Genomic architecture of seasonal adaptation during speciation
Seasonality appears to be a major driver of speciation, particularly in plant-eating insects that specialize on hosts that are only available during limited seasonal windows. The apple maggot fly, Rhagoletis pomonella, provides a textbook example, historically feeding on native hawthorns in North America but now shifting to exploit apples, an introduced crop. Apples fruit earlier, the flies have only one generation per year, and the apple host race has evolved earlier seasonal timing via changes in pupal diapause regulation.
From Dambroski HR and JL Feder. J. Evol. Biol. 20 2101–2112. doi:10.1111/j.1420-9101.2007.01435.x
This separation in time is driving ecological speciation among the host races. Further, the entire Rhagoletis species complex appears to be an adaptive radiation with highly genetically similar species that differ markedly in seasonal timing, suggesting that this is a major mechanism for speciation. A number of studies have now shown that ecological speciation can drive genetic divergence across the genome, including in R. pomonella. Scott Egan (Rice), Jeff Feder (Notre Dame), and I have now assembled genome-wide RAD marker data sets demonstrating that a very simple experimental shift in seasonality explains much of this genomic variation.
We are now pursuing the phenotypic targets of selection, which appear to be diapause-related. Earlier seasonality in apple flies causes pupae to experience a longer, warmer pre-winter period that represents a novel environment for the pupae. This novel environment exposes formerly covert, maladaptive variation in diapause regulation that is now visible to natural selection in the derived apple race. Using respirometric biomarkers and other developmental landmarks, we are identifying these ecophysiological phenotypes and correlating with genomic markers. These studies are integrated with our related efforts to identify candidate genes and pathways for adaptive diapause regulation.