Evolutionary implications of male killing and feminization

To date, male killing has been reported in various species of insects, including fruitflies, mosquitoes, butterflies, moths, ladybird beetles, and parasitic wasps Furthermore, the causal agents of the male killing belong to taxonomically diverse microorganisms, such as bacteria belonging to Wolbachia, Rickettsia, Arsenophonus, Spiroplasma, and Flavobacterium and unicellular prokaryotes belonging to Microsporidia (Hurst and Majerus, 1993; Hurst and Jiggins, 2000; Hurst et al ., 2003) . Therefore, male killing is considered to be a trait that is easy to evolve (Hurst et al , 2003) On the other hand, endosymbiont-induced complete feminization has only been reported in E. hecabe among insects . Even among all arthropods, microbe-induced feminization has only been found in a few species, such as wood-lice and shrimps (Rigaud, 1997; Dunn et al ., 1993) . Despite its rare occurrence, feminization is a more advantageous strategy for maternally transmitted endosymbionts than male killing, because all the offspring of infected mothers can transmit the infection to subsequent generations in the case of feminization, whereas only half the offspring can transmit the infection to subsequent generations in the case of male killing

The male killing observed in Ostrinia is considered to be death of genetic males due to the feminizing effect of Wolbachia Some endosymbionts may have a feminizing effect on genetic male hosts and this effect can often be lethal . In E. hecabe, this feminizing effect may somehow be nonlethal, such that genetic males completely revert to functional females Evolutionary transitions in either the host (E. hecabe) or the endosymbiont (Wolbachia) can account for the nonlethal complete feminizing effect on genetic males of E. hecabe, i e , E. hecabe may have evolved a trait in genetic males such that they are not killed by feminiza-tion or Wolbachia may have evolved a trait not to kill genetic males while feminizing them Overall, it is undoubtedly the case that male killing and feminization are both outcomes of the close associations between endosymbionts and the sex-determining systems of their hosts . By untangling such complex interactions between endosymbionts and their hosts, we may be able to reveal unknown aspects of sex determination or sex differentiation in insects

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