees, and palm trees, still hold an important place in several ecosystems. But many species of gymnosperms that lived in prehistoric forests are now extinct, having been replaced by angiosperms.


Evolution of Angiosperms

Undisputed fossil records place the massive appearance and diversification of angiosperms in the middle to late Mesozoic era. Angiosperms (&ldquoseed in a vessel&rdquo) produce a flower containing male and/or female reproductive structures. Fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse, flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants.

Figure (PageIndex<1>): Fossil evidence of angiosperms: This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. A large number of pollinating insects also appeared during this same time.

Although several hypotheses have been offered to explain this sudden profusion and variety of flowering plants, none have garnered the consensus of paleobotanists (scientists who study ancient plants). New data in comparative genomics and paleobotany have, however, shed some light on the evolution of angiosperms. Rather than being derived from gymnosperms, angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were basal angiosperms related to tropical grasses. Both views draw support from cladistic studies. The so-called woody magnoliid hypothesis (which proposes that the early ancestors of angiosperms were shrubs) also offers molecular biological evidence.

The most primitive living angiosperm is considered to be Amborellatrichopoda, a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree. A few other angiosperm groups, known as basal angiosperms, are viewed as primitive because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more primitive because they share morphological traits with both monocots and eudicots.


Species Concept: History, Types and Categories | Taxonomy

The species, as we know, is the fundamental unit of taxonomic hierarchy. Davis (1978) called them ‘Building bricks’ in Biological classification. In biological phenomenon biosystematics concept is the oldest one. It is the lowest category of hierarchy which is consistently used and recognized by all the botanists. According to Stebbins (1977) is the basic unit of evolutionary process.

It starts with the great Philosopher Plato who proposed concept of eidos or species and believed that all objects are shadows of the ‘eidos’. Mayr (1957) suggested that variations in species are found arid presented on typological species concept.

Principle of logical division by Aristotle based in part upon Plato’s idea was the basis of Taxonomy serving as schema upon which “species concept” is based. Species was considered to be a relative term applicable to various levels in a classification scheme.

A logical relationship was also established between genus and species. Then species was defined on a priori basis and regarded as unchanging and fixed. After the knowledge of a number of organisms, people started facing difficulty as there are species which belong to different genera.

Various attempts have been made to define a species. Broadly 5 different concepts are given:

(i) Nominalistic Species Concept:

It believes that “Nature produces individuals and nothing more”. (Linneaus species concept).

(ii) Typological Species Concept or Taxonomic species concept:

According to it species is “A very natural group of organisms hence a natural taxon in classification has an invariant generalized or idealised pattern shared by all members of the group”.

(iii) Biological Species Concept:

It was presented by Dobzansky in 1937. He suggested that “species is a group of interbreeding natural populations, that are reproductively isolated from such other group.”

(iv) Evolutionary Species Concept:

According to this concept, species is “a spatio temporal lineage of populations that evolves separately from other lineages and has its own ecological niche”.

(v) Ecological Species Concept:

It was suggested by L. Van Valen in 1976. According to it species is “A group of individuals maintained ecologically but not reproductively”.

The species concept kept on changing from time to time. Ernst Mayr 1963 stressed that the non-breeding of natural populations rather than the sterility of individuals be taken as the decisive species criterion.

Species as Individual:

After the knowledge of some monotypic taxa philosophical problems, biological and evolutionary problems like gene flow etc. the questions arises as whether species might not better be regarded as individuals rather than classes.

The general accepted concept is, ‘species is a unit of taxonomic convenience, and that the populations, in the sense of a geographically constrained group of individuals with some unique amorphous characters, is the unit of evolutionary significance, ‘species is regarded as real by most of the taxonomists.

The hypermodern species concept by Platnick (1976) and Ghiselin (1976) defines species as “firms” in economic analogy.

Current Species Concepts:

A. Morphological Species Concept:

Morphologic species or Morphospecies concept is also called as classical phenetic species concept or Linnaean or classical species concept.

The concept suggests that:

(a) Species are the smallest groups that are consistently and persistently distinct and distinguishable by ordinary means.

(b) Species is easily recognized kind of organisms, and in macroscopic plants and animals their recognition should rest on simple gross observation (May be with hand lens only).

(c) A species is a community of a number or related communities, where distinctive morphological characters are one in the opinion of a competent systematist.

Morphological or classical species concept of readily recognized and morphologically defined species is practical and efficient system for information retrieval in most of the flowering plants.

This concept is useful and meaningful even for those plants where hybridization is common (e.g., Quercus).

B. Biological Species Concept:

This concept is held conceptually by most systematist at the present time.

(a) A group of interbreeding populations.

(b) Reproductively isolated from other such group.

Utility of this concept is that it deals with reproductive isolations. Krukeberg (1969) stressed that the bio-species also differ in their ecological contexts.

Love (1954) accorded species status to morphologically indistinguishable cytotypes.

Grant (1966) used the biological species concept and created a new diploid species of Gilia (Polemoniaceae) experimentally through artificial relation over then generations in 16 years.

Mayr (1982) modified the definition of biological species concept to stress ecological aspects along with reproductive isolation and told that species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature. However, Hengeveld (1988) criticized the use of word niche in definition as it is difficult to define it accurately.

The common criticisms of this concept are:

(i) Accurate determination of interbreeding among population.

(ii) Real extent of gene flow among population.

(iii) Common occurrence of interspecific hybridization between species of flowering plants.

(iv) Inapplicability of the concept to asexual species.

Hull (1970) critically evaluated the biological species concept and found certain difficulties in using it. He felt that the numerical phenetic species concept has more problems, particularly on deciding which phenetic unit is the one to be called species.

C. Genetic Species Concept:

The genetic species concept assumes that the biological factors of gene flow and reproductive isolation are operative but the way to define species is by a measure of the genetic differences or distance among population.

It is the numerical phenetic species concept using a quantitative measure or genetic rather the morphological or other distance. The problem comes in knowing the genetic difference between populations.

Palmer and Zamir:

Palmer and Zamir (1982) for the first time studied the genetic distance from DNA sequence of chloroplast DNA.

D. Paleontological Species Concept:

Paleontologists work upon fossil species generally paleospecies or chronospecies in which arbitrary time limits are used to delimit paleontological species. Vaughan (1905) termed a collection of paleospecies in a monophyletic succession as genes. Paleospecies are usually time-oriented morphospecies.

