Double trouble: Two distinct endosymbiotic manipulations of the spider Mermessus fradeorum

Spiders, like most arthropods, are host to a plethora of heritable endosymbiotic bacteria.  Endosymbionts ensure vertical transmission either by conveying a facultative benefit to the host or by manipulating host reproduction to eliminate males that serve as evolutionary dead-ends for maternally-inherited bacteria.  Common modes of reproductive manipulation include parthenogenesis, male killing, feminization, and cytoplasmic incompatibility.  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 this potentially unique symbiont-spider relationship, field-collected Mermessus fradeorum (Linyphiidae) were reared in the laboratory and screened for common endosymbionts.  Two distinct infections were detected in laboratory populations: a double infection of both Rickettsia and Wolbachia in one lineage and a single Wolbachia infection in a second lineage.  Mating assays revealed that Rickettsia-Wolbachia infected mothers produced extremely female-biased offspring.  Antibiotic treatment of Rickettsia-Wolbachia infected females successfully eliminated both endosymbionts and restored the sex ratio to the expected 1:1 in subsequent generations.  Conversely, mothers infected with only Wolbachia produced offspring with an even sex ratio and antibiotic elimination of Wolbachia revealed no differences in fecundity, size, or survivorship between infected and uninfected offspring.  However, in a two-way factorial mating assay, strong cytoplasmic incompatibility was detected such that mating between cured females and infected males produced 70% fewer offspring than all other pairings.  A simple probe into the nature of endosymbiont infection in M. fradeorum has revealed rich and complex layers of reproductive manipulation that likely drive host evolution and ecology.