Introduction
No one knows the exact nature of the LECA, but we think that this ancestor was a unicellular, aquatic, motile creature with one or two flagella. Thus, in some respects the LECA was simple. But in other ways, it was already quite complex, with a nucleus, mitochondria, secretory apparatus, RNAi, and reproducing both asexually and sexually. Thus, when we think of where sex first evolved, it was in the water, involving swimming cells (Levin and King, 2013; Umen and Heitman, 2013). And when we think of how sex first evolved, this involved changes in ploidy and the process of meiosis, given their conserved nature throughout eukaryotes. And while cell-cell and nuclear-nuclear fusion play prominent roles in sexual reproduction today, there may have been an era in which endoreplication cycles followed by meiosis drove the processes of ploidy change during ancestral modes of sexual reproduction. In this view, cell-cell fusion may be ancient, but perhaps not as ancient as other features of sexual reproduction.
Why sex is so pervasive is thought to result from potential benefits conferred by sexual reproduction. These include purging the genome of deleterious mutations and shuffling the genome via independent chromosomal assortment and recombination to give rise to a diverse repertoire of meiotic progeny. Sex may also enable organisms to keep pace with or outrun pathogens, including those both external and those internal (such as transposons). There is sound experimental evidence from studies in Caenorhabditis elegans and in naturally occurring snails in New Zealand for this last hypothesis in which sex allows species to keep pace with their pathogens (King et al., 2009; King et al., 2011; Morran et al., 2011; Vergara et al., 2013). However, these potential benefits of sex are pitted against well-known costs of sexual reproduction: that only 50% of a parental genome is transmitted to any given progeny, the time and energy required to locate mates, and the breaking apart of well adapted genomic configurations.
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The core features of sexual reproduction are conserved in organisms as diverse as the model budding yeast Saccharomyces cerevisiae and humans, despite a billion years or more of evolution separating us from our last common shared ancestor. These conserved features include: 1) ploidy changes from haploid to diploid to haploid (or diploid to haploid to diploid), 2) the process of meiosis that enables meiotic recombination and halves the ploidy of the genome, and 3) cell-cell fusion between mating partners (a and α cells) or gametes (the sperm and the egg). This ubiquity of the conserved features of sex again speaks to the antiquity of the process.
Beyond the commonalities in the mechanisms of sex, there are also shared features to the modes of sexual reproduction. This includes outbreeding between genetically divergent members of the population, but also types of inbreeding that can involve the ability of the yeast S. cerevisiae to undergo mating type switching that allows mother cells to mate with their daughter cells. And in humans there are the examples of consanguineous marriages, resulting for example from cousin-cousin pairings, which lead to considerable inbreeding with the risk of exposure of recessive alleles in a homozygous configuration. We will return to this theme of the balance between outbreeding and inbreeding.
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