Question

Explain the different ways apomictic seeds can develop, Give an example of each.

Answer 1

Apomixis is the formation of new individuals through asexual reproduction without involving the formation and fusion of gametes.

The two common types of apomixis are recurrent agamospermy and adventive embryony.

(i) Recurrent agamospermy: Agamospermy is the formation of seed and has an embryo formed without meiosis and syngamy. It is of two types—noncurrent and recurrent. In noncurrent agamospermy, the embryo is haploid. Therefore, the seed having it is non-viable. In recurrent agamospermy, all the cells of the embryo sac are diploid as it is formed directly either from a
nucellar cell (apospory) or diploid megaspore mother cell (diplospory). The diploid egg and other diploid cells of the embryo sac can grow into normal embryos. Embryo formation directly from the diploid egg without fertilisation is called diploid parthenogenesis. Examples: Rubus, apple, poa

(ii) Adventive embryony (sporophytic budding): Formation of an embryo directly from the diploid sporophytic cells such as the nucellus and the integument of the ovule is called adventive embryony. Examples: Citrus, Opuntia. During embryogenesis, an embryo develops from the zygote inside the embryo sac, and the embryo sac becomes an endosperm. An apomictic
embryo, if developed, increases the number of embryos inside the seed, called polyembryony.

In gymnosperms, polyembryony can also occur because of cleavage of the growing embryo. It is called cleavage polyembryony. Occurrence of polyembryony because of fertilisation of more than one egg is called simple polyembryony. Formation of extra embryos through sporophytic budding is called adventive polyembryony. Polyembryony is quite common in onion,
groundnut, mango, lemon and orange.