Apomixis
Encyclopedia
In botany
, apomixis was defined by Winkler
as replacement of the normal sexual reproduction by asexual reproduction
, without fertilization. This definition notably does not mention meiosis
. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet
, or replacement of the flower by bulbils are types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.
In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. asexual reproduction through seeds
.
Apogamy is a related term that has had various meanings over time. In plants with independent gametophyte
s (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis
of gametophyte cells.
See also Male apomixis in a conifer below.
, maintaining distinctions from other congeneric apomicts, while having much smaller differences than is normal between species of most genera
. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as aggregate species, typically listed in Flora
s with the convention "Genus species agg." (e.g., the bramble
, Rubus fruticosus agg.). In some plant families
, genera with apomixis are quite common, e.g. in Asteraceae
, Poaceae
, and Rosaceae
. Examples of apomixis can be found in the genera Crataegus
(hawthorns), Amelanchier (shadbush), Sorbus
(rowan
s and whitebeam
s), Rubus
(brambles or blackberries), Poa
(meadow grasses), Hieracium (hawkweed
s) and Taraxacum (dandelions).
Although the evolution
ary advantages of sexual reproduction
are lost, apomixis can pass along traits fortuitous for evolutionary fitness. As Clausen eloquently
put it (page 470) "The apomicts actually have discovered the effectiveness of mass production long before Mr Henry Ford
applied it to the production of the automobile. ... Facultative apomixis ... does not prevent variation; rather, it multiplies certain varietal products." Facultative apomixis means that apomixis does not always occur, i.e. sexual reproduction also can happen. It appears likely that in plants all apomixis is facultative, i.e. that "obligate apomixis" is an artifact of the observation methods. Böcher noted that facultative apomictic plants increased sexual seed set under stress conditions.
classification of the different types is not possible. Consequently there are almost as many different usages of terminology for apomixis in angiosperms as there are authors on the subject. For English speakers, Maheshwari 1950 is very influential. German speakers might prefer to consult Rutishauser 1967. Some older text books on the basis of misinformation (that the egg cell in a meiotically unreduced gametophyte can never be fertilized) attempted to reform the terminology to match parthenogenesis as it is used in zoology
, and this continues to cause much confusion.
Agamospermy occurs mainly in two forms: In gametophytic apomixis, the embryo
arises from an unfertilized egg cell (i.e. by parthenogenesis
) in a gametophyte
that was produced from a cell that did not complete meiosis
. In adventitious embryony (sporophytic apomixis), an embryo is formed directly (not from a gametophyte) from nucellus or integument tissue (see nucellar embryony
).
The most complex of these types of apomixis in flowering plants is recurrent apomixis, now more often called gametophytic apomixis. It is divided into diplospory (generative apospory) in which the embryo sac arises from a cell of the archesporium, and apospory (somatic apospory) in which the embryo sac arises from some other nucellus cell. Considerable confusion has resulted because diplospory is often defined to involve the megaspore mother cell
only, but a number of plant families have a multicellular archesporium and the embryo sac could originate from one of the other cells.
, where the seeds are derived entirely from the pollen
with no genetic contribution from the female "parent". Similar mechanisms occur infrequently in other plants and are known as androgenesis or androclinesis.
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
, apomixis was defined by Winkler
Hans Winkler
Professor Hans Winkler was a German botanist. He was Professor of Botany at the University of Hamburg, and a director of that university's Institute of Botany. He is remembered for coining the term 'genome' in 1920, by making a portmanteau of the words gene and chromosome...
as replacement of the normal sexual reproduction by asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
, without fertilization. This definition notably does not mention meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet
Plantlet
Plantlets are young or small plants used as propagules. They are usually grown from clippings of mature plants. Many plants such as spider plants naturally create stolons with plantlets on the ends as a form of asexual reproduction.-External links:...
, or replacement of the flower by bulbils are types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.
In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. asexual reproduction through seeds
SEEDS
SEEDS is a voluntary organisation registered under the Societies Act of India....
.
Apogamy is a related term that has had various meanings over time. In plants with independent gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
s (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
of gametophyte cells.
See also Male apomixis in a conifer below.
