A team effort?: Differential symbiont infections provide insights into the cooperatively-induced feminization of a linyphiid spider

Most terrestrial arthropods harbor maternally-transmitted bacterial symbionts, often harboring multiple distinct symbiont lineages simultaneously as coinfections. Symbiont coinfections may facilitate symbiont competition for resources but also cooperation to ensure mutual transmission. As these symbionts rely primarily on female reproduction for their spread, many alter host biology in ways that benefit hosts or skew reproduction to favor infected females. Some symbionts feminize their hosts so that genetic males develop as functional phenotypic females that produce female-biased offspring. These microbial feminizers have been identified in various arthropod hosts, including the linyphiid spider Mermessus fradeorum. This spider is variably infected with up to five heritable symbionts: a Rickettsiella that sabotages host reproduction via cytoplasmic incompatibility, Tisiphia, and three strains of Wolbachia. Curiously, when spiders harbor all five symbionts, they are feminized, although the relative contributions of each symbiont to feminization are not clear. We determined the role of these symbionts in host feminization using spider lines that were naturally differentially infected with unique symbiont combinations. Crosses between females of these various “symbiotypes” with uninfected males revealed that feminization induction requires infection with one strain of Wolbachia, wMfra1. However, coinfection with all five symbionts results in the strongest and least variable feminization phenotype. This suggests that the other symbionts, which cannot feminize hosts without wMfra1,  cooperate to amplfy feminization efficacy reliably. This research provides insights into the feminization of M. fradeorum and potential cooperation between coinfecting symbionts.