Apomixis

Abstract

Abstract Seeds contribute significantly to world food supply. Flowering plants mainly produce seeds by sexual reproduction, which is a driver of genetic diversity. The combination of meiosis during male and female gamete formation, and subsequent gamete fusion at fertilisation in the ovule of the flower to form the embryo compartment of the seed, leads to segregation of parental alleles in seedling progeny. A second fertilisation event generates the endosperm, a nutritive tissue supporting embryo growth and seedling germination. Remarkably, some flowering plants form seeds asexually by apomixis. Apomixis comprises an ensemble of developmental processes that together alter female reproductive functions in the ovule, converting the sexual programme to an asexual one. The result is that the apomictically derived embryo develops solely from cells in maternal ovule tissues and therefore seedling progeny are genetically identical to the mother. Apomixis is largely absent in important food crops. Harnessing apomixis as a technology in plant breeding would increase food yield and security. Key Concepts: Apomixis is an asexual mode of seed formation that produces clonal progeny with a maternal genotype. It primarily influences reproductive events in the ovule of the flower. Apomixis is absent in major crops, however, deployment of apomixis as a crop‐breeding tool would economically maintain hybrid vigour that is currently lost in successive seed generations. Apomixis comprises three developmental components that deviate from the normal sexual pattern: avoidance of meiosis during egg cell development, fertilisation‐independent embryo formation, and generation of viable endosperm with or without fertilisation. Angiosperms exhibit two mechanistically different types of apomixis termed sporophytic and gametophytic. Sporophytic apomixis involves direct formation of an embryo from a diploid somatic (nonsexual) ovule cell, and a viable seed forms when the adjacent sexual gametophyte is fertilised and forms endosperm. Gametophytic apomixis involves formation of a meiotically unreduced (i.e. diploid) female gametophyte, followed by embryo formation from the diploid egg by parthenogenesis (i.e. without fertilisation). In gametophytic apomixis, endosperm formation may occur autonomously (i.e. without fertilisation) or pseudogamously (i.e. in response to fertilisation of the central cell). Most apomicts retain the ability to produce some seed via sexual reproduction and studies in some apomicts indicate sexual reproduction is the default reproductive mode on which apomixis is superimposed. Apomixis is controlled by dominant loci and those controlling the avoidance of meiosis and fertilisation‐independent embryo development are genetically separable in some species and tightly linked in others. Apomixis loci are proposed to recruit or hijack the sexual machinery in ovule cells undergoing apomixis, modifying the timing of the sexual programme so that meiosis and fertilisation are avoided leading to asexual seed formation.