Apomixis

Apomixis is a form of reproduction in which an embryo forms through an essentially asexual route, without the formation of true gametes and without the act of fertilization, even though it occurs within the structural context of what looks like normal sexual reproduction. Put another way, apomixis is 'sexual reproduction' in appearance only: it takes place using the flower's reproductive machinery, but without meiosis and without syngamy actually occurring. The embryo develops directly within the ovule, and the ovule matures into a seed exactly as it would after true fertilization, so apomictic seeds, unlike ordinary asexual propagules, still allow the species to disperse via seed while producing offspring genetically identical to the parent.

Two closely related processes underlie apomixis: apogamy, where a cell or organ of the gametophyte generation (for instance the egg cell itself, or another haploid cell of the embryo sac) directly produces an embryo-like structure without any fertilization event; and apospory, where a diploid sporophytic cell (typically in the nucellus) directly produces a diploid gametophyte-like structure without undergoing meiosis at all; the textbook examples, orange and mango, are plants in which such nucellar cells give rise to embryos in the seed.

The main categories of apomixis are:

a. Recurrent apomixis : The embryo sac arises, generation after generation, from either an archesporial cell or another nucellar cell, without a normal reduction division. In diplospory, the unreduced (diploid) embryo sac develops directly from the diploid megaspore mother cell itself (as in Taraxacum); in apospory, it instead develops from ordinary nucellar cells lying outside the megaspore lineage.

b. Non-recurrent apomixis : Here the megaspore mother cell does undergo a normal meiotic division, producing a genuinely haploid embryo sac, but the embryo that eventually forms arises either from the (unfertilized) egg cell by parthenogenesis, or from some other haploid gametophytic cell via apogamy. Because this route still involves meiosis, plants arising this way are typically sterile and cannot themselves reproduce sexually (as in Nicotiana).

c. Adventive Embryony : Embryos develop directly from somatic cells of the nucellus or the integuments, entirely outside the embryo sac, and typically alongside a normally-formed zygotic embryo in the same seed, common in mango, orange, and lemon. Because it produces multiple embryos within one seed, adventive embryony is a major cause of the condition called polyembryony. …