Selfish element maintains sex in natural populations of a parasitoid wasp

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Abstract
Genomic conflicts between heritable elements with different modes of inheritance are important in the maintenance of sex and in the evolution of sex ratio. Generally, we expect sexual populations to exhibit a 1:1 sex ratio. However, because of their biology, parasitoid wasps often exhibit a female–biased sex ratio. Sex–ratio distorters can further alter this optimum, sometimes leading to the complete loss of sexual reproduction. In the parasitoid wasp Trichogramma kaykai ca. 4–26% of females in field populations are infected with a bacterial sex–ratio distorter, Wolbachia, allowing virgin mothers to produce daughters. In some micro–Hymenoptera these infections have led to the complete loss of sex, but in field populations of T. kaykai the proportion of individuals infected remains relatively stable. We tested several hypotheses to explain this low infection level, including ineficient and horizontal transmission of Wolbachia, suppressor genes negating the effect of Wolbachia and the presence of male–biasing sex–ratio distorters. Here, a male–biasing sex–ratio distorter, a parasitic B chromosome, causing females to produce only sons, keeps the frequency of Wolbachia low. The male–biasing factor of T. kaykai is the second known case of a B chromosome manipulating the reproduction of a parasitoid wasp.