E. Evolutionary Species Concept:

Evolution in genetical species keeps them sharply separate. Evolutionary species concept was advocated by Simpson (1961) who suggested that “An evolutionary species is a lineage” (an ancestral descendant sequence of population), evolving separately from others and with its own unitary evolutionary role and tendencies.

This concept avoids the difficulties with determining actual or potential levels of interbreeding as gene flow and allows some degree of inter-specific hybridization. Simpson (1961) also suggested that niche included multidimensional relationship of a taxon to its environment and not just its microgeographic situation.

Van Valen (1976) defined it as “A species is a lineage occupying an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range.”

F. Cladistic Species Concept:

Wiley (1978) modified the evolutionary species concept and defined “A species is a single lineage ancestral descendant population of organisms which maintains its identify from other such lineages and which has its own evolutionary tendencies and historical fate”.

He stressed upon “historical fate” instead of “evolutionary role”. It is a shift from ecological view point to a historical context resulting from apomorphic changes within single branch of a cladogram. Donoghue (1985) and Mishler (1985) called it as phylogenetic species concept. Willis (1981) stated that each species is an internally similar part of a phylogenetic tree.

G. Biosystematic Species Concept:

It includes experimental taxonomic studies also. The variation taking place in reciprocal transplant and hybridization etc., are to be included successfully. Most common examples are Ecotypes, Ecospecies, Coenospecies etc.

Closely, related but ecologically distinct population which are largely interfertile are called ecotypes.

Similar but hybrids between them are of reduced viability.

These are not interfertile at all (even artificially).

Those complex of species which cannot be sorted out taxonomically from a population and in which there is hope of eventual resolution are called species aggregate or Microspecies. Species group or Superspecies.

Many categories are proposed for interpretation of reproduction limits of taxa.

Camp and Gilly (1943) suggested at least 12 kinds, i.e.:

Two of them are popular, i.e.,

“A species which is genetically and morphologically homogeneous and all members are interfertile”.

“A species made up of races which, if selfed, produce morphologically stable populations, but when crossed many produce several types of viable and fertile off-springs”.

Apomictic groups are included in apogameon and agameon. Danser (1929) used the term comparium and commiscuum for coenospecies but was on the ability to hybridize and on geographic factors.

Heslop-Harison (1954) and Gilmour used Gamodeme (coenospecies) and Syngamodeme (Comparium) terms. Grant (1957) defined syngamone as the sum total of species of semispecies linked by frequent or occasional hybridization in nature, a hybridizing group of species, the most inclusive interbreeding population.

H. Multi-Species Concept:

It was given by Van Valen (1976). According to it ‘a set of broadly sympatric species (some region) that exchange genes in nature’ is called a species.

Compilospecies is one which is morphologically distinct (Taxonomic species concept) but not reproductively isolated (single species according to biological concept), e.g., Salvia mellifera and S. apiana which are morphologically distinct but reproductively not isolated.

Sibling Species:

The two species which are morphologically not isolated but reproductively distinct are called sibling species, e.g., Gillia inconspicua and G. transmontana.

According to Grant (1981) Microspecies is the population of predominantly uniparental plant groups which are themselves uniform or slightly uniform and are differentiated morphologically from one another. These species are restricted to a limited geographical area.

The term microspecies was first introduced by Jord (1873) and often known as Jardanons in comparison to Linnaeons which are the normal species as suggested by Linneaus.

The species are capable of cross fertilization. Microspecies are of following types:

(i) Agamospermous microspecies:

The species reproduce by agamospermy, e.g., Rubus.

(ii) Autogamous microspecies:

The species are autogamous and chromosomally homozygous, e.g., Erophila.

(iii) Clonal microspecies:

The species reproduce only vegetative propagation, e.g., Phragmites.

(iv) Heterogamic microspecies:

The reproduction in the species is by genetic system, e.g., Oenothera biennis.

I. General Species Concept:

The species concept that is most acceptable for more sexually reproducing flowering plants is the biological concept. The reproductive barriers that keep species apart are most important for limiting gene exchanging and for maintaining the integrity of each unit.

The flow-chart determining population and species as suggested by Dayer and SlobodchiKoff (1974) is given below:

Unlike animals many plants hybridize freely in nature and are thus not reproductively isolated. In Epilobium (160 species) interspecific hybridization is very common. According to biological species concept the whole group is placed under complex species.

Any consideration of solution to the species problem now should consider that:

(a) All populations tend to vary and that no two are even alike.

(b) Some of these variations are adaptive and are of survival value.

(c) Forces of nature result in the extinction of some individuals while other survives the same force.

(d) Some of the variations shown by individuals within a population must be hereditary if successive generations are to be modified from the ancestral types.

(e) Environment of the individuals must not be static.

Earlier the taxonomists were not knowing that the individual species possess certain degree of capacity for variation. They believed that taxonomy means that the description of a single plant is the description of the species to which it belonged. But now the scenario is quite different.

Now variation in the same species also has to be studied. So population study is a must. Mayr (1942, 47, 52) proposed the rule that we should name populations and not species, though an individual can be chosen as the nomenclatural type.

Population is the group of plants that grow in a particular habitat at a particular time. Several populations constitute the species. In such cases there are bound to be discontinuities between the different parts of the population. The more in dispersed such population systems are, the more will be difference between them.

A local breeding population growing together with inbreeding capacity to form a common gene pool is recognized by the biosystematists as the unit of evolutionary change.

Such ‘Panmictic’ population between whose members there is potentially free gene flow show variations in many features, even though they belong to the same species. Populations are therefore dynamic and show variations with in the population and variation between the populations of the same species.

Variations between individuals caused by factors like:

(a) Modification due to external environment,

Variation is a deviation in structural, functional or developmental characters of an organism from its parents, from others in the same population.

Genotypic variation is the difference in genotype within population or species as a result of mutation, recombination or gene interaction with some evolutionary significance.

Phenotypic variation is variation in structural and functional characters resulting from environmental factors on one or more genotypes this may not be of evolutionary significance.

Variety is the term applicable to diversity, variant population, variability, character variation etc. (Table 1).

Variant is an individual or a group of individuals within a population that is definiable and recognizable. It is a neutral term used without taxonomic significance. Mostly the term is applied to an individual or group of individuals with one or only a few distinctive characteristics. It may or may not be equivalent of the taxonomic category or variety.

Diversity refers to a number of types of organisms or taxa in the plant kingdom. Total diversity of plant kingdom is approximately 2,50,000 (vascular plants). Among which 2, 35,000 are angiosperms, 600 gymnopserms and 10,000 pteridophytes. Diversity can be seen in various types also, e.g., in Rosaceae at least 9 fruit types are found.

