Sex is a universal property of life encompassing any process that incorporates foreign DNA into functional genomes. Sex is essential for the conservation of biological information across time and, together, is a powerful mechanism of genetic innovation. Unlike prokaryotes, in which sex has no link with reproduction, eukaryotes perform sexual reproduction, an alternation of cellular fusion (syngamy) and meiosis better referred to as meiotic sex. Meiosis possibly evolved in early eukaryotes as a way to reversibly shift from haploidy to diploidy in response to environmental signals. Meiotic sex might then have emerged for maintenance of multi-chromosome, large-sized genomes. The evolution of mating types belonging to either of two sexes/genders prevented fusion of sister cells, thus fostering genetic recombination. Meiotic sex sets strong boundaries between species, at the same time promoting genetic isolation and the emergence of novel species. Species boundary in prokaryotes is conventional. Despite high cost, meiotic sex is almost universal in extant eukaryotes and was most likely present in the last common ancestor.
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