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The Evolution of Meiotic Sex and Its Alternatives


The Evolution of Meiotic Sex and Its Alternatives

Ghader Mirzaghaderi et al. Proc Biol Sci.


Meiosis is an ancestral, highly conserved process in eukaryotic life cycles, and for all eukaryotes the shared component of sexual reproduction. The benefits and functions of meiosis, however, are still under discussion, especially considering the costs of meiotic sex. To get a novel view on this old problem, we filter out the most conserved elements of meiosis itself by reviewing the various modifications and alterations of modes of reproduction. Our rationale is that the indispensable steps of meiosis for viability of offspring would be maintained by strong selection, while dispensable steps would be variable. We review evolutionary origin and processes in normal meiosis, restitutional meiosis, polyploidization and the alterations of meiosis in forms of uniparental reproduction (apomixis, apomictic parthenogenesis, automixis, selfing) with a focus on plants and animals. This overview suggests that homologue pairing, double-strand break formation and homologous recombinational repair at prophase I are the least dispensable elements, and they are more likely optimized for repair of oxidative DNA damage rather than for recombination. Segregation, ploidy reduction and also a biparental genome contribution can be skipped for many generations. The evidence supports the theory that the primary function of meiosis is DNA restoration rather than recombination.

Keywords: apomixis; automixis; paradox of sex; restitutional meiosis; selfing.


Figure 1.
Figure 1.
Processes during meiotic prophase I. Proteins in Saccharomyces cerevisiae (after [–15]) involved in each phase are shown inside the figure. Most cases are resolved without recombination (exchange of flanking regions, see red versus blue arrows). (Online version in colour.)
Figure 2.
Figure 2.
Normal (a) and non-reductional (b) meiotic divisions resulting in reduced (n) and unreduced (2n) meiotic products (gametes/spores) for a diploid parent with two chromosome pairs. Maternal and paternal chromosomes are shown in red and blue, respectively. FDR and SDR maintain different levels of parental heterozygosity. (Online version in colour.)
Figure 3.
Figure 3.
Forms of asexual reproduction with meiosis; unreduced eggs develop parthenogenetically. (a) Automixis in animals keeps female meiosis, but restores diploidy either via gamete duplication or via fusion of meiotic products, followed by loss of heterozygosity. (b) Apomixis in plants has three major pathways: in apospory and adventitious embryony, the meiotic pathway runs in parallel to somatic development, whereby reduced egg cells are being fertilized. Diplospory involves a female restitutional meiosis either via FDR or SDR (figure 2). Gene conversion is indicated by green and yellow circles on homologous chromosomes. (Online version in colour.)

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