Apomixis in Plants

Apomixis is a form of asexual reproduction in which flowering plants produce seeds without the usual sexual process. More precisely, the embryo develops without fertilisation of an egg, either from an unreduced egg or directly from maternal tissues within the ovule. It therefore mimics sexual reproduction by producing seeds without the normal fusion of male and female gametes to form the embryo.

How apomictic embryos develop

In ordinary sexual reproduction, meiosis produces haploid cells, and fertilisation restores the diploid chromosome number. Apomixis bypasses the normal sexual pathway through different developmental routes.

• In one route, a diploid egg forms without reduction division and develops into an embryo without fertilisation.
• In another route, cells of the nucellus, a maternal tissue surrounding the embryo sac, divide and develop directly into embryos.
• Nucellar embryo formation occurs in some varieties of Citrus and mango; several embryos may consequently occur within a single seed.

Distinction from related processes

The defining feature of apomixis in this context is asexual embryo formation within a seed, not simply the absence of visible pollination.

• Parthenocarpy means fruit development without fertilisation and generally produces seedless fruits; apomixis concerns the production of embryos and seeds.
• Vegetative propagation produces plants from structures such as stems, roots or leaves, whereas apomixis provides a seed-based route of asexual reproduction.
• Polyembryony means the occurrence of more than one embryo in a seed; it can accompany apomixis but is not synonymous with it.
• Apomixis does not necessarily eliminate every role of fertilisation: in some forms, fertilisation is still required for endosperm development.

Importance for crop improvement

Apomixis is valuable because it can preserve a successful maternal genetic combination through seed generations.

• Seeds collected from sexually reproducing hybrid crops can show segregation of parental traits, so subsequent plants may not retain the original hybrid's desirable characteristics.
• If a suitable hybrid reproduces through apomixis, its hybrid characters can be maintained without the usual segregation associated with sexual reproduction.
• This creates the potential for farmers to reuse hybrid seed while retaining desirable traits, although developing apomictic hybrids remains a crop-improvement objective rather than a universal feature of crops.