I am interested in the processes that maintain variation in parasite resistance traits. Despite the benefit of evolving resistance to parasites, susceptibility is highly variable in hosts. This variation may be maintained by genetic linkage of resistance traits to other variable traits, via developmental or physiological tradeoffs, pleiotropic effects (whereby specific genes affect more than one trait), or tight linkage of genes. These genetic associations can have significant implications on trait evolution, since selection acts differently on different traits.

A number of studies have demonstrated a possible link between parasite resistance and dark pigmentation in several species. For example, in the fruit fly Drosophila falleni, artificial selection for individuals with low pigmentation resulted in strains that were almost twice as susceptible to infection by the parasitic nematode Howardula aoronymphium as wildtype flies. In other species, melanism is thought to be linked to other traits including temperature regulation, warning coloration, and behavior. Therefore, natural selection on these other traits may have indirect effects on immune functioning.

I am interested in investigating how strongly this correlation between melanism and resistance traits holds and what implications it has for evolution and maintenance of variation in both traits. The species I study are the flies Drosophila falleni, D. putrida, and D. neotestacea, all of which have variable abdominal melanin spot patterns in nature and are known to be parasitized by the nematode H. aoronymphium. Susceptible individuals become infected as larvae as they feed and develop on mushrooms. 

I have found significant negative correlations between melanin pigmentation and nematode infection in males from local populations of D. putrida and D. neotestacea. I also found a negative correlation between body size and pigmentation in D. putrida males. In other species of Drosophila, females are known to prefer mating with larger males. This may present a conflict between natural and sexual selection since strong female preference for larger mates may result in lighter and more susceptible males, due to the negative correlation between body size and pigmentation. This process may impede the evolution of parasite resistance and possibly explain the high variation in resistance traits. For my dissertation work, I hope to carry out a number of experiments to test this hypothesis and to explore how the genetic nature of these correlations might shape trait evolution.