Sex ratio bias caused by endosymbiont infection in the dwarf spider Oedothorax retusus

Spiders exhibit a remarkable variety of reproductive phenotypes such as induced parthenogenesis and reproductive skew in primary sex ratio. However, observations of distorted sex ratios are mainly inferred from field catches of adult individuals, whereas detailed information on clutch primary sex ratio and sex ratio inheritance, resulting from multiple generations of laboratory rearing, is scarce. One of the potential causes of sex ratio variation is infection with maternally inherited endosymbiont bacteria that alter a mother’s offspring sex ratio to increase their own fitness. Although studies show that spiders are infected with several endosymbiont species, it was only recently discovered that endosymbiont bacteria can cause a female sex ratio bias in this order. To explore the distribution of biased sex ratios and endosymbiont infection patterns, we investigated sex ratio variation and bacterial presence in Oedothorax retusus Westring 1851. Significant sex ratio variation was detected in six matrilines originating from wild-caught females, one of which consistently showed a female bias in offspring production. Congruent with a bacterial effect, the sex ratio bias showed a clear maternal inheritance, and treatment with antibiotics reversed the sex ratio to equal numbers of males and females. Female-biased clutches were found to exhibit a significantly lower number of hatched spiderlings than unbiased clutches, suggesting the occurrence of male-killing. All matrilines showed infection with the Cardinium endosymbiont, while two matrilines, including the female biased one, were additionally infected with Wolbachia and Rickettsia. These findings indicate that bacterial endosymbionts are responsible for the sex ratio variation in this species, and suggest that effects of endosymbiont bacteria in the order Araneae could be more widespread than previously assumed.