The evolution of apomixis

Abstract

SUMMARYThe selective forces responsible for the evolution of gametophytic apomixis in outbreeding plant populations are analysed in terms of a simple single gene model. In the absence of selection, apomixis inevitably becomes fixed in a population. This conclusion holds regardless of the dominance relations of the alleles specifying apomictic versus sexual seed formation. Substantial heterotic viability selection is required to prevent fixation of recessive and codominant apomictic alleles and maintain a stable polymorphism at the mating system locus. These findings suggest that gametophytic apomixis should be a common mode of reproduction in plant species. Possible factors accounting for the relative paucity of apomictic plants are discussed. It is concluded that one of the major factors hindering the spread of apomixis is its usually complex inheritance and the need to accumulate, in the one individual, two or more mutations affecting meiosis and the reproductive system.

Non-Mendelian transmission of apomixis in maize–Tripsacum hybrids caused by a transmission ratio distortion

“…Except for rare exceptions, diploid genotypes in agamic complexes are sexual, whereas apomixis is found at higher ploidy levels (Asker & Jerling, 1992). The need for such separation is in agreement with the existing models; these predict that, when in competition at the same ploidy level and under a one dominant gene hypothesis, apomixis spreads to fixation and sexuality disappears (Pernès, 1972;Maynard Smith, 1978;Marshall & Brown, 1981).…”

“…Models (Pernès, 1972;Maynard Smith, 1978;Marshall & Brown, 1981) show that, under a single dominant gene hypothesis and at the same level of ploidy, obligate apomixis rapidly invades populations. As a result, sexuality would disappear.…”

“…Asexual reproduction has been shown to present strong selective advantages in many circumstances (Maynard Smith, 1978;Marshall & Brown, 1981). However, amphimixis, that is a life cycle with alternating syngamy and meiosis, is strikingly predominant over the various forms of asexual reproduction.…”

The role of tetraploids in the sexual–asexual cycle in dandelions (Taraxacum)

“…This is the basic reason why hermaphroditic asexuals replace outcrossing sexuals rapidly in population genetic models (Charlesworth, 1980;Marshall and Brown, 1981;Joshi and Moody, 1998). However, our data show that the frequency of polyploid offspring on diploid sexuals is low, on average less than 2%.…”

“…The relative levels of the gene flows do not only affect the composition of the gene pools, but will also affect the coexistence of sexuals and asexuals. Population genetic models predict a rapid decline of the sexual outcrossing population when apomixis is controlled by a single dominant gene (Charlesworth, 1980;Marshall and Brown, 1981;Joshi and Moody, 1998).…”