It's about time: the evidence for host plant‐mediated selection in the apple maggot fly, Rhagoletis pomonella, and its implications for fitness trade‐offs in phytophagous insects

The apple maggot fly, Rhagoletis pomonella, Walsh (Diptera: Tephritidae), provides a unique opportunity to address the issue of host‐related fitness trade‐offs for phytophagous insects. Rhagoletis pomonella has been controversial since the 1860's when Benjamin Walsh cited the fly's shift from hawthorn (Crataegus spp.) to apple (Malus pumila) as an example of an incipient sympatric speciation event. Allozyme and mark‐release‐recapture studies have subsequently confirmed the status of apple and hawthorn flies as partially reproductively isolated and genetically differentiated ‘host races’, the hypothesized initial stage in sympatric divergence. Here, we review the ecological and genetic evidence for host‐plant mediated selection in R. pomonella. We reach the following three major conclusions: First, although developmental timing is not everything, it is a good deal of the story. Differences in the fruiting phenologies of apple and hawthorn trees exert different selection pressures on the diapause and eclosion time characteristics of the host races. In particