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Apomixis
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In botany, apomixis (also called apogamy) is asexual reproduction, without fertilization. In plants with independent gametophytes (notably ferns), apomixis refers to the formation of sporophytes by parthenogenesis of gametophyte cells. Apomixis also occurs in flowering plants, where it is also called agamospermy. Apomixis in flowering plants mainly occurs in two forms: In agamogenesis (also called gametophytic apomixis), the embryo arises from an unfertilized egg that was produced without meiosis.
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In botany, apomixis (also called apogamy) is asexual reproduction, without fertilization. In plants with independent gametophytes (notably ferns), apomixis refers to the formation of sporophytes by parthenogenesis of gametophyte cells. Apomixis also occurs in flowering plants, where it is also called agamospermy. Apomixis in flowering plants mainly occurs in two forms: In agamogenesis (also called gametophytic apomixis), the embryo arises from an unfertilized egg that was produced without meiosis. In adventitious embryony , a nucellar embryo is formed from the surrounding nucellus tissue. Apomictically produced seeds are genetically identical to the parent plant.
The phenomenon where the usual sexual reproduction in plants has been completely replaced by a type of asexual reproduction is called apomixis. The term apomixes was first used by Winkler in 1908. He defined apomixis as the substitution for the usual sexual reproduction of a form of reproduction which does not involve meiosis and syngamy.
Thus, apomixis is an abnormal sexual reproduction in which the embryo develops from the egg cells associated with it without fertilization and with or without meiosis.
Types of Apomixis
Apomixis is of various types such as:-
1.Parthenogenesis: Development of an embryo directly from the egg cells or male gametes without pollination and fertilization is called parthenogeneasis. It is of two types:
a)Haploid parthenogenesis: Parthenogenesis of normal haploid egg into an embryo is termed as haploid parthenogenesis. Plants developd from such embryo are haploid and usually sterile. It has been noticed in Solanum nigrum.
b)Androgenesis: In Nicotiana and Crepis embryos have been reportrd from male gametes. Such an embryo development is called Androgenesis.
c)Diploid parthenogenesis: When the embryo sac develops without reduction diviosion so that embryo sac and all cells within it are diploid, are called diploid parthenogenesis. It gives a diploid plant and noticed in Taraxacum spp.
2.Apogamy: An embryo may some times develop from the cells other than egg cells i.e. synergids or antipodal cells of embryo sac. This is called apogamy. If the embryo develops from haploid synergids or antipodal cells, it is called haploid apogay. On the contrary if embryo sac develop from some diplod cells it is called diploid apogamy. E.g. Antenaria, Alchemilla , Allium3.Apospory: Development of cells other than embryo sac cells such as integument and nucellus. Into embryos is known apospory.
Depending upon the type of cells from where the embryo has developed apomixis is of following types:-
1.Nonrecurrent apomixis : In this type the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac is formed. The embryo arise either from the egg or from other cell of the gametophyte. The plants produced from these embryos are haploid and generally sterile. They do not reproduce any more. Solanum nigram, Lilium spp., Orchis maculate, Nicotiana tabacum etc. show this type of apomixis.
2.Recurrent apomixis: In this type, the embryo sac generally arises either from an archesporial cell or from some other part of the nucellus. Here all the nuclei of the embryo sac are diploid, and there is no meiotic division. This type of apomixis is observed in Eupatorium glandulosum, Parthenium argentatum etc.
3.Adventitive embryony: This type of apomixis is also known as sporophytic budding. Here the developed embryo sacs may be haploid or diploid, but the embryos do not arise from the cells of the gametophytes, and they arise only from the cells of nucellus or the integument. Adventive embryony has been reported in Citrus, Euphorbia dulcis, Mangifera indica etc.
As apomictic plants are genetically identical from one generation to the next, each has the characters of a true species, 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 Floras with the convention "Genus species agg." (e.g., the bramble, Rubus fruticosus agg.). Examples of apomixis can be found in the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Hieracium (hawkweeds) and Taraxacum (dandelions). Although the evolutionary advantages of sexual reproduction are lost, apomixis does pass along traits fortuitous for individual evolutionary fitness.
A unique example of male apomixis has recently been discovered in the Saharan Cypress, Cupressus dupreziana, where the seeds are derived entirely from the pollen with no genetic contribution from the female "parent" (Pichot, et al., 2000, 2001).
See also
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