Geographically patterned variation in diapause and its relationship to other climate-associated phenotypes and genotypes of Drosophila americana

To declare a trait adaptive, it is necessary to show heritability and beneficial fitness effects of a phenotype. Association between phenotypic and environmental variance is insufficient to identify adaptive traits, because non-selective forces build phenotype-environment associations without beneficial fitness effects. However, if the same phenotype-environment associations occur in independent lineages subject to similar environmental stresses, the traits shared among lineages may be more likely to have beneficial fitness effects and is a good target for further study of adaptation.

I tested the hypothesis that among species sharing similar climate and habitat distributions, shared traits are good candidates for adaptations to shared selective pressures. I used Drosophila americana as a model because it is endemic to temperate environmental conditions similar to those of other well-studied populations and species of Drosophila and has other qualities that make it amenable to studies of putative adaptations. These include an ancient association with cold climates and previously defined regions of clinally distributed genetic variation maintained by selection.

I demonstrated an association between reproductive diapause, and clinally distributed genetic variation in D. americana. The fused X-4 chromosome is sufficient to increase the probability of diapause in D. americana, establishing heritability of the trait and linking genetic variation under selection to a phenotype.

I also investigated geographically patterned variation in putatively adaptive traits, to test whether adaptation to cold climate involves the same traits in related, but independently adapted lineages of Drosophila. I found that between D. americana and temperate populations of D. melanogaster some, but not all traits putatively adaptive to cold climate exhibit shared associations with climate variables.

To test the fitness effects of diapause in cold environments, I assessed the response of diapause in D. americana to temporal changes in the environment. In comparing diapause incidence between isofemale lines of D. americana derived from flies collected either prior to 2009 or following the record cold winter of 2009-2010, I found that Southern populations previously accustomed to warmer climate significantly increased in diapause incidence.

My results demonstrate the interaction between variation in genotype, phenotype, and fitness for reproductive diapause in D. americana. I verified that identifying traits shared among species living in similar environments is an effective approach to target putative adaptations, and that, while traits shared among species living in similar environments are good candidates for adaptations, evidence of selective advantage is necessary before a trait can be truly considered adaptive.