The why's of sex

Sex isn’t quite what it seems – while superficially wasteful in an evolutionary sense (why inherit on only one half of your genes, when you can inherit all of them asexually, or why waste resources in mating when you don’t need a mate asexually?), theory and empirical studies have argued for the evolutionary advantages of sex in the context of favoring recombination, efficacy of natural selection in purging or fixing new mutations, and thus evolving faster (see my previous post that describes this).

The rate and molecular signatures of adaptation M J McDonald et al. Nature 1–4 (2016) doi:10.1038/nature17143

The rate and molecular signatures of adaptation
M J McDonald et al. Nature 1–4 (2016) doi:10.1038/nature17143

However, there has been little empirical evidence that points directly towards the adaptive advantages conferred by sex. McDonald et al. (2016) sought genomic evidence of what we’ve known for a good part of population genetics since the modern synthesis – does sex speed adaptation? Using experimental evolution strains of S. cerevisae across 12 asexual (mitotic, budding) and 6 sexual (meiotic, sporulation) populations for approximately 1000 generations. Fitness assays of evolved populations were performed against ancestral strains. Genomes of every 90th generation were sequenced across four sexual, and one asexual population.
Key findings from this study include (1) the striking significant increases in fitness of sexually evolving populations, compared to the asexual populations, (2) an average of 44 de novo mutations per population, with similar proportions of nonsynonymous, synonymous, and intergenic mutations between sexual and asexual populations, (3) but a significant difference in rates of fixation of de novo mutations – fewer mutations fix in sexual populations, indicating that sex improves the efficiency of selection, disallowing harmful mutations to fix, and (4) the potential ubiquity of epistasis wherein a de novo mutation mayhaps be beneficial in one genomic background, but deleterious in another.

Future studies are needed to fully understand the consequences of this interplay between sex and balancing selection, and to investigate how epistasis interacts with recombination to alter the dynamics of sequence evolution. By combining precise control of the sexual cycle with whole-population time-course sequencing, this experimental system offers the potential to understand how these factors affect the rate, molecular outcomes, and repeatability of adaptation.

McDonald, Michael J., Daniel P. Rice, and Michael M. Desai. “Sex speeds adaptation by altering the dynamics of molecular evolution.” Nature 531.7593 (2016): 233-236. DOI: 10.1038/nature17143

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