Manipulate or facilitate: How endosymbiotic bacteria thrive in Linyphiid spiders

Sheet-weaving Linyphiid spiders are widely distributed, agriculturally important predators.   Like the majority of insect taxa, spiders are host to a variety of maternally inherited endosymbiotic bacteria including Cardinium, Rickettsia, Wolbachia, and Spiroplasma that ensure their own transmission through the female germline either by manipulating host reproduction or conveying a facultative benefit.  Depending upon selection pressures, the selfish interests of endosymbionts may be in concert or conflict with that of the host.   Recent broad-taxa screening studies indicate that endosymbionts are particularly common among spiders; however, little is known about how these bacteria affect their spider hosts.   To investigate the continuum of bacterial phenotypes in Linyphiid spiders, I reared two naturally-infected lines of the Linyphiid spider Mermessus fradeorum: one infected with Wolbachia and one superinfected with Wolbachia and Rickettsia.  Superinfected M. fradeorum produced a radical female bias, whereas M. fradeorum infected with only Wolbachia produced a more even sex ratio.  Subsequent generations of superinfected progeny retained a strong female bias.  The superinfected line was not parthenogenetic:  virgin females produced no viable egg sacs.  The exact mechanism of reproductive manipulation is not yet clear, but I hypothesize that Rickettsia induces the female bias through male killing or feminization. Future chromosome observations will reveal if feminization of genetic males has occurred.  The function of Wolbachia remains unclear, but ongoing efforts to cure M. fradeorum of Wolbachia and Rickettsia will allow me to unambiguously determine whether Rickettsia causes female bias and to identify any additional phenotypic effects of both symbionts within this spider.