Population Variability:

It is of three fundamental types, i.e., developmental, environmentally induced and genetic variation. Developmental variability can be seen in Oak (Quercus).

Where in Scarlet Oak (Quercus coccinia) the leaves on adventitious shoot (Sucker) are similar to normal leaves of Red Oak (Q. rubra) whereas, the leaves on shaded sucker shoot, of turkey Oak. (Q. leavis) are similar to sun (normal) leaves of scarlet oak). Other example is of Bean plant where the first leaves are opposite and simple which later on become alternate and pinnately compound.

Genetic variation in the population is the result of mutation and recombination. Mutation may be as small as substitution of single nucleotide pair in the DNA molecule or as big as chromosomal aberration (structure or number).

Recombination is a reassortment of chromosomes, crossover segments etc. It generally takes place in meiosis and fertilization. It results in hybrid formation, pollination, dispersal, population size etc. Recombination also results from hybridization and introgression. Introgression is process of successive hybridization causing the migration of genetic material from one species (infraspecific) into another.

Character variation in plants is fundamental to taxonomic training and research.

Speciation is the process by which new species are formed from other ancestral ones. The most widely accepted hypothesis explaining the process of speciation is the geographical theory of speciation or allopatric theory of speciation. According to it the first step in speciation is reproductive isolation brought about by physical separation or geographical separation of population (allopatric population).

Separation results commonly from shrinkage of the original range of a species so that the populations become physically isolated. Separation may also results by large range or dispersal when new populations become established beyond the range of pollination of original gene pool.

The second step is the independent differentiation and evolution of these reproductively isolated populations. Eventually, if the ranges of the populations merge again and the reproductive isolation persists, then the speciation process is considered complete.

Due to first step in differentiation ecotypes are formed. Agrostis tenuis, a temperate grass which also grown on lead concentrated soil, and becomes lead tolerant.

This character is not lost even in the absence of the metal. If the two populations grown side by side, may cross and produce hybrids that cannot tolerate the lead concentration on one side or cannot compete with the normal plants of the species on the other side. Lack of hybrid survival constitute – absolute barrier to gene exchange.

Speciation may take place abruptly also by development of polyploids and mutations Futuyma (1983) suggested that “Speciation more thoroughly a wash in unfolded and often contradictory speculation than any other single topic in evolutionary theory”.

Isolating Mechanism:

There are many isolating mechanisms which operate to prevent or reduce the gene flow between populations:

(a) Reproductive Isolation:

It depends upon some aspects of plants themselves such as lock and key pollination mechanism, difference in flowering time of the two populations etc., e.g., Salvia apiana and S. mellifera are two species growing in different zones but show resemblances also. S. mellifera blooms in April, smaller flowers pollinated by smaller bees, flies and butterflies, while in S. apiana the flowers are large and pollinated by larger carpenter bees. S. apiana grows in drier soil. According to Stace (1989) the two species are separated by the ecological and mechanical differences.

(b) Spatial Environmental Isolation:

It is also known as geographic isolation. According to Grant (1981) “Spatial or geographic isolation exists between any allopatric species whose respective geographical areas are separated by gaps greater than the normal radius of dispersal of their pollens or seed”, i.e., Liriodendron tulipifera of Eastern United State and L. chinense of South eastern Asia, which cannot breed under natural conditions but have been crossed artificially.

Types of Species:

Grant (1981) recognized 5 types of species along with Evolutionary species of Simpson (1961):

(Morphological species, phenetic species) The taxa, group of morphologically similar individuals.

(2) Biological species: (Genetic species):

Sexually reproducing population system.

(3) Microspecies: (Agamospecies):

Population is uniparental organism.

(4) Successional species: (Paleospecies):

(5) Biosystematic species: (Ecospecies, Coenospecies):

(6) Evolutionary species:

Combined sexually reproducing populations iniparental groups and phyletic lineages.

Taxonomic Hierarchy:

To achieve the ranking in Taxonomy a hierarchy of categories is suggested. It was Linnaeus (1753, 1754) who for the first time suggested such hierarchy and so it is known as Linnaean Hierarchy. It was developed long before the knowledge of evolution during 15th or 16th centuries. Species is the fundamental unit of it.

Linnaeus hierarchy was slightly modified over the years and is as follows:

Now many intermediate categories are included in it such as subfamily, sub genus, section, tribe etc.

The Linnaean hierarchy is viewed as system of classes within classes and named nested classes by Buck and Hull (1966). In Linnaean hierarchy category means a particular level or rank in the taxonomic hierarchy, e.g., genus or class, and taken collectivity all these available categories represent all the different levels. Taxon means a cluster of individuals grouped together based on the sharing of features in common. The taxa are referred to particular categories, e.g., species or variety available in the hierarchy.

In the accepted system of nomenclature every individual plant is treated as belonging to a number of taxa of consecutively subordinate ranks each with subcategories or taxa of consecutively as ascending rank e.g., an individual plant of swamp rose would belong to the consecutively higher ranks of the species.

For example : Ranunculus muricatus species is muricatus, Genus is Ranunculus, Family: Ranunculaeae, Order: Ranales: Class: Magnoliopsida and Division: Magnoliophyta, according to recent system of classification. The suffix for each rank is different, e.g., aceae suffix shows a name of family.

Conceptually, the ranks of taxa cannot be defined precisely, just arranged hierarchically. A group of plants, however, can be circumscribed and delimited as named ranks, e.g., rose in the genus Rosa. Asters for the genus Aster. Natural groups are treated as ranks at the generic, familial and higher categories, e.g., Pines (Pinus), Oaks (Quercus), Clover (Trifolium).

The only Taxonomic group with an inherent rank in the species aside from intraspecific taxa.

The name of an infraspecific taxon is a combination of the name of a species and an infraspecies epithet connected by a term denoting its rank. Infraspecific epithet is formed as those of species and, when adjectival in form and not used as substantives, they agree grammatically with the generic name, e.g., Saxifraga aizoom var, aizoom, subvart, brevifolia forma multicaulis subforma surculosa Engler and Irmschen or Saxifraga aizoom subforma surculosa, Engler and Irmschen.

Infraspecific Ranks:

Linnaeus used only one infraspecific taxon, the variety. But over the years various taxonomists used a number of infraspecific ranks in their respective Flora. International Code of Botanical Nomenclature recognizes five infraspecific Ranks, Subspecies, Variety, Subvariety, Form and Subform.

But many workers found it to be insufficient and well over 100 different infraspecific ranks have been proposed time to time. Now-a-days only three of the five suggested by ICBN are in use. Subvariety and subform are largely abandoned.

