apomixis and polyembryony

APOMIXIS AND POLYEMBRYONY

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

It is of two types :

(i) Agamospermy and

(ii) Vegetative Propagation

Agamospermy is a mode of apomixis in which seeds are formed but are asexual in nature as the embryo develops directly without gametic fusion.

There are several types of agamospermy :

(i) Adventitive Embryony (Sporophytic budding) : Embryo arises from diploid sporophytic cells such as nucellus or integuments (other than egg), e.g., Citrus, Opuntia, Mango.

(ii) Recurrent Agamospermy : In this method, a diploid embryo sac is formed from megaspore mother cell which has a diploid egg or oosphere. The diploid egg grows parthenogenetically into diploid embryo, e.g., Apple, Pear, Allium. Diploid embryo sac can develop directly from either the diploid megaspore mother cell (diplospory) or diploid nuclear cell (apospory).

(iii) Non-recurrent Agamospermy : Embryo develops parthenogenetically from haploid egg, e.g., Banana.

(iv) Apogamy: It is formation of sporophyte or embryo directly from cells of gametophyte. In higher plants, only diploid apogamy is successful. In lower plants, haploid apogamy is equally successful.

Polyembryony is the phenomenon of having more than one embryo in a seed, discovered in Citrus by Leeuwenhock. There may be more than one egg cell in an embryo sac or more than one embryo sac in an ovule.

All the egg cells may get fertilised. Synergids and antipodal cells may also form embryos (mixed polyembryony). Occurrence of polyembryony due to fertilisation of more than one egg is called simple polyembryony.

Formation of extra embryos through sporophytic budding is called adventitive polyembryony.

Polyembryony is quite common in onion, groundnut, mango, lemon and orange. In some of these cases stimulus of pollination may be required. Citrus seed has 2-40 embryos, one normal and the rest adventitive mostly nucellar.

Importance of Apomixis:

Hybrid varieties of several of our food and vegetable crops are being extensively cultivated because they provide higher and better yield. However, there is one major drawback.

Hybrid seeds have to be produced every year because seeds collected from hybrid plants, if sown subsequently, do not maintain hybrid characters due to segregation of traits.

Production of hybrid seeds every year is costly thus increasing the cost of crop production. This can be avoided if apomixis is introduced in hybrid seeds.

Concept Builder

(i) The study of seed is called spermology.

(ii) Chalazosperm is perisperm like tissue in chalazal region. It is a substitute for endosperm e.g. Cynastrum.

(iii) Seed viability in Oxalis -few days

(iv) Apomixis is common in some species of Asteraceae and grasses.