Evolutionary and Ecological Physiology of Diapause
Developmental arrest in a diapausing life history stage provides an escape from conditions that would be highly deleterious were an insect actively growing and reproducing. Overwintering diapause stages are typical in temperate insects, for example. However, there are costs associated with lingering in a largely inactive state. Most insects do not feed during diapause, and thus depend upon energetic reserves built up prior to diapause initiation. Because insects are ectothermic, the speed at which these reserves are depleted depends on environmental temperature: higher temperatures increase metabolic demand. When temperatures fluctuate seasonally, the timing of diapause initiation and termination determines metabolic demand (i.e., there is an interaction between seasonal timing and climatic selection). Two important implications of this observation are: 1) the evolution of seasonal timing may often be accompanied by the evolution of energy storage and metabolism, and 2) systems exhibiting natural diversity in seasonal timing may provide the variation necessary to elucidate general mechanisms and genetics of energy storage and metabolism.
Working with the apple maggot fly, Rhagoletis pomonella, Daniel Hahn, Jeff Feder, and I are studying these evolutionary, physiological, and genetic aspects of diapause energetics. The recent evolution of an apple-infesting host race from an ancestral hawthorn-infesting population of R. pomonella is a textbook example of contemporary, sympatric speciation. The derived apple race has evolved earlier seasonal timing to track the relatively earlier fruiting date of apples compared to hawthorns.
From Dambroski HR and JL Feder. J. Evol. Biol. 20 2101–2112. doi:10.1111/j.1420-9101.2007.01435.x
We hypothesize that this shift in timing places greater metabolic demand on the apple host race, and we are testing for two potential adaptations: greater lipid storage or decreased metabolic rate in pre-diapause and diapausing pupae. We are exploiting this recently evolved variation to address the importance of seasonal adaptation in the speciation process, and to identify genetic loci affecting basic pathways of nutrient storage and metabolism. We are applying a combination of common garden experiments, genetic association studies, and functional genomic approaches. Serra Goudarzi and I have found that apple flies are in fact fatter than hawthorn flies. We are currently analyzing F1 common garden data and microsatellite marker association data with Jeff Feder and Sheina Sim to determine whether this difference is genetically based.
I am also interested in the evolutionary genetics and physiology of seasonal timing itself. Taking similar methodological approaches, I am working to 1) characterized developmental differences in diapause break between apple and hawthorn host races, and 2) identify changes in gene expression that correlate with transitions between diapause and non-diapause life history stages in R. pomonella. John Fuller and I have used measures of metobolic rate mapped onto visual markers of morphogenesis to identify diapause and post-diapause developmental stages with a high degree of accuracy. The long term goal is to use candidates for the initiation of these developmental events to dissect the genetic basis of seasonal timing adaptation, both among host races and across geographic clines.