Apomixis and the paradox of sex in plants

Abstract

Background The predominance of sex in eukaryotes, despite the high costs of meiosis and mating, is still an evolutionary enigma. Many theories have been proposed, none of them being conclusive on its own, and they are partly not well applicable to land plants. Sexual reproduction is obligate in embryophytes for the great majority of species. Scope This review will compare the main forms of sexual and asexual reproduction in ferns and angiosperms, based on the generation cycling of sporophyte and gametophyte (leaving vegetative propagation aside). The benefits of sexual reproduction for maintenance of genomic integrity compared to asexuality will be discussed in the light of developmental, evolutionary, genetic and phylogenetic studies. Conclusions Asexual reproduction represents modifications of the sexual pathway, with various forms of facultative sexuality. For sexual land plants, meiosis provides direct DNA repair mechanisms of oxidative damage in reproductive tissues. The ploidy alternations of meiosis-syngamy cycles, and prolonged, multicellular stages in the haploid phase in the gametophytes provide a high efficiency of purifying selection against recessive deleterious mutations. Asexual lineages might buffer effects of such mutations via polyploidy, and can purge the mutational load via facultative sexuality. The role of organelle-nuclear genome compatibility for maintenance of genome integrity is still not well understood. The costs of mating are in plants in general low because of predominant hermaphroditism. Phylogenetic patterns in the Archaeplastid clade suggest that high frequencies of sexuality in land plants are concomitant with a stepwise increase of intrinsic and extrinsic stress factors. Furthermore, expansion of genome size in land plants would increase the potential mutational load. Sexual reproduction appears to be essential for keeping long term genomic integrity, and only rare combinations of extrinsic and intrinsic factors allow for shifts to asexuality.

Resolving Relationships of Bolbitis sinensis Species Complex Using RAD Sequencing

“…However, for species with large and complex genomes, maintaining DNA replication fidelity through asexual reproduction can be challenging. Several studies have proposed that asexual populations have weaker responses to natural selection, leading to the accumulation of more deleterious mutations over generations [29][30][31]. The relatively high frequencies of asexual reproduction without ploidy cycling in ferns might be a consequence of frequent polyploidization, which allows for the buffering of recessive deleterious mutations over long time periods [31].…”

“…Several studies have proposed that asexual populations have weaker responses to natural selection, leading to the accumulation of more deleterious mutations over generations [29][30][31]. The relatively high frequencies of asexual reproduction without ploidy cycling in ferns might be a consequence of frequent polyploidization, which allows for the buffering of recessive deleterious mutations over long time periods [31]. However, this remains a hypothesis, and more studies are needed to verify these observations and better understand the genetic and evolutionary dynamics within the B. sinensis species complex.…”