The definitions of the three infraspecific ranks as given by Du Rietz are:

“A population of several biotypes forming a more or less distinct regional facies of a species”. It is thus a geographical race, ecotype, topodeme or genoecodeme.

“A population of one or several biotypes, forming more or less distinct local facies of a species”. It is thus a local or ecological race, an ecotype genoecodeme of a lower order, or an Ecodeme.

“A population of one or several biotyes occurring sporadically in a species population in one or several distinct characters”. It is thus a genodeme orrelatively minor genetic variant occurring mixed with other such distinct variants.

Family and Higher Categories of Species Concept:

In classification, the categories considered at the higher level are family, order class, division and kingdom.

It is the largest group or population which is well defined, traditionally as Plant and Animal. But Whitekar (1969) suggested a 5 kingdoms classification and divided all the living organisms into 5 kingdom, i.e., Monera, Protista, Fungi, Plantae and Animalia.

Very recently a new or sixth kingdom also has been included by Yarg, Kaine and Woese (1985) who considered very primitive methane Bacteria discovered by Woese and Fox (1977), Fox (1977) into this separate kingdom.

Previously Eichier (1883) proposed Algae, Fungi, Bryophyta, etc as divisions. Bold and Wynne (1985) divided Algae into many divisions, i.e., Chlorophyta, Rhodophyta, Phaeophyta whereas, Cyanophyta is now treated as Cyanobacteria and is placed in division Monera. Lewin (1981), divided algae into prochlorophyta (Prokaryotic algae).

In angiosperms which were traditionally divided into dicot and monocots, Dicots are called as Magnoliophyta by Cronquist, Takhtajan and Zimmermann etc. (1966).

Bremer and Wanntorp (1978) recognised six “major groups” based on a cladistic reinterpretation of Takhtajan’s (1969) phyletic classification.

In Angiosperms Orders are not well defined except for a few such as Caryophyllales. Cronquist, Throne, Takhtajann and Dahlgreen etc., used orders in various ways.

Different workers put stress on certain families only. If we look into the history of it, we can see that since the time of Theophrastus the families were there but were named on the basis of their similarities, e.g., mints (Labiatae) or carrots (Umbelliferae) etc. Tournefort (1700) attempted to make informal groups of genera with some similar characters and tried to describe them.

Linnaeus (1753) also has not used any term as family. The term ‘family’ was first used by A.L. de Jussieu in his Genera plantarum (1989) where he grouped the genera under one head, i.e., family. A.L. de Jussieu is also known as “Father of the familial concept” in flowering plants.

Genus is next to species as far as taxonomic hierarchy is considered. Though there are many intervening categories as series, section, subgenus etc., as suggested by ICBN but they are not fundamental units. Sub-sections are good and helpful when a genus has a large number of Species (Infra generic), e.g., Senecio (Asteraceae) has more than 1,000 species.

According to Robinson a genus is a group of species which from likeness point of view appear to be more nearly related to each other than they are to other species.

The concept of Genus is the oldest among all taxonomic categories. Since very early times the plants in groups were named as Elms, Poplars, Willow, Pines, Oaks, Roses, Palms, etc. Tournefort (1700) used 698 genera of plants, Linnaeus (1737) about 935 genera etc., So it was very difficult to remember all of them.

The necessity of grouping them in accordance with their characters arose and family concept had taken birth, Jussieu (1989) recognized 100 families but now modern taxonomists recognize more than 400 families.

Tournefort (1700) is known as “Father of generic concept”. He proposed that out of six features of a plant, i.e., root, stem, leaves, flower, fruit and seeds, at least 5 should be considered for generic circumscription.

Linnaeus in Philosophia Botanica (1751) used 3 characters for generic concepts, i.e.:

(a) Natural character (complete description of plant).

(b) Factitious character (selection of certain characters suitable for discrimination among genera in an artificial system of classification).

(c) Character essentials (characters allowing easiest description).

According to the nomenclature code the generic name is a uninominal singular word and should be a noun. It may be masculine, neuter or feminine (-us and -pogon) are masculine -um is neuter and -a (are for feminine genera). The first letter of the generic name should be written in Capital letter of English.

The generic name may be:

(i) Name of a Person (Commemoration):

The name is dedicated to a person e.g., Bauhinia (for Bauhim) Fuchsia (for Fuchs), Victoria (for Queen Victoria), Dillenia (for Dillinius), Linnaea (for Linnaeus), Collaea (after Colla) and Ottoa (alter Otto) etc.

The generic name is after a place such as Arabis (for Arabia), Salvadora (for EL Salvadore) etc.

(iii) Name of a character:

The generic name may be based on an important character of the plant, e.g.,

Hygrophila for marshy plant.

Trifolium for trifoliate leaves etc.

The name is directly adopted from a language other than latin, e.g.,

Generally, the name should be in Latin language. The generic name of a tree ends with -us as tree is considered to be feminine, e.g., Pinus, Quercus, Cedrus, Pyrus, Prunus etc. A generic name cannot be a technical term used in morphology unless it was published before January 1, 1912. The generic name should be accompanied with a specific epithet following the binomial system of Linnaeus.

According to ICBN Generic name should comply the following:

(1) To use Latin termination in so far as possible.

(2) To avoid names not readily adaptable to the Latin language.

(3) Not to make names which are very long and difficult to pronounce in Latin.

(4) Not to make names by combining words from different languages.

(5) To indicate, if possible, by the formation or ending of the name the affinities or analogies of the genus.

(6) To avoid adjectives used as nouns.

(7) Not to use a name similar to or derived from the epithet in the name of one of the species of the genus.

(8) Not to dedicate genera to person quite unconnected with botany or at least with natural science.

(9) To give feminine forms to all personal generic names, whether they commemorate a man or woman.

(10) Not to form generic names by combining parts of two existing generic names, because such names are likely to be confused with nothogeneric names (Hybrid names).

Species is the fundamental unit or category of taxononomic hierarchy. According to Davis (1978) it is the building bricks in biological classification. Species category is one of the oldest concepts used historically by people.

It is the lowest category in the hierarchy that is consistently used and recognized by all people of the world. Concept of species was used by Plato (a philosopher) for the first time in terms of eidos. After many years (Aristotle) species was defined on an a priori basis and was regarded as fixed and unchanging.

John Ray in his Historia Plantarum presented species concept (1668- 1704). Davis and Heywood (1963) suggested that any change in the species is due to environmental factor or inherent factors.

