Bacterial Solutions to the Problem of Sex

J. Arjan G. M. de Visser

“Why sex?” is a question that has occupied the minds of evolutionary biologists for more than a century. The evolution of organisms that mix their genes during reproduction is considered one of the major transitions in evolution [1], because it fundamentally changed how genes are transmitted to the next generation. In asexual reproduction, offspring inherit a more or less unaltered genome from the parent. In sexual reproduction, genetic material is first reduced in the gametes (sperm or ovaries) and then fused with that of another individual, before new offspring may develop. Sexual reproduction has led to the evolution of males and females with different and sometimes even opposing reproductive interests and behavior. The consequence of sex, which many evolutionary biologists think explains its existence, is the production of genetic variation by mixing genes from different individuals.

Nineteenth-century German biologist August Weismann [2] was the first to realize that sex can produce variation and that variation allows organisms to better respond to selection. Three models have since been proposed as general explanations for the advantages of sex by elaborating Weismann's idea: (i) sex facilitates adaptation to a novel environment by combining beneficial gene variants, or alleles, from different genomes (Fisher-Muller (FM) model [3,4]); (ii) sex helps organisms stay adapted to existing environmental conditions by combining and efficiently removing deleterious alleles (mutational deterministic (MD) model [5]); or—a combination of these two ideas—(iii) sex helps to liberate beneficial alleles from linked deleterious alleles in the genome in which they arose (mutational load (ML) model [6,7]).

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de Visser JAGM (2007) Bacterial Solutions to the Problem of Sex. PLoS Biol 5(9): e245

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