Apomixis and evolution
As apomictic plants are genetically identical from one generation to the next, each has the characters of a true speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, maintaining distinctions from other congeneric apomicts, while having much smaller differences than is normal between species of most genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as aggregate species, typically listed in Flora
Flora (book)
A Flora is a book or other work which describes the plant species occurring in an area or time period, often with the aim of allowing identification. Some classic and modern floras are listed below....
s with the convention "Genus species agg." (e.g., the bramble
Bramble
Brambles are thorny plants of the genus Rubus, in the rose family . Bramble fruit is the fruit of any such plant, including the blackberry and raspberry. The word comes from Germanic *bram-bezi, whence also German Brombeere , Dutch Braam and French framboise...
, Rubus fruticosus agg.). In some plant families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, genera with apomixis are quite common, e.g. in Asteraceae
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...
, Poaceae
Poaceae
The Poaceae is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes and sedges...
, and Rosaceae
Rosaceae
Rosaceae are a medium-sized family of flowering plants, including about 2830 species in 95 genera. The name is derived from the type genus Rosa. Among the largest genera are Alchemilla , Sorbus , Crataegus , Cotoneaster , and Rubus...
. Examples of apomixis can be found in the genera Crataegus
Crataegus
Crataegus , commonly called hawthorn or thornapple, is a large genus of shrubs and trees in the rose family, Rosaceae, native to temperate regions of the Northern Hemisphere in Europe, Asia and North America. The name hawthorn was originally applied to the species native to northern Europe,...
(hawthorns), Amelanchier (shadbush), Sorbus
Sorbus
Sorbus is a genus of about 100–200 species of trees and shrubs in the subfamily Maloideae of the Rose family Rosaceae. Species of Sorbus are commonly known as whitebeam, rowan, service tree, and mountain ash...
(rowan
Rowan
The rowans or mountain-ashes are shrubs or small trees in genus Sorbus of family Rosaceae. They are native throughout the cool temperate regions of the Northern Hemisphere, with the highest species diversity in the mountains of western China and the Himalaya, where numerous apomictic microspecies...
s and whitebeam
Whitebeam
The whitebeams are members of the Rosaceae family, comprising subgenus Aria of genus Sorbus, and hybrids involving species of this subgenus and members of subgenera Sorbus, Torminaria and Chamaemespilus. They are deciduous trees with simple or lobed leaves, arranged alternately...
s), Rubus
Rubus
Rubus is a large genus of flowering plants in the rose family, Rosaceae, subfamily Rosoideae. Raspberries, blackberries, and dewberries are common, widely distributed members of the genus. Most of these plants have woody stems with prickles like roses; spines, bristles, and gland-tipped hairs are...
(brambles or blackberries), Poa
Poa
Poa is a genus of about 500 species of grasses, native to the temperate regions of both hemispheres. Common names include meadow-grass , bluegrass , tussock , and speargrass. "Poa" is Greek for fodder...
(meadow grasses), Hieracium (hawkweed
Hawkweed
Hawkweed refers to any species in the very large genus Hieracium and its segregate genus Pilosella, in the sunflower family ....
s) and Taraxacum (dandelions).
Although the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary advantages of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
are lost, apomixis can pass along traits fortuitous for evolutionary fitness. As Clausen eloquently
Eloquence
Eloquence is fluent, forcible, elegant or persuasive speaking. It is primarily the power of expressing strong emotions in striking and appropriate language, thereby producing conviction or persuasion...
put it (page 470) "The apomicts actually have discovered the effectiveness of mass production long before Mr Henry Ford
Henry Ford
Henry Ford was an American industrialist, the founder of the Ford Motor Company, and sponsor of the development of the assembly line technique of mass production. His introduction of the Model T automobile revolutionized transportation and American industry...
applied it to the production of the automobile. ... Facultative apomixis ... does not prevent variation; rather, it multiplies certain varietal products." Facultative apomixis means that apomixis does not always occur, i.e. sexual reproduction also can happen. It appears likely that in plants all apomixis is facultative, i.e. that "obligate apomixis" is an artifact of the observation methods. Böcher noted that facultative apomictic plants increased sexual seed set under stress conditions.