According to Cytogenetists (Shull) “The species are only quasi-natural entities’.In Microorganisms the species is not a valid concept. There the unit is called strain.

In binomial nomenclature, the first epithet of the name is generic and the second epithet is specific. All specific epithets should begin with small letter of English. The specific name may be pronoun, adjective etc.

It may be dedicated to the name of a person, e.g., sahnii (for Prof. Birbal Sahni). roylei (for Royale) hookeri (for Hooker) etc.

The name of the place should be an adjective ending with -ens, -ensis, – cumor -cus or -iana etc. e.g., kashmiriana, indicum, indicus, canadensis, nepalensis, nepalens, etc.

(iii) Name of a character:

The character may be represented as alba (for white flower) sativa (edible nature) etc. Depending upon the gender it may be alba, album or albus or sativa, sativum or sativus etc.

(iv) Noun in apposition:

Sometimes the specific epithet carries its own name, e.g., in Pyrus malus. Malus is the Greek name of apple and in Allium cepa, cepa is the Latin name of onion.

Generally, the specific epithet is of one word but if it has two words then a hyphen (-) should be placed in between, e.g., Hibiscus rosa-sinensis Capsella bursa-pestoris etc.

Till recently, species was thought out to be the smallest unit of (Taxa) taxonomic ranks. But now subspecies and varieties are also available. The names of subspecies are trinomial, the third epithet being of subspecies. A variety may be quadrinomial as the fourth epithet will be for varieties but in case there is no subspecies, it will be trinomial, e.g., Brassica oleracea var. capitata.

According to ICBN. specific epithets should comply the following:

(i) To use Latin terminations insofar as possible.

(ii) To avoid epithets which are very long and difficult to pronounce in Latin.

(iii) Not to make epithets by combining words from different languages.

(iv) To avoid those formed if two or more hyphenated words.

(v) To avoid those which have the same meaning as the generic name (pleonasm).

(vi) To avoid those which express a character common to all or nearly the species of a genus.

(vii) To avoid in the same genus those which are very much alike especially those which differ only in their last letters or in the arrangement of two letters.

(viii) To avoid those which have been used before in any closely allied genus.

(ix) Not to adopt epithets from unpublished name found in correspondence, traveller’s notes, herbarium labels, or similar sources, attributing them to their authors, unless these authors have approved publications.

(x) To avoid using the names of little known or very restricted localities unless the species is quite local.


Floral Formulas

Since there are so many terms about flowers, and at the same time, flower structure and diversity always were of immense importance in botany, two specific ways were developed to make flower description more compact. First is a flower formula. This is an approach where every part of flower is designated with a specific letter, numbers of parts with digits, and some other features (whorls, fusion, position) with other signs.

  • Symmetry: * means radial symmetry, while X means bilateral symmetry.
  • Whorls: K is the calyx, C is the corolla, A is the androecium, and G is the gynoecium. The number that follows each letter represents the number of parts in that whorl. For the gynoecium, a line under the number indicates an inferior ovary, while a line above the number indicates a superior ovary.
  • Fusion: In most representations, connation is indicated by circling the number, while adnation is indicated by drawing a line connecting the numbers of the fused whorls.

Here are a few examples of floral formulas, followed by their interpretation:

(ast K_<4>C_<4>A_<2+4>G_>): flower actinomorphic, with four sepals, four petals and six stamens in two whorls, ovary superior, with two fused carpels

(uparrow K_<(5)>[C_<(1,2,2)>A_<2,2>]G_>): flower zygomorphic, with five fused sepals, five unequal fused petals, two-paired stamens attached to petals, superior ovary with two subdivided carpels

(ast K_<(5)>C_<(5)>[A_<5>G_>]): actinomorphic flower with five fused sepals and five fused petals, five stamens attached to pistil, ovary inferior, with three fused carpels

The following signs are used to enrich formulas:

PLUS &ldquo+&rdquo is used to show different whorls minus &ldquo(-)&rdquo shows variation &ldquo(vee)&rdquo = &ldquoor&rdquo

BRACKETS &ldquo[]&rdquo and &ldquo()&rdquo show fusion. In most representations, connation is indicated by circling the fused whorl, while adnation is indicated by drawing a line underneath the formula connecting those whorls.

COMMA &ldquo,&rdquo shows inequality of flower parts in one whorl

MULTIPLICATION &ldquo( imes)&rdquo shows splitting

INFINITY &ldquo(infty)&rdquo shows indefinite number of more than 12 parts

Flower diagram is a graphical way of flower description. This diagram is a kind of cross-section of the flower. Frequently, the structure of pistil is not shown on the diagram. Also, diagrams sometimes contain signs for the description of main stem (axis) and flower-related leaf (bract). The best way to show how to draw diagram is also graphical (Figure (PageIndex<1>)) formula of the flower shown there is (ast K_<5>C_<5>A_<5>G_>).

Figure (PageIndex<1>): How to draw a diagram (graphical explanation): compare numbers on plant and on diagram. On the left of the image, a simplified aerial view of the flower represents the floral diagram. In the center is a star-like 5-lobed stigma surrounded by 5 4-lobed anthers. There are 5 pink crescent shapes that form a circle around this (the petals) and 5 pale green crescents that form a circle around these (the calyx). Three darker green, bracket like shapes represent leaves and a single dark green circle represents the peduncle.

Review of Terms and Formula Designations

FLOWER PARTS occur in whorls in the following order&mdashsepals, petals, stamens, pistils.

(The only exceptions are flowers of Eupomatia with stamens then perianth, Lacandonia with pistils then stamens, and some monocots like Triglochin, where stamens in several whorls connect with tepals.)

PEDICEL flower stem

RECEPTACLE base of flower where other parts attach

HYPANTHIUM cup-shaped receptacle (Figure (PageIndex<2>))

PERIANTH = CALYX + COROLLA

SEPALS small and green, collectively called the CALYX, formula: K

PETALS often large and showy, collectively called the COROLLA, formula: C

TEPALS used when sepals and petals are not distinguishable, they form SIMPLE PERIANTH, formula: P

ANDROECIUM collective term for stamens: formula: A

STAMEN = FILAMENT + ANTHER

ANTHER structure containing pollen grains

FILAMENT structure connecting anther to receptacle

GYNOECIUM collective term for pistils/carpels, formula: G. Gynoecium can be composed of:

    A single CARPEL = simple PISTIL, this is MONOMERY

(Note that variant #4, several compound pistils, does not exist in nature.)

