Selfish element maintains sex in natural populations of a parasitoid wasp

Genomic conflicts between heritable elements with different modes of inheri tance are important in the maintenance of sex and in the evolution of sex r atio. Generally, we expect sexual populations to exhibit a 1:1 sex ratio. H owever, because of their biology, parasitoid wasps often exhibit a female-b iased sex ratio. Sex-ratio distorters can further alter this optimum, somet imes leading to the complete loss of sexual reproduction. In the parasitoid wasp Trichogramma kaykai ca. 4-26% of females in field populations are inf ected with a bacterial sex-ratio distorter, Wolbachia, allowing virgin moth ers to produce daughters. In some micro-Hymenoptera these infections have l ed to the complete loss of sex, but in field populations of T-kaykai the pr oportion of individuals infected remains relatively stable. We tested sever al hypotheses to explain this low infection level, including inefficient an d horizontal transmission of Wolbachia, suppressor genes negating the effec t of Wolbachia and the presence of male-biasing sex-ratio distorters. Here, a male-biasing sex-ratio distorter, a parasitic B chromosome, causing fema les to produce only sons, keeps the frequency of Wolbachia low. The male-bi asing factor of T. kaykai is the second known case of a B chromosome manipu lating the reproduction of a parasitoid wasp.