Apomixis in flowering plants (angiosperms)
Agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms and a simple hierarchicalHierarchy
A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...
classification of the different types is not possible. Consequently there are almost as many different usages of terminology for apomixis in angiosperms as there are authors on the subject. For English speakers, Maheshwari 1950 is very influential. German speakers might prefer to consult Rutishauser 1967. Some older text books on the basis of misinformation (that the egg cell in a meiotically unreduced gametophyte can never be fertilized) attempted to reform the terminology to match parthenogenesis as it is used in zoology
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
, and this continues to cause much confusion.
Agamospermy occurs mainly in two forms: In gametophytic apomixis, the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
arises from an unfertilized egg cell (i.e. by parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
) in a gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
that was produced from a cell that did not complete meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. In adventitious embryony (sporophytic apomixis), an embryo is formed directly (not from a gametophyte) from nucellus or integument tissue (see nucellar embryony
Nucellar embryony
Nucellar embryony is a form of seed reproduction that occurs in certain plant species, including many citrus varieties. During the development of seeds from plants that possess this genetic trait, the nucellar tissue which surrounds the megagametophyte can produce additional embryos which are...
).
Types of apomixis in flowering plants
Maheshwari used the following simple classification of types of apomixis in flowering plants:- Nonrecurrent apomixis: In this type "the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac is formed. The new embryo may then arise either from the egg (haploid parthenogenesis) or from some other cell of the gametophyte (haploid apogamy)." The haploid plants have half as many chromosomes as the mother plant, and "the process is not repeated from one generation to another" (which is why it is called nonrecurrent). See also parthenogenesis and apogamy below.
- Recurrent apomixis (also called gametophytic apomixis): In this type, the embryo sac has the same number of chromosomes as the mother plant because meiosisMeiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
was not completed. It generally arises either from an archesporial cell or from some other part of the nucellus. - Adventive embryony, also called sporophytic apomixis, sporophytic budding, or nucellar embryonyNucellar embryonyNucellar embryony is a form of seed reproduction that occurs in certain plant species, including many citrus varieties. During the development of seeds from plants that possess this genetic trait, the nucellar tissue which surrounds the megagametophyte can produce additional embryos which are...
: Here there may be an embryo sac (gametophyte) in the ovuleOvuleOvule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
, but the embryos do not arise from the cells of the gametophyte; they arise from cells of nucellus or the integument. Adventive embryony is important in several species of CitrusCitrusCitrus is a common term and genus of flowering plants in the rue family, Rutaceae. Citrus is believed to have originated in the part of Southeast Asia bordered by Northeastern India, Myanmar and the Yunnan province of China...
, in GarciniaGarciniaGarcinia is a plant genus of the family Clusiaceae native to Asia, Australia, tropical and southern Africa, and Polynesia. The number of species is highly disputed, with various sources recognizing between 50 and about 300 taxa as specifically valid...
, Euphorbia dulcis, Mangifera indicaMangifera indicaMangifera indica is a species of mango in the Anacardiaceae family. It is found in the wild in India and cultivated varieties have been introduced to other warm regions of the world...
etc. - Vegetative apomixis: In this type "the flowers are replaced by bulbils or other vegetative propagules which frequently germinate while still on the plant". Vegetative apomixis is important in AlliumAlliumAllium is a monocot genus of flowering plants, informally referred to as the onion genus. The generic name Allium is the Latin word for garlic....
, Fragaria, some grasses, etc.
The most complex of these types of apomixis in flowering plants is recurrent apomixis, now more often called gametophytic apomixis. It is divided into diplospory (generative apospory) in which the embryo sac arises from a cell of the archesporium, and apospory (somatic apospory) in which the embryo sac arises from some other nucellus cell. Considerable confusion has resulted because diplospory is often defined to involve the megaspore mother cell
Megaspore mother cell
A megaspore mother cell is a diploid cell in which meiosis will occur, resulting in the production of four megaspores. This megasporcyte is formed from two distinct processes – megasporogenesis and megagametogenesis....
only, but a number of plant families have a multicellular archesporium and the embryo sac could originate from one of the other cells.
Some terms related to apomixis
- Parthenogenesis: Development of an embryo directly from an egg cell without fertilization is called parthenogenesis. It is of two types:
- Haploid parthenogenesis: Parthenogenesis of a normal haploid egg (a meiotically reduced egg) into an embryo is termed haploid parthenogenesis. If the mother plant was diploid, then the haploid embryo that results is monoploid, and the plant that grows from the embryo is sterile. If they are not sterile, they are sometimes useful to plant breeders (especially in potato breeding, see dihaploidy). This type of apomixis has been recorded in Solanum nigram, Lilium spp., Orchis maculate, Nicotiana tabacum etc.