To determine the number of CARPELS in a compound PISTIL, count LOCULES, points of placentation, number of STYLES, STIGMA and OVARY lobes.PISTIL Collective term for carpel(s). The terms CARPEL and PISTIL are equivalent when there is no fusion, if fusion occurs then you have 2 or more CARPELS united into one PISTIL.

CARPEL structure enclosing ovules, may correspond with locules or placentas

OVARY basal position of pistil where OVULES are located. The ovary develops into the fruit OVULES develop into seeds after fertilization.

LOCULE chamber containg OVULES

PLACENTA place of attachment of OVULE(S) within ovary

STIGMA receptive surface for pollen

STYLE structure connecting ovary and stigma

FLOWER Floral unit with sterile, male and female zones

ACTINOMORPHIC FLOWER A flower having multiple planes of symmetry, also called radially symmetrical, formula: (ast)

ZYGOMORPHIC FLOWER A flower having only one plane of symmetry, also called bilaterally symmetrical, formula: (uparrow)

PERFECT FLOWER A flower having both sexes

MALE / FEMALE FLOWER A flower having one sex, formula: ♂ / ♀

MONOECIOUS PLANTS A plant with unisexual flowers with both sexes on the same plant

DIOECIOUS PLANTS A plant with unisexual flowers with one sex on each plant, in effect, male and female plants

SUPERIOR OVARY most of the flower is attached below the ovary, formula: (G_>)

INFERIOR OVARY most of the flower is attached on the top of ovary, formula: (G_>)

(Inferior ovary only corresponds with monomeric or syncarpous flowers.)

WHORL flower parts attached to one node


Families in Angiosperms

  • Jump to families starting with:
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Prospects

In the immediate future, we will deliver a further data release through which we expect to reach the milestone of sampling 50 |$\%$| of all angiosperm genera. This target will be achieved through substantial novel data production by PAFTOL and collaborators, augmented by data mined from public sources. In-depth phylogenetic analyses of our data and their evolutionary implications are also underway.

Beyond this point, we see three priority areas in which future platform developments might be concentrated, resources permitting. Firstly, taxon sampling to the genus level must be completed. Our original target of sampling all angiosperm genera remains, but the mode of reaching this is likely to evolve. We anticipate an acceleration in production of Angiosperms353 data by the broader community. The completion of generic-level sampling will require both the integration of community data in the broader angiosperm tree of life as well as strategic investment in filling inevitable data gaps for orphan groups. Secondly, numerous opportunities for refinement exist across our methods. For example, insights from our data might permit the optimization of the Angiosperms353 probes to improve gene capture. Efficiency of gene assembly from sequence data can also be improved bioinformatically ( McLay et al. 2021). However, as costs of sequencing decline, target sequence capture in vitro may no longer be necessary, the target genes simply being mined from sufficiently deeply sequenced genomes. Thirdly, for the full integrative potential of Angiosperms353 genes to be achieved, infrastructure for aggregating and sharing this coherent body of data must be improved. While the Kew Tree of Life Explorer provides a proof-of-concept, it is the public data repositories (e.g., NCBI, ENA) that offer the greatest prospects of a mechanism to achieve this. To fully parallel the earlier success of public repositories for facilitating single-gene phylogenetic trees (e.g., rbcL, matK), new tools are needed to assist with efficient upload and annotation of target capture loci and associated metadata.

Even with a completed genus-level angiosperm tree of life well within reach, the monumental task of sampling all species remains. The scale of this challenge is 24-fold greater than the genus-level tree toward which we are currently working. However, with sufficient investment, increased efficiencies and community engagement, such an ambition could potentially be realized. Collections-based institutions are poised to play a critical role in this endeavor through increasingly routine molecular characterization of their specimens, perhaps as part of digitization programs and are already facilitating the growing trend toward species-complete sampling in phylogenomic studies (e.g., Loiseau et al. 2019 Murphy et al. 2020 Kuhnhäuser et al. 2021). Our platform demonstrates how large-scale phylogenomic projects can capitalize on natural history collections to achieve a much more complete sampling than hitherto possible.

The growing movement to sequence the genomes of all life on Earth, inspired by the Earth Biogenome Project ( Lewin et al. 2018), significantly boosts the prospects for completing the tree of life for all species but is hampered by the focus on “gold standard” whole genomes requiring the highest quality input DNA. Our platform offers the opportunity to bridge the gap between the ambition of these projects and the vast phylogenomic potential of natural history collections. However, as life on Earth becomes increasingly imperilled, we cannot afford to wait. To meet the urgent demand for best estimates of the tree of life, we must dynamically integrate phylogenetic information as it is generated, providing synthetic trees of life to the broadest community of potential users ( Eiserhardt et al. 2018). Our platform facilitates this crucial synthesis by providing a cross-cutting data set and directing the community toward universal markers that seem set to play a central role in completing an integrated angiosperm tree of life.


Acknowledgements

We would like to thank Drs. J. Chris Pires, Scott T. Woody, Richard M. Amasino, Heinz Himmelbauer, Fred G. Gmitter, Timothy R. Hughes, Rebecca Grumet, CJ Tsai, Karen S. Schumaker, Kevin M. Folta, Marc Libault, Steve van Nocker, Steve D. Rounsely, Andrea L. Sweigart, Gerald A. Tuskan, Thomas E. Juenger, Douglas G. Bielenberg, Brian Dilkes, Thomas P. Brutnell, Todd C. Mockler, Mark J. Guiltinan, and Mallikarjuna K. Aradhya for providing tissue and DNA of various species used in this study. We would also like to thank the Georgia Genomics Facility and Georgia Advanced Computing Resource Center (GACRC) for technical support, particularly Dr. Shanho Tsai and Yecheng Huang of the GACRC for their efforts.

Funding

The work conducted by the U.S. Department of Energy Joint Genome Institute is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 to JS. We thank the Joint Genome Institute and collaborators for access to unpublished genomes of B. rapa, S. viridis, P. virgatum, and P. hallii. This work was supported by the National Science Foundation (NSF) (MCB –1339194), by the Office of the Vice President of Research at UGA, and by The Pew Charitable Trusts to RJS. CEN was supported by a NSF postdoctoral fellowship (IOS – 1402183).

Availability of data and materials

Genome browsers for all methylation data used in this paper are located at Plant Methylation DB (http://schmitzlab.genetics.uga.edu/plantmethylomes). Sequence data for MethylC-seq, RNA-seq, and small RNA-seq are located at the Gene Expression Omnibus, accession GSE79526. Code and other relevant data is available on GitHub (https://github.com/chadn737/Widespread-natural-variation-of-DNA-methylation-within-angiosperms.git).

