According to Winkler (1908, 1934), the substitution for sexual reproduction of an asexual process which does not involve any nuclear fusion. The term apomixis (apo = without; mixis = mingling) is a general one, and covers all types of asexual reproduction which tend to replace, or to act as substitute for the sexual method.
This phenomenon covers all the non-sexual methods of reproduction in which the reproducing spores are developed without fusion, whereas the normal sexual cycle (amphimixis) involves two important processes: (a) meiosis, that transforms a diploid (2n) sporophytic cell into four haploid (n) gametophytic cells, and (b) fertilization, where two haploid gametes of opposite sex fuse and re-establish the diploid sporophytic generation.
Thus, in a sexual cycle, a diploid (2n) or sporophytic generation alternates with the haploid (n) or gametophytic generation. In angiosperms, the gametophytic generations are very short and are represented by embryo sac on the female side and microspore or pollen grain on the male side. The remaining part of the life-cycle represents the sporophytic generation.
The plants where the usual sexual reproduction has been completely replaced by a type of asexual reproduction are called apomictic, and the phenomenon, apomixis.
There are two main categories of apomixis, i.e.:
1. Agamospermy and
2. Vegetative reproduction.
In this category of apomixis the plants have retained seed as the agent of propagation but the embryo is formed by some process in which normal meiosis and syngamy have been eliminated. This phenomenon is known as agamospermy.
There are three different types of agamospermy:
1. Adventive embryony:
In this type of agamospermy, embryos arise directly from the diploid sporophytic cells either of nucellus or integuments. The sexual embryo sac develops in normal way and the zygotic embryo either degenerates or competes with the apomictic embryos.
In this type of agamospermy, a diploid embryo sac is formed from a megaspore mother cell, without a regular meiotic division (e.g., in Aerva tomentosa). In this type an archesporium differentiates, but the megaspore mother cell develops into an unreduced embryo sac. The embryo is formed by the unfertilized egg, i.e., parthenogenesis or some other cell of the embryo sac, i.e., apogamety.
This phenomenon was reported in angiosperms for the first time by Rosenberg (1907). Here a somatic cell in the nucellus directly forms an unreduced embryo sac, and the diploid egg parthenogenetically develops into embryo. Here the megaspores gradually degenerate and the aposporic embryo sac may develop from a functional megaspore mother cell.
However, for the sake of convenience Dr. P. Maheshwari (1950) has sub-divided apomixis into three types, i.e.,
(i) Nonrecurrent apomixis,
(ii) recurrent apomixis, and
(iii) Adventive embryony.
i. Non-recurrent apomixes:
In this type the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac is formed. Here the embryo arises either from the egg (i.e., haploid parthenogenesis) or from some other cell of the gametophyte (i.e., haploid apogamy).
The plants produced by this method are, haploid and generally sterile and do not reproduce sexually any more. This type of apomixis has been seen in several species such as Solarium nigrum, Lilium, Bergenia, Erythraea centaurium. Orchis maculata, Nicotiana tabacum, etc.
ii. Recurrent apomixes:
In this type, the embryo sac generally arises either from an archesporial cell (i.e., generative apospory) or from some other part of the nucellus (i.e., somatic apospory). Here all the nuclei of the embryo sac are diploid, and there is no meiotic division.
The embryo arises either from the egg (diploid parthenogenesis) or from some other cell of the gametophyte (diploid apogamy). Generative apospory has been observed in Eupatorium glandulosum, Parthenium argentatum, etc. Somatic apospory has been reported in Hieracium excelens, H. flagellare and H. aurantiacum.
iii. Adventive 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 nucellus or the integument. There is no alternation of generations, because the diploid tissues of the present sporophyte directly give rise to the new embryo.
Adventive embryony has been frequently reported in Citrus, Euphorbia dulcis, Capparis frondosa, Mangifera indica and Hiptage madablota.
Significance of apomixes:
The most important apomictic crop plants are citrus, mango, mangos-teen and black berries. As a reproductive system, it offers the possibility of the indefinite propagation of specially favourable biotypes, which may be highly heterozygous or sexually sterile.
In obligate apomixis, this advantage is served at the expense of the long term evolutionary flexibility which is the gift of sexuality. However, in facultative apomixis, where sexual and apomictic members co-exist, the phenomenon is of special significance.
In this type of apomixis, the new individual arises from a group of undifferentiated or differentiated cells, where neither embryo nor seed are produced. This type of reproduction takes place by means of bulbs, bulbils, tubers, runners, suckers, etc. Such propagules are formed by the sporophyte only.
Gustafsson (1946) has distinguished three types of vegetative reproduction in angiosperms:
1. In this type, the propagules are formed outside the floral regions; though the sex organs are formed, yet no fertilization or seed setting takes place, e.g., in Agave americana and Elodea canadensis.
2. In this type, the propagules are formed outside the floral regions, and the plants are sexually sterile, as found in Fritillaria imperialis and Lilium bulbiferum.
3. In this type, the propagules are formed on the floral branches either in addition to the flowers or in place of them. This phenomenon is commonly known as vivipary.
This term is also used for those plants (e.g., mangrove vegetation), in which sexually formed seeds germinate on the mother plant. Here the term vivipary will be used as vegetative vivipary. The vegetative vivipary is commonly found in grasses, e.g., Festuca, Pea, and in Allium.
The vegetative vivipary is actually an adaptation for the multiplication of a genotype under a set of environmental conditions which prevent or check to some extent the opportunity for normal pollination.
However, most of the vegetative viviparous races have not lost the capacity for flowering and normal seed setting, e.g., a grass, Deschampsia caespitosa is sexually reproducing in Sweden, while growing by vegetative vivipary in California.
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