Inbotany,apomixis isasexual development ofseed or embryo withoutfertilization. However, other definitions include replacement of the seed by aplantlet or replacement of the flower bybulbils.
Apomictically produced offspring are genetically identical to the parent plant, except in nonrecurrent apomixis. Its etymology is Greek for "away from" + "mixing".
Normal asexual reproduction of plants, such as propagation fromcuttings or leaves, has never been considered to be apomixis. In contrast toparthenocarpy, which involvesseedless fruit formation without fertilization, apomictic fruits have viable seeds containing a proper embryo, with asexual origin.
In flowering plants, the term "apomixis" is used in a restricted sense to meanagamospermy, i.e.clonal reproduction through seeds. Although agamospermy could theoretically occur ingymnosperms, it appears to be absent in that group.[1]
Apogamy is a related term that has had various meanings over time. In plants with independentgametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation ofsporophytes byparthenogenesis of gametophyte cells.
Male apomixis (paternal apomixis) involves replacement of the genetic material of an egg by the genetic material of the pollen.
Some authors included all forms of asexual reproduction within apomixis, but that generalization of the term has since died out.[1]
Because apomictic plants are genetically identical from one generation to the next, each lineage has some of the characters of a truespecies, maintaining distinctions from other apomictic lineages within the samegenus, while having much smaller differences than is normal between species of most genera. They are therefore often calledmicrospecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together asspecies aggregates, typically listed infloras with the convention "Genus species agg." (such as thebramble,Rubus fruticosus agg.). In someplant families, genera with apomixis are quite common, for example inAsteraceae,Poaceae, andRosaceae. Examples of apomixis can be found in the generaCrataegus (hawthorns),Amelanchier (shadbush),Sorbus (rowans andwhitebeams),Rubus (brambles or blackberries),Poa (meadow grasses),Nardus stricta (matgrass),Hieracium (hawkweeds) andTaraxacum (dandelions). Apomixis is reported to occur in about 10% of globally extantferns.[2] Amongpolystichoid ferns, apomixis evolved several times independently in three differentclades.[2]
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 can also happen. It appears likely that all apomixis in plants is facultative;[4][5] in other words, that "obligate apomixis" is an artifact of insufficient observation (missing uncommon sexual reproduction).
The gametophytes ofbryophytes, and less commonlyferns andlycopods can develop a group of cells that grow to look like a sporophyte of the species but with theploidy level of the gametophyte, a phenomenon known as apogamy. The sporophytes of plants of these groups may also have the ability to form a plant that looks like a gametophyte but with the ploidy level of the sporophyte, a phenomenon known as apospory.[6][7]
See also androgenesis and androclinesis described below, a type of male apomixis that occurs in a conifer,Cupressus dupreziana.
Agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms[4] and a simplehierarchical classification of the different types is not possible. Consequently, there are almost as many different usages of terminology for apomixis inangiosperms as there are authors on the subject. For English speakers, Maheshwari 1950[8] is very influential. German speakers might prefer to consult Rutishauser 1967.[9] Some older text books[10] 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 the termparthenogenesis as it is used inzoology, and this continues to cause much confusion.
Agamospermy occurs mainly in two forms: Ingametophytic apomixis, theembryo arises from an unfertilized egg cell (i.e. by parthenogenesis) in agametophyte that was produced from a cell that did not complete meiosis. Inadventitious embryony (sporophytic apomixis), an embryo is formed directly (not from a gametophyte) fromnucellus or integument tissue (seenucellar embryony).
Maheshwari[8] 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 sacmegagametophyte 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 manychromosomes as the mother plant, and "the process is not repeated from one generation to another" (which is why it is called nonrecurrent). See alsoparthenogenesis andapogamy below.
Recurrent apomixis, is now more often calledgametophytic apomixis: In this type, the megagametophyte has the same number of chromosomes as the mother plant because meiosis was not completed. It generally arises either from anarchesporial cell or from some other part of thenucellus.
Adventive embryony, also calledsporophytic apomixis,sporophytic budding, ornucellar embryony: Here there may be a megagametophyte in theovule, 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 ofCitrus, inGarcinia,Euphorbia dulcis,Mangifera indica etc.
Vegetative apomixis: In this type "the flowers are replaced bybulbils or other vegetative propagules which frequently germinate while still on the plant". Vegetative apomixis is important inAllium,Fragaria,Agave, and some grasses, among others.
Gametophytic apomixis in flowering plants develops in several different ways.[11] A megagametophyte develops with an egg cell within it that develops into an embryo throughparthenogenesis. The central cell of the megagametophyte may require fertilization to form theendosperm,pseudogamous gametophytic apomixis, or inautonomous gametophytic apomixis endosperm fertilization is not required.
Indiplospory (also calledgenerative apospory), the megagametophyte arises from a cell of thearchesporium.
Inapospory (also calledsomatic apospory), the megagametophyte arises from some other (somatic) cell of the nucellus.
Considerable confusion has resulted because diplospory is often defined to involve themegaspore mother cell only, but a number of plant families have a multicellular archesporium and the megagametophyte could originate from another archesporium cell.
