UC Davis

Parasites impose strong selective pressures on animals, influencing traits from behavior to reproductive strategies. In particular, parasite-mediated selection has been proposed as a major driver of sexual reproduction, as genetic recombination can generate variation that facilitates rapid evolutionary responses to parasites. Accordingly, asexual vertebrates are predicted to be more susceptible to parasites, yet some persist over longer than expected evolutionary timescales. The Amazon molly (Poecilia formosa) is a gynogenetic, clonal fish that occurs sympatrically with its sexual parental species, including the Sailfin molly (P. latipinna). Here we investigate the link between behavioral differences and parasite infection in wild-caught Amazon and Sailfin mollies across two populations. Fish were exposed to a simulated predator attack, and space-use behavior was quantified as time spent in open versus sheltered areas. We subsequently determined their infection status and parasite load via dissection. We found that Amazon and Sailfin mollies differed in their qualitative, but not quantitative resistance to parasites. Amazon mollies had worse qualitative resistance (i.e., were more likely to be infected with at least one parasite) and spent more time in the open, "riskier" portion of the arena. However, consistent with previous studies, Amazon and Sailfin mollies did not differ in quantitative resistance (i.e., parasite load). Parasite prevalence and load differed between our two collection sites, suggesting an important role for environmental context. These results suggest that there is a correlation between behavior and parasite exposure in Amazon mollies, while immune or physiological traits may allow them to limit parasite loads once infected. While the scope of this study makes it difficult to tell whether increased risk-taking behavior leads to parasite exposure or vice versa, our results highlight important differences between sexual and asexual species in parasite-laden environments.