APOMIXIS IN FLOWERING PLANTS
Reproduction in plants is of two types:
- Sexual or Amphimixis
- Asexual or Apomixis
Angiosperms have a Diplontic life cycle. The main part of the lifecycle is Sporophyte & the Gametophyte is reduced.
Apomixis mimics sexual reproduction but produces seeds without fertilization. Seen in some species of Family Asteraceae & Grasses.
It is of 3 types:
- Apogamy: Formation of sporophyte/embryo from any cell of the gametophyte/embryo sac without fertilization. Cells of the embryo sac i.e., antipodal cells or the synergids. The embryo formed is haploid.
- Apospory: Formation of gametophyte i.e., the embryo sac from any cell of the sporophyte without meiosis. Cells of the sporophyte are cells of nucellus & integuments. The embryo sac formed has diploid cells. The egg is also diploid. Diploid egg either undergo fertilization or undergoes Parthenogenesis.
- Parthenogenesis: Formation of the embryo from the egg without fertilization.
Apomixis can be divided into different types according to occurrence:
- Non-Recurrent: The plant produced due to apomixis is unable to reproduce further.
Seen in the case of:
Haploid Parthenogenesis: Development of embryo from the egg without fertilization. The egg formed is haploid.
2. Recurrent: The plant produced due to apomixis can reproduce further.
Seen in the case of:
Diplospory: The megaspore mother cell forms embryo sac without meiosis. The egg formed is diploid.
3. Adventive embryony: Also referred as Sporophytic budding. The embryo is formed from the cells of nucellus or integuments. Examples are Mango, Citrus, Opuntia. Due to this Polyembryony occurs.
Polyembryony is the formation of many embryos in the seed.
Adventive Polyembryony is the formation of many embryos in the embryo sac from the cells of integuments and nucellus.
Cleavage Polyembryony is the formation of many embryos due to division in the zygote. Seen in Gymnosperms.
Simple Polyembryony is the formation of many embryos due to fertilization of many eggs. Seen in Grasses.