Apomixis Classification and Significance

Apomixis

Seeds are formed but the embryos develop without fertilization. The plants resulting from them are identical in genotype to the parent plant. In apomixis, sexual reproduction in either suppressed or absent. When sexual reproduction does occur, the apomixis is termed as ” Facultative”. But when sexual reproduction is absent it is referred to as obligate.

CLASSIFICATION

Adventive Embryony

Embryos develop directly from vegetative cells of the ovule, such as nucellus, integument and chalaza. Adventive embryony occurs in Mango (Mangifera indica) citrus etc.,

Apospory

Some vegetative cells of the ovule develop into unreduced embryosacs after meiosis. The embryo may develop from egg cell or some other cell of this embryo sac. It occurs in some species of Hieraceum, Malus, crepis, Ranum culus etc.

Diplospory’

Embryo sac is produced from the megaspore, which may be haploid or more generally diploid. Generally the meiosis is no modified that the megaspore remains diploid. Diplospory leads to parthenogenesis or apogamy.

Parthenogenesis

The embryo develops from egg cell. Depending upon whether the embryosac is haploid or diploid, parthenogenesis is termed as haploid or diploid parthenogenesis. Haploid parthenogenesis occurs accidentally and has been reported in Solanum nigera, Nicotiana, Crepis and Maize. Diploid parthenogenesis occurs in many grasses e.g. Taraxacum.

Apogamy

Synnergids or antipodal cells develop into an embryo. Like parthenogenesis, apogamy may be haploid or diploid depending upon the haploid or diploid state of the embryosac. Diploid apogamy occurs in Anternna, Alchemilla, Allium and many other plant species.

Significance of Apomixis

Apomixis is a nuisance when the breeder desires to obtain sexual progeny. But it is of great help when the breeder desires to maintain varieties. Thus in breeding of apomicitc species, the breeder has to avoid apomictic progeny when he is making crosses or producing inbred lines. But once a desirable genotype has been selected, it can be multiplied and maintained through apomictic progeny. This would keep the genotype of a variety intact.

Classification based on stability

Based on the stability of apomixis in subsequent generations, they are classified as,

1. Recurrent (stable ) apomixis

2. Non-recurrent (unstable) apomixis

Diploid apospory, diploid parthenogenesis and diploid apogamy are recurrent apomixis. Haploid apospory, haploid parthenogenesis and haploid apogamy are nonrecurrent apomixis.

Androgamy

Refers to the development of embryo from one of the male gametes, inside or outside the embryo sac and it is haploid in nature. Apomicts conserve the genetic constitution of the parent. Heliozygosity and consequent hybrid vigour can be permanently fixed through apomixis. Genetically uniform individuals can be rapidly multiplied, as it does not involve segregation. Breeding for apomicts is called ” Single line breeding”.

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