Authors’ contributions

Conceptualization: CEN, AJB, and RJS Performed experiments: CEN, NAR, KDK, AR, JTP, and RJS Data Analysis: CEN, AJB, LJ, and MSA Writing – Original Draft: CEN Writing – Review and Editing: CEN, AJB, SAJ, NMS, and RJS Resources: QL, JMB, JAU, CE, JS, JG, SAJ, and NMS. All authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

Ethics approval

Ethics approval was not needed for this study.


Flowering Plants

Authors: Cassia grandis, Mauricio Mercadante, cc-by-nc-sa
Kerry Woods
Kevin J. Caley
Editor:
Emily Monosson
Source:
Encyclopedia of Earth

Flowering plants, or angiosperms are the largest grouping within the plant kingdom (Kingdom Plantae or Viridiplantae) in terms of the numbers of described species. Approximately 260,000 species of flowering plant have been named so far, constituting nearly 90% of all known species of plants. Even so, taxonomists describe many new species annually, and estimates of total angiosperm diversity reach as high as 400,000 species. There are about 450 families of flowering plants, displaying extremely diverse life histories and ecological adaptations. In addition to dominating plant biodiversity, angiosperms are the dominant photosynthetic organisms (primary producers) in most terrestrial ecosystems (an important exception to this rule are the boreal forests, which are often dominated by conifers). All important food plants are angiosperms.

Angiosperms are also the youngest of the plant divisions, having arisen relatively late in the history of terrestrial plant life. The first land plants are about 450 million years old, but the earliest definitive angiosperm fossils are only about 130 million years old, placing their known origins within the Early Cretaceous period. However, indirect evidence leads some scientists to estimate that angiosperms may have originated as early as 250 million years ago, that is, at the end of the Permian period.

Angiosperms are anatomically distinguished from other plant groups by several developmental and anatomical features. They produce flowers, which are very short branches bearing a series of closely spaced leaves modified to facilitate pollination (sepals and petals) or to bear the organs involved in sexual reproduction (stamens and pistils). Developing seeds are completely enclosed in an ovary derived from a portion of the pistil (the word angiosperm is of Greek derivation, meaning covered seed). Ovary tissues mature to form a fruit that is generally involved in protecting the seed and facilitating its dispersal (only angiosperms bear true fruits). Seeds at some point in their development contain a distinctive tissue, the triploid endosperm, which serves as a nutritional reserve for the developing embryo.

Angiosperms comprise such a large group, it is difficult to single out a mere few of the most intriguing, but here are some of the most important and diverse (and delicious!) flowering plant families.

The composite or daisy family (Asteraceae) has about 24,000 named species, and may be the largest plant family. The orchid family (Orchidaceae) rivals the daisy family in diversity, with about 17,000 species named. The grass family (Poaceae), with over 10,000 species, includes three of the four most productive human-food plants: rice (Oryza), wheat (Triticum) and maize (Zea). The fourth, the potato (Solanum tuberosum), is a member of the nightshade family (Solanaceae). The bean (or legume) family (Fabaceae, also known as Papilionaceae) includes about 19,000 species, many of which are important in human food because their symbiotic association with certain nitrogen-fixing bacteria leads to unusually high protein content.

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This article was adapted from the Encyclopedia of Earth.
Available under CC BY-SA-2.5


Callose and its Role in Pollen and Embryo Sac Development in Flowering Plants

I.B.2. Callose and its Role during Cytokinesis

While the role of the callose SCW during microgamete development remains unclear, less uncertainty surrounds the role of callose in intersporal walls, where initial separation of the individual microspores provides one obvious function. The provision of a subsequently degradable temporary wall, to allow release of microspores from the tetrad, is a possible second function (e.g. see Echlin and Godwin, 1968 ). However, more significant in terms of its impact on angiosperm evolution is where the intersporal callose walls form and when, because this will determine how many germination pores the mature pollen grain will have. Pore number is arguably the most important character in flowering plant taxonomy as the major division of angiosperms is not monocot versus dicot but one based on pollen type. Basal angiosperms are characterized by having one-pored or uniaperturate pollen, whereas the more highly derived angiosperms (eudicots) have three-pored or triaperturate pollen ( Angiosperm Phylogeny Group , 1998 ). The tricolpate pollen of Arabidopsis is an example of one type of triaperturate pollen.

Figure 4 illustrates the two general patterns for the appearance of callose walls after microspore meiosis seen in flowering plants ( Furness et al., 2002 Furness and Rudall, 2004 ). As already mentioned, Arabidopsis microspores undergo simultaneous cytokinesis at the end of meiosis II. In simultaneous cytokinesis, the spindles of the first and second meiotic divisions appear to interact to form a tetrahedral tetrad ( Fig. 4A Furness et al., 2002 ). In the alternative pattern, successive cytokinesis, a dyad stage corresponding to the first cytoplasmic division, occurs after meiosis I. The second meiotic division follows. But because the spindles can adopt various orientations with respect to each other, a variety of different arrangements of the tetrad are produced ( Fig. 4B–E ).

Fig. 4 . Diagram of the development of common tetrad types. (A) Simultaneous division. (B–E) Successive division. Arrowheads indicate equatorial plane of spindle solid circle indicates nucleus solid circle inside a small circle indicates nucleus directly below the one above.

(Reproduced with permission of the University of Chicago Press from Furness et al., 2002 .)

Microspore cytokinesis is highly variable among the basal angiosperms, a group of plants that includes species such as the familiar water-lilies (Nymphaeales) and the monocots, of which the grasses (Poaceae) are a well-known example ( Angiosperm Phylogeny Group, 1998 ). In these plants both simultaneous and successive forms of cytokinesis are observed, as well as a number of modified versions where, for instance, an ephemeral cell plate forms after meiosis I and subsequently disperses, with simultaneous cytokinesis occurring after meiosis II ( Furness et al., 2002 ). In palms (Arecaceae), the simultaneous and successive types of cytokinesis can even occur within the same stamen ( Sannier et al., 2006 ). Regardless of the type of cytokinesis, most basal angiosperms produce pollen with a single pore at one pole or an aperture pattern based on this arrangement.

