My current work is involved with how correlated traits can shape evolution since existing relationships may influence evolutionary trajectories. An interesting trait to study in this regard is the production and patterning of melanism which varies considerably inter- and intraspecifically, and is associated with multiple functions such as temperature regulation, UV protection and crypsis. Several studies have also elucidated relationships between melanism and seemingly unrelated traits such as behavior and pathogenic resistance. 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. Therefore, natural selection affecting melanism with regards to its other functions may have indirect effects on immune functioning.
Immunity is an intriguing trait to investigate, since despite the benefit of evolving resistance traits, 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 may be pulling traits in opposing directions. Another dimension of complexity is added when considering the association of immunity with melanism which has its own assortment of sources for variation.
Through my dissertation work, I hope to investigate how strongly the 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 mushroom feeding 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. Infection in susceptible individuals occurs during the larval stages, and can lead to detrimental fitness effects such as mortality and sterility.
My previous work has reveled significant negative correlations between melanin pigmentation and nematode infection in males from natural local populations of D. putrida and D. neotestacea. This study revealed the propensity for this correlation to occur in nature and in additional Drosphila species. Additionally, I also found a positive correlation between body size and pigmentation in D. putrida males. Future work will involve seeing whether these correlations exist due to an underlying developmental or genetic constraint using artificial selection as a tool to break the correlations if possible. Other projects aim to investigate what the underlying genetic architecture of this correlation may be and how the traits in question may be affected by other fitness factors such as desiccation and starvation tolerance.
