Many populations of the buprestid leaf-mining beetle, Brachys tessellatus, from central South Carolina, USA, show highly skewed sex ratios, ranging from 1.3 to 6.0 females per male. We have identi®ed a Rickettsia bacterium that is associated with sex ratio distortion (SRD) and selective killing of male embryos in B. tessellatus. Molecular assays of infection by this bacterium are highly associated with SRD within families, and treatment with an antibiotic (tetracycline) increases the number of male eggs that hatch and develop. The 16S rDNA sequence indicates that this is a novel Rickettsia, most closely related to Rickettsia bellii (a tick-associated bacterium) and a pea-aphid Rickettsia. It is also related to a Rickettsial bacterium that causes male-killing in an unrelated ladybird beetle species. Low levels of parthenogenesis are also observed in this system (about 10% of females) and may be the result of selection due to male rarity, or a direct result of infection by the Rickettsia.Keywords: sex ratio distortion, Rickettsia, male-killing bacteria, beetle, endosymbiotic, parthenogenesis. IntroductionInherited microorganisms are associated with a variety of reproductive alterations in their hosts, including cytoplasmic incompatibility, thelytokous parthenogenesis, feminization of genetic males, and male-killing (reviewed in Werren & O'Neill, 1997). These alterations are generally adaptive for the microorganisms because they are typically inherited maternally through the egg cytoplasm, but are not transmitted via sperm. As a result, inherited microbes often have an asymmetrical inheritance pattern with high maternal (female) inheritance and low or absent paternal (male) inheritance. Selection therefore favours such microorganisms to bias sex ratio or sex determination towards females, the transmitting sex (Werren & O'Neill, 1997).Male-killing microorganisms are widespread in nature (see Hurst et al., 1997 for a review), and involve a wide range of microbial taxa and eukaryotic hosts. Examples include gamma proteobacteria such as Arsenophonus nasoniae, the male-killing bacterium of Nasonia (Gherna et al., 1991), alpha proteobacteria such as Rickettsia in ladybeetles (Werren et al., 1995b) and Wolbachia in 1 Acraea butter¯ies (Hurst et al., 1999),¯avobacteria in beetles (Hurst et al., 1997), spiroplasms in Drosophila (Williamson et al., 1999) and microsporidia (protozoa) in mosquitoes (Hurst, 1991) and amphipods (Dunn & Hatcher, 1997). The pattern suggests that male-killing by resident inherited microbes can readily evolve whenever the ecological circumstances are appropriate. Male-killing can be selectively favoured under two general circumstances: (a) when lethality of males increases the ®tness of infected females in the family or (b) when male-killing provides an inoculum of microbes for horizontal (infectious) transmission. Although there is some evidence that male-killing may provide an inoculum for horizontal transmission in some systems (Hurst, 1991), a ®tness advantage to infected female siblings ...