- Diploid parthenogenesis: When the embryo sac develops without completing meiosisMeiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
, so that the embryo sac and all cells within it are meiotically unreduced (aka diploid, but diploid is an ambiguous term), this is called diploid parthenogenesis, and the plant that develops from the embryo will have the same number of chromosomes as the mother plant. Diploid parthenogenesis is a component process of gametophytic apomixis (see above).
- Androgenesis: This term is confusing because it is used for two different processes that both have the effect of producing a haploid organism (plant) that has "male inheritance". The first process is a natural one that has been noted as a rare phenomenon in many plants (e.g. Nicotiana and Crepis); embryos arise from male gametes. The process involves fusion of the male and female gametes and replacement of the female nucleus by the male nucleus. The second process that is referred to as androgenesis involves (artificial) culture of haploid plants from anther tissue or microspores.
- Apogamy: Although this term was (before 1908) used for other types of apomixis, and then discarded as too confusing, it is still sometimes used when an embryo develops from a cell of the megagametophyte other than the egg cell. In flowering plants the megagametophyte is the embryo sac, and the cells involved in apogamy would be synergids or antipodal cells.
- Addition hybrids, called BIII hybrids by Rutishauser: An embryo is formed after a meiotically unreduced egg cell is fertilized. The ploidy level of the embryo is therefore higher than that of the mother plant. This process occurs in some plants that are otherwise apomictic, and may play a significant role in producing tetraploid plants from triploid apomictic mother plants (if they receive pollen from diploids). Because fertilization is involved, this process does not fit the definition of apomixis.
- PseudogamyPseudogamyPseudogamy refers to aspects of reproduction. It has come to have different meanings in zoology and in botany.- In zoology :In zoology it is used for a type of parthenogenesis in which the sperm stimulates the egg cell to develop into an embryo, but no male inheritance is transmitted...
refers to any reproductive process that requires pollinationPollinationPollination is the process by which pollen is transferred in plants, thereby enabling fertilisation and sexual reproduction. Pollen grains transport the male gametes to where the female gamete are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself...
but does not involve male inheritance. It is sometimes used in a restrictive sense to refer to types of apomixis in which the endospermEndospermEndosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...
is fertilized but the embryoEmbryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
is not. A better term for the restrictive sense is centrogamy. - Agamospecies, the concept introduced by Göte TuressonGöte TuressonGöte Wilhelm Turesson was a Swedish evolutionary botanist who made significant contributions to ecological genetics, and coined the terms ecotype and agamospecies. He conducted extensive work to demonstrate that there is a genetic basis to the differentiation of plant populations...
: "an apomict population the constituents of which, for morphological, cytological or other reasons, are to be considered as having a common origin," i.e., basically synonymous with "microspecies.
Male apomixis in a conifer
A unique example of male apomixis as the regular reproductive method has recently been discovered in the Saharan Cypress, Cupressus duprezianaCupressus dupreziana
Cupressus dupreziana, the Saharan Cypress, is a very rare coniferous tree native to the Tassili n'Ajjer mountains in the central Sahara Desert, southeast Algeria, where it forms a unique population of trees hundreds of kilometres from any other trees. There are only 233 specimens of this critically...
, where the seeds are derived entirely from the pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
with no genetic contribution from the female "parent". Similar mechanisms occur infrequently in other plants and are known as androgenesis or androclinesis.
See also
- ParthenocarpyParthenocarpyIn botany and horticulture, parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules. The fruit is therefore seedless. Stenospermocarpy may also produce apparently seedless fruit, but the seeds are actually aborted while still small...
, the production of seedless fruits. - ParthenogenesisParthenogenesisParthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
, the animalAnimalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
equivalent of apomixis. - CytomixisCytomixisCytomixis is a process of chromosome transfer that can occur during pollen formation in flowering plants. It results in pollen grains with chromosome numbers greater or less than the haploid number, possibly including some that are diploid or polyploid, and often producing some non-viable pollen...
, a process of nuclear fusion that occurs during pollen meiosis