Infection by a vertically-transmitted microsporidian parasite is associated with a female-biased sex ratio and survival advantage in the amphipodGammarus roeseli

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Abstract
SUMMARY: Vertically transmitted parasites may have positive, neutral or negative effects on host fitness, and are also predicted to exhibit sex-specific virulence to increase the proportion or fitness of the transmitting sex. We investigated these predictions in a study on the survival and sex ratio of offspring of the amphipodGammarus roeselifrom females infected by the vertically transmitted microsporidiaNosema granulosis. We found, to our knowledge, the first evidence for a positive relationship betweenN. granulosisinfection and host survival. Infection was associated with sex ratio distortion, not by male-killing, but probably by parasite-induced feminization of putativeG. roeselimales. This microsporidia also feminizes another amphipod host,Gammarus duebeni, which is phylogenetically and biogeographically distant fromG. roeseli. Our study suggests that the reproductive system of gammarids is easily exploited by these vertically-transmitted parasites, although the effects of infections on host fitness may depend on specific host-parasite species interactions.

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