Diplospory is further subdivided according to how the megagametophyte forms:
Allium odorum–A. nutans type. The chromosomes double (endomitosis) and then meiosis proceeds in an unusual way, with the chromosome copies pairing up (rather than the original maternal and paternal copies pairing up).
Taraxacum type: Meiosis I fails to complete, meiosis II creates two cells, one of which degenerates; three mitotic divisions form the megagametophyte.
Ixeris type: Meiosis I fails to complete; three rounds of nuclear division occur without cell-wall formation; wall formation then occurs.
Blumea–Elymus types: A mitotic division is followed by degeneration of one cell; three mitotic divisions form the megagametophyte.
Antennaria–Hieracium types: three mitotic divisions form the megagametophyte.
Eragrostis–Panicum types: Two mitotic division give a 4-nucleate megagametophyte, with cell walls to form either three or four cells.
Apomixis occurs in at least 33 families of flowering plants, and has evolved multiple times from sexual relatives.[12][13] Apomictic species or individual plants often have a hybrid origin, and are usually polyploid.[13]
In plants with both apomictic and meiotic embryology, the proportion of the different types can differ at different times of year,[11] andphotoperiod can also change the proportion.[11] It appears unlikely that there are any truly completely apomictic plants, as low rates of sexual reproduction have been found in several species that were previously thought to be entirely apomictic.[11]
The genetic control of apomixis can involve a single genetic change that affects all the major developmental components, formation of the megagametophyte, parthenogenesis of the egg cell, and endosperm development.[14] However, the timing of the various developmental processes is critical to successful development of an apomictic seed, and the timing can be affected by multiple genetic factors.[14]
Apomeiosis: "Without meiosis"; usually meaning the production of a meiotically unreduced gametophyte.
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 normalhaploid egg (a meiotically reduced egg) into an embryo is termed haploid parthenogenesis. If the mother plant was diploid, then the haploid embryo that results ismonoploid, 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, seedihaploidy). This type of apomixis has been recorded inSolanum nigrum,Lilium spp.,Orchis maculata,Nicotiana tabacum, etc.
Diploid parthenogenesis: When the megagametophyte develops without completing meiosis, so that the megagametophyte and all cells within it are meiotically unreduced (a.k.a. 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 ofgametophytic apomixis (see above).
Androgenesis andandroclinesis are synonyms. These terms are used for two different processes that both have the effect of producing an embryo that has "male inheritance".
The first process is a natural one. It may also be referred to asmale apomixis orpaternal apomixis. It involves fusion of the male and female gametes and replacement of the female nucleus by the male nucleus. This has been noted as a rare phenomenon in many plants (e.g.Nicotiana andCrepis), and occurs as the regular reproductive method in the Saharan Cypress,Cupressus dupreziana.[15][16][17] Recently, the first example of natural androgenesis in a vertebrate, a fish,Squalius alburnoides was discovered.[18] It is also known in invertebrates, particularly clams in the genusCorbicula, and these asexually reproducing males are noted to have a wider range than their noninvasive non-hermaphroditic cousins, more similar to hermaphroditic invasive species in the genus, indicating that this does sometimes have evolutionary benefits.[19]
The second process that is referred to as androgenesis or androclinesis involves (artificial) culture of haploid plants fromanther tissue ormicrospores.[20] Androgenesis has also been artificially induced in fish.[21]
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 cells involved in apogamy would be synergids or antipodal cells.
Addition hybrids, calledBIII hybrids by Rutishauser:[9] 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.
Pseudogamy refers to any reproductive process that requirespollination but does not involve male inheritance. It is sometimes used in a restrictive sense to refer to types of apomixis in which theendosperm is fertilized but theembryo is not. A better term for the restrictive sense iscentrogamy.[20]
Agamospecies, the concept introduced byGöte Turesson: "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.[22]
Cytomixis – Migration of the nucleus from one plant cell to another, a process of nuclear fusion that occurs during pollen meiosis
Klepton – Species that requires input from another biological taxon to complete its reproductive cycle, a phenomenon known in zoology where mating with another taxon is required to complete reproduction
^abNygren, A. (1967). "Apomixis in the angiosperms". In W. Ruhland (ed.).Handbuch der Pflanzenphysiologie. Vol. 18. Berlin: Springer-Verlag. pp. 551–596.
^abKoltunow, A.M.; Johnson, S.D.; Bicknell, R.A. (2000). "Apomixis is not developmentally conserved in related, genetically characterizedHieracium plants of varying ploidy".Sexual Plant Reproduction.12 (5):253–266.doi:10.1007/s004970050193.S2CID23186733.
Gvaladze G.E. (1976). Forms of Apomixis in the genusAllium L. In: S.S. Khokhlov (Ed.): Apomixis and Breeding, Amarind Pub., New Delhi-Bombay-Calcutta-New York pp. 160–165
Bhojwani S.S.& Bhatnagar S.P. (1988). The Embryology of angiosperms. Vikas Publishing house Pvt.Ltd. New Delhi.
Heslop-Harrison, J. (1972) "Sexuality in Angiosperms,"pp. 133–289, In Steward,F.C. (ed.) Plant Physiology, Vol. 6C, Academic Press New York.
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