High rate of large-scale hemizygous deletions in asexually propagating Daphnia: implications for the evolution of sex.

The origin and maintenance of sex remains one of the most debated topics in evolutionary biology. Investigations of the molecular genetic consequences of asexuality, such as direct estimation of mutation and recombination rates in asexual lineages, are critical for explaining the prevalence of sex in nature. In this study, we use long-term mutation accumulation lines of asexually propagating Daphnia pulex and D. obtusa to examine the role of hemizygous deletion and ameiotic recombination (crossover and gene conversion) in the evolution of asexual taxa. Large-scale hemizygous deletions ranging from 2 to 30 kb are found to occur at a rate of 6.7 × 10(-5) locus(-1) generation(-1) in D. pulex, which is one order of magnitude higher than the rate of large-scale deletions in Saccharomyces cerevisiae and Drosophila melanogaster. A conservative estimate of gene conversion and crossing over in the apomictic germ line cells of Daphnia yields a rate of 3.3 × 10(-5) locus(-1) generation(-1). Our exceptionally high rate of large-scale segmental deletions suggests that the long-term survival of asexual Daphnia lineages is likely compromised by a high deleterious mutation pressure and that selection against deleterious mutations may play an important role in the evolution and maintenance of sex.