Apomixis

Apomixis is the production of seeds without fertilisation and is a form of asexual reproduction that mimics sexual reproduction.

Normal seed production in flowering plants involves fertilisation, where male and female gametes fuse. However, certain plants have evolved the ability to produce seeds without fertilisation. This phenomenon is termed Apomixis.

In recurrent apomixis, the nuclei of the embryo sac are diploid (2n). Meiosis does not occur. Two main sub-categories exist under Agamospermy:

1. Diplospory (Generative Apospory)

• A diploid embryo sac is formed directly from the megaspore mother cell (MMC) without undergoing regular meiotic division.
• The embryo sac is therefore unreduced (2n).
• Also called Generative Apospory.

2. Apospory (Somatic Apospory)

• The megaspore mother cell (MMC) undergoes normal meiosis, producing four megaspores.
• However, all four megaspores gradually degenerate (disappear).
• A nucellar (somatic) cell becomes activated and develops into a diploid (2n) embryo sac.
• Also called Somatic Apospory.

3. Adventive Embryony (Sporophytic Budding)

• An embryo arises directly from diploid sporophytic cells of the ovule - specifically from the nucellus or the integument.
• It is also called Sporophytic Budding because the gametophytic phase is completely absent.
• These adventive embryos develop alongside the normal zygotic embryo, giving rise to Polyembryony.

In non-recurrent apomixis, the megaspore mother cell undergoes normal meiotic division, producing a haploid (n) embryo sac. However, fertilisation does NOT occur.

The embryo develops in one of two ways:

Plants produced by non-recurrent apomixis are generally sterile and cannot reproduce sexually. - Haploid parthenogenesis is of considerable value in genetics as it enables the production of true-breeding homozygous forms.

Example: Nicotiana

This is essentially the same as Adventive Embryony described under Recurrent Apomixis (Section 4.3). It refers to embryo formation from somatic cells of the ovule alongside normal zygotic embryo development, resulting in polyembryony.

Examples: Lemon (Citrus limon), Orange (Citrus sinensis), Mango (Mangifera indica)

• Apomixis helps preserve desirable hybrid characters for many generations.
• Farmers can reuse hybrid seeds without losing crop quality.
• Since meiosis is absent, there is no segregation of characters.
• It helps in rapid and economical crop improvement.
• Apomixis produces genetically identical plants (maternal clones).
• Non-recurrent apomixis is useful in genetic and breeding studies.
• Scientists are trying to introduce apomixis into crops like rice, wheat, and maize for better food production.