Apomixis & Polyembryony - Practice Questions & MCQ

Apomixis:

• Apomixis (Gk. apo- without, mixis- mixing) is a mode of reproduction which does not involve the formation of zygote through gametic fusion. 
• Hence, there is no fertilization involved.
• In plants apomixis commonly mimics sexual reproduction but produces seeds without fertilisation, e.g., some species of Asteraceae and grasses.
• It is of following types:
  • Agamospermy
  • Adventive embryony 

Agamospermy:

• Agamospermy (Gk. a- without, gamos- marriage, sperma- seed) is the formation of seed that has an embryo formed without meiosis and syngamy. 
• It is of two types, non-recurrent and recurrent.
• In non-recurrent 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. 
• These can be formed:
  • directly from a nucellar cell (apospory) 
  • Indirectly from diploid megaspore mother cell (diplospory)
• The diploid egg, as well as other diploid cells of the embryo sac, can grow into normal embryos. 
• Formation of an embryo directly from a diploid egg without fertilization is called diploid parthenogenesis, e.g., Rubus, Apple, Poa.

Adventive Embryony:

• An embryo develops directly from a diploid cell other than an egg like that of nucellus and integument, e.g., Citrus, Opuntia. 
• It gives rise to a con­dition called polyembryony or the phenomenon of having more than one embryo. 
• 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. 
• In gymnosperms, polyembryony can also occur due to cleavage of growing em­bryo. 
• It is called cleavage polyembryony.
• The occurrence of polyembryony due to 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, Orange.