In the more highly derived eudicots, a group that includes ∼75% of present day species and most of the plants traditionally classified as dicots (Arabidopsis is an example), microspores are formed by simultaneous cytokinesis and pollen grains are triaperturate or have a related aperture pattern ( Furness and Rudall, 2004 Angiosperm Phylogeny Group, 1998 ). Those basal angiosperms that do have triaperturate pollen – for example members of the family Illiciaceae such as star-anise – also have simultaneous cytokinesis, just as in Arabidopsis ( Sampson, 2000 Furness et al., 2002 ). It thus appears that triaperturate pollen grains can only be formed by simultaneous cytokinesis, whereas uniaperturate pollen grains can be formed by either simultaneous or successive cytokinesis.

What’s the basis for this relationship between callose wall formation and the number of germination pores? Pollen apertures first appear during ontogeny of the exine layer of the pollen wall, but their locations are defined by the geometry of the tetrad and are controlled by the meiotic spindle ( Sheldon and Dickinson, 1986 Ressayre et al., 1998, 2005 ). Combinations of different variable elements during meiosis can account for most of the widespread patterns. These elements include the timing of cytokinesis (successive, simultaneous or intermediate), the orientation of the meiotic axes (tetrahedral, tetragonal, decussate, linear or T-shaped Fig. 4 ), and the way callose is deposited to form the intersporal walls (which can be either centripetally as in Arabidopsis or centrifugally as in most monocots ( Nadot et al., 2006 ). Generally, apertures are formed at the last points of cytoplasmic contact between meiotic products, with polar apertures being additionally defined by the position of the spindle pole at second meiosis ( Ressayre et al., 2005 ). As an example, in a eudicot tetrad the 12 apertures (4 microspores × 3 apertures per microspore) form in pairs at midpoints (equatorially) along the edges of the tetrahedron created by the six bipolar spindles ( Fig. 4A ). Places where apertures will later form are initially marked by patches of microtubules and endomembranes that appear to be involved in blocking primexine template synthesis at these sites ( Dickinson and Sheldon, 1984 Munˇoz et al., 1995 Ressayre et al., 2002 ).

There is great interest in the genes and proteins that are involved in male meiotic cytokinesis. By their very nature, mutations that affect the function of these genes are also likely to reduce pollen viability, so genetic screens for male sterility have been extensively used in their identification (e.g. see Caryl et al., 2003 Johnson-Brousseau and McCormick, 2004 ).

As expected, some male sterility mutations do specifically affect cytokinesis and are linked to formation or dissolution of callose. In Arabidopsis, the GLS genes, AtGSL1 and AtGSL5, appear to play essential and overlapping roles in synthesizing the intersporal walls ( Enns et al., 2005 ). Microgamete development in mutant atgsl1 or atgsl5 plants proceeds normally up to the completion of meiosis I (i.e. a bicellular microsporocyte surrounded by a callosic SCW), suggesting that GSL1 and GSL5 are not required during these early stages. However, while plants homozygous for gsl1 produce normal pollen, 15% of the pollen from gsl5-homozygous plants and 30% of the pollen from gsl1/+ gsl5/gls5 plants (i.e. plants homozygous for the gsl1 mutation and heterozygous for the gsl5 mutation) and gsl1/gls1 gsl5/+ plants are small and shrivelled with misplaced and misshapen apertures. As these pollen grains have completed meiosis, this indicates that a deficiency of GSL1 and GSL5 has effects on later stages of pollen development when callose is not normally present. Significantly, gls1/+ gsl5/gsl5 plants also produce enlarged multinucleate pollen grains with more than three pores: the absence of intersporal walls in these pollen grains indicates that GSL1 and GSL5 are required for the synthesis of the intersporal walls but not the callosic SCW ( Enns et al., 2005 ). This phenotype can be contrasted with the effect of mutations in ATGSL2, where there is no common SCW but cytokinesis proceeds normally ( Nishikawa et al., 2005 ).

Failure to form intersporal walls is also seen in Arabidopsis plants carrying the allelic mutations tetraspore and stud ( Hűlskamp et al., 1997 Spielman et al., 1997 ). Because microspore cytokinesis does not take place in these mutants, large, multinucleate coenocytic pollen grains with aberrant numbers of misplaced apertures are produced ( Spielman et al., 1997 ). Consistent with the proposed role of microtubules in regulating both the placement of cleavage planes and the sites of future pore formation, the product of TETRASPORE is a kinesin, a motor protein that binds to microtubules and is involved in vesicle trafficking ( Yang et al., 2003 ).

Once cytokinesis is complete, breakdown of the individual SCWs generally results in microspore release. Some species, however, naturally release their pollen as a tetrad of fused pollen grains. This is seen in members of the mountain pepper family (Winteraceae), as well as in the water lilies (Nymphaeales), bulrushes (Typhaceae), heaths (Ericaceae), evening primroses (Onagraceae) and acacias (Fabaceae) ( Smyth, 1994 Copenhaver et al., 2000 ). In these families there is often little or no callose in the intersporal cross walls of the tetrad, which allows the exine layers of adjacent microspores to fuse and prevents the pollen grains from being separated ( Prakash et al., 1992 Scott et al., 2004 ). Arabidopsis plants with one of three non-allelic quartet (qrt) mutations (qrt1-3) also produce fused tetrads of pollen grains ( Preuss et al., 1994 Copenhaver et al., 2000 Rhee et al., 2003 ). Although defective or delayed callose degradation has been suggested as a reason why qrt microspores fail to separate ( Echlin and Godwin, 1968 Izhar and Frankel, 1971 ), analysis of the qrt mutants suggests that the tetrads are instead held together by changes in pectic components in the primary cell wall around the microsporocyte ( Rhee and Somerville, 1998 ). Consistent with this, QRT1 has recently been shown to encode a pectin methylesterase and QRT3 an endo-polygalacturonase ( Rhee et al., 2003 Francis et al., 2006 ).


Data Availability

All data generated in this study are publicly released under a Creative Commons Attribution 4.0 International (CC BY 4.0) license and the Toronto guidelines on pre-publication data sharing (Toronto International Data Release Workshop Authors 2009). The data are accessible via the Kew Tree of Life Explorer (https://treeoflife.kew.org) and our secure FTP (http://sftp.kew.org/pub/treeoflife/). Raw sequence reads are deposited in the European Nucleotide Archive (https://www.ebi.ac.uk/ena/browser/home) under umbrella project PRJEB35285. Scripts and other files relating to our phylogenomic pipeline are available at our GitHub (https://github.com/RBGKew/KewTreeOfLife). Supplementary materials cited in this paper plus Data Release 1.0 data sets duplicated from our secure FTP (assembled genes, gene alignments, gene trees, species tree, examples of scripts) are available from the Dryad Digital Repository (https://doi.org/10.5061/dryad.ns1rn8ps7).


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