Cytoplasmic male killing elements in Adalia bipunctata (Linnaeus) (Coleoptera: Coccinellidae)

A wild sample of Adalia bipunctata larvae and pupae were collected from Surrey, and F1 virgin adults mated. The sex ratio produced by these pairs was found to be variable, 22 pairs produced a sex ratio consistent with a 1:1 sex ratio, but four produced a strong female bias. Daughters and males from biased broods were mated to adults from non-biased broods of different parentage. Males and females from unrelated families were crossed. Where the female was taken from a biased clutch, the biased sex ratio trait recurred in a high proportion of cases. Where males from such broods were crossed, or females from broods with no history of bias were mated, sex ratios consistent with 1:1 were obtained. Pairs producing a strong female bias showed hatch rates which were approximately half that of crosses producing non-biased sex ratios. Treatment with tetracycline in golden syrup cured females who showed the biased sex ratio trait. Daughters conceived after this treatment produced normal sex ratios. Females fed purely on golden syrup, as a control, continued to produce female biased sex ratios until treated with tetracycline. We therefore suggest that an unknown bacterium, transmitted through egg cytoplasm but not sperm, is responsible for the early mortality of male embryos and thus a bias in the secondary sex ratio. The contribution that the study of the Coccinellidae can make to theories concerning the incidences and evolution of male killing elements is discussed in relation to egg cannibalism.