Male-killing Wolbachia in Drosophila: a temperature-sensitive trait with a threshold bacterial density.

Inherited microorganisms that disturb the reproduction of their host have been characterized from a number of host taxa. To understand the general principles underlying the genetic and mechanistic basis of interactions, study of different agents in model host species is required. To this end, the nature and genetics of the maternally inherited sex-ratio trait of Drosophila bifasciata were investigated. Successful curing of affected lines with antibiotics demonstrated this trait was associated with the presence of a bacterium, and molecular systematic analysis demonstrated an association between the presence of the trait and infection with an A group Wolbachia. The penetrance and heritability of the trait did not vary with maternal age. Exposure to elevated temperatures did reduce trait penetrance but did not affect heritability. Examination of the effect of temperature on bacterial density in eggs revealed a decrease in bacterial density following exposure of the parent to elevated temperature, consistent with the hypothesis that male killing in D. bifasciata requires a threshold density of Wolbachia within eggs. The male offspring produced following exposure to elevated temperatures were infected with Wolbachia on emergence as adults. Crossing studies demonstrated a weak cytoplasmic incompatibility phenotype exhibited by Wolbachia in these males. The results are discussed with respect to the incidence of male killing within the clade Wolbachia, the general nature of Wolbachia-host interactions, and the prospects for using this association to investigate the mechanism of male killing.