Bacterial Infections of Hemocytes Associated with the Maternally Inherited Male-Killing Trait in British Populations of the Two Spot Ladybird, Adalia bipunctata

Adalia bipunctata, the two spot ladybird, carries a vertically transmitted bacterial agent which kills male progeny during embryogenesis. Some matrilines of A. bipunctata give rise to strongly female-biased sex ratios. 16S rDNA sequence analysis revealed a bacterium of the genus Rickettsia associated with this trait, a conclusion which is corroborated here. Using light microscopy, an association between a bacterium located in A. bipunctata hemocyte cytoplasm and matrilines which show the sex ratio trait was found. This element was not found in hemocytes taken from females from normal sex ratio lines, nor in hemocytes taken from males. The association is confirmed by study of the inheritance of the sex ratio trait. Only daughters of sex ratio crosses that bear this cytoplasmic bacterium also show the sex ratio trait, with other daughters being normal with respect to sex ratio. Transmission electron microscopy of hemocytes revealed a walled bacterium, bearing features of members of the genus Rickettsia, free in the cytoplasm of hemocytes taken from infected lines, but not in those taken from uninfected lines.