Whilst most animals invest equally in males and females when they reproduce, a variety of vertically transmitted parasites has evolved the ability to distort the offspring sex ratios of their hosts. One such group of parasites are male-killing bacteria. Here we report the discovery of females of the ladybird Anisosticta novemdecimpunctata that produced highly female-biased offspring sex ratios associated with a 50% reduction in egg hatch rate. This trait was maternally transmitted with high efficiency, was antibiotic sensitive and was infectious following experimental haemolymph injection. We identified the cause as a male-killing Spiroplasma bacterium and phylogenetic analysis of rDNA revealed that it belongs to the Spiroplasma ixodetis clade in which other sex ratio distorters lie. We tested the potential for interspecific horizontal transfer by injection from an infected A. novemdecimpunctata line into uninfected individuals of the two-spot ladybird Adalia bipunctata. In this novel host, the bacterium was able to establish infection, transmit vertically and kill male embryos.
