MBE Advance Access published online on February 14, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn046
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Research Article |
Sexual selection and maintenance of sex: Evidence from comparisons of rates of genomic accumulation of mutations and divergence of sex-related genes in sexual and hermaphroditic species of Caenorhabditis
Dept of Biology, McMaster University, Hamilton, Ontario, Canada
Corresponding author: Singh R.S., Telephone number: (905) 525 9140 ext 24378, Fax number: (905) 522 6066, Email address: singh{at}mcmaster.ca
Received for publication November 29, 2007. Revision received February 7, 2008. Accepted for publication February 10, 2008.
Several hypotheses have been proposed to explain the persistence of dioecy despite the reproductive advantages conferred to hermaphrodites, including greater efficiency at purging deleterious mutations in the former. Dioecy can benefit from both mutation purging and accelerated evolution by bringing together beneficial mutations in the same individual via recombination and shuffling of genotypes. In addition, mathematical treatment has shown that sexual selection is also capable of mitigating the cost of maintaining separate sexes by increasing the overall fitness of sexual populations, and genomic comparisons have shown that sexual selection can lead to accelerated evolution. Here we examine the advantages of dioecy vs. hermaphroditism by comparing the rate of evolution in sex related genes and the rate of accumulation of deleterious mutations using a large number of orthologs (11, 493) in the dioecious C. remanei and the hermaphroditic C. briggsae. We have used this dataset to estimate the deleterious mutation rate per generation, U, in both species and find that while it is significantly higher in hermaphrodites, both species are at least two orders of magnitude lower than the value required to explain the persistence of sex by efficiency at purging deleterious mutations alone. We also find that genes expressed in sperm are evolving rapidly in both species; however, they show a greater increase in their rate of evolution relative to genes expressed in other tissues in C. remanei, suggesting stronger sexual selection pressure acting on these genes in dioecious species. Interestingly, the persistence of a signal of rapid evolution of sperm genes in C. briggsae suggests a recent evolutionary origin of hermaphrodism in this lineage. Our results provide empirical evidence of increased sexual selection pressure in dioecious animals, supporting the possibility that sexual selection may play an important role in the maintenance of sexual reproduction.
Key Words: Sexual Selection • Deleterious Mutation Rate • Sperm • Caenorhabditis