The mechanism of reproduction determines the genetic makeup (homozygous or heterozygous) of the crops. For hybrid production, understanding the method of reproduction is crucial.
Sexual reproduction is either inhibited or nonexistent in apomixis. Apomixis is considered facultative when sexual reproduction is repressed (i.e. present), and obligatory apomixis when sexual reproduction is not suppressed (i.e. missing).
The most prevalent type of apomixis is facultative apomixis. When a haploid cell produces an embryo. Apomixis is nonrecurrent since its progeny cannot be sustained further, but apomixis is recurrent when embryos emerge from diploid cells.
Agamospermy is a synonym for apomixis.
Apomixis is of two types:
A) Adventitious embryony: Embryos are formed immediately from the nucellus, integument, and chalaza cells of the ovule. No embryo sac is produced, e.g. Mango, citrus, orchid, and other nucellar seedlings
B) Gametophytic apomixis : Embryos are created without fertilisation from egg cells or other cells found in embryo sacs. Apomeiosis is a mechanism that develops unreduced embryo sacs in recurrent apomixis.
Apomeiosis is of two types:
1) Apospory: Unreduced embryo sacs are formed by a sequence of mitotic divisions without meiosis from certain vegetative cells of the ovule. Hieraceum, Malus, Crepis, Ranunculus, Orchids, and other plants are examples.
2) Diplospory : The megaspore, which can be haploid or, more commonly, diploid, produces the embryo sac. Diploid megaspores are formed by modified meiosis in apomictic species.
Parthenogenesis or pseudogamy may be used to produce the embryos in such embryo sacs.
1) Parthenogenesis : Without pollination, the embryo develops from the embryo sac. Pollen lands on the stigma of a flower during pollination. There are two forms of parthenogenesis.
a) Gonial parthenogenesis embryo is developed from egg cell.
b) Somatic parthenogenesis Embryo is developed from some cells of the embryo sac other than the egg cell. Generally ‘Parthenogenesis is used as a synonym of somatic parthenogenesis.
ii) Pseudogamy :The words “pseudo” and “gamous” both imply “false” and “marriage.” Pollination occurs, but no fertilisation of the egg cell occurs; instead, fertilisation of the secondary nucleus does. There are two forms of pseudogamy.
a) gonal pseudogamy Embryo is developed from egg cell.
b) somatic pseudogamy Embryo is developed from some other cells of embryo sac.
Vybrid : The progeny of a hybrid between two facultative apomicts is known as Vybrid. Every generation, the vybrid reproduces by facultative apomixis and is maintained by harvesting the seeds of only F-like apomictic plants.
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