Mississippi State University Starkville, Mississippi
Facultative bacterial mutualists can improve insect health through various means, including aiding in nutrition and immunity. Frequently, however, mutualists also act selfishly, by over-proliferating within individual hosts or by manipulating reproduction to favor their persistence in host populations. In Drosophila hosts, heritable Spiroplasma symbionts exemplify these trade-offs through phenotypes like parasite protection and male-killing, which are thought to favor persistence given efficient but imperfect maternal transmission. Here, we examine costs, benefits, and transmission of an unstudied Spiroplasma symbiont of Drosphila atripex, sAtripex (sAtr), which is evolutionarily divergent from all well-studied Drosophila-associated Spiroplasma. In its native host, sAtr marginally decreases host lifespan, while in Drosophila melanogaster, host longevity is severely reduced and may be associated with upregulation of the host IMD immune pathway. We found no evidence of reproductive manipulation by male-killing or cytoplasmic incompatibility, but like other Spiroplasma strains, sAtr does provides protection against parasitoid wasps. Despite the costs, we noticed that this symbiont is never spontaneously lost in our lab stocks, and in formal tests we found perfect transmission in both hosts, suggesting features of its growth or localization may be different to the previously studied Drosophila-associated strains. Together, our findings emphasize the importance of host-symbiont compatibility in establishing costs, as well as the outsized role transmission plays in maintaining relationships tilted heavily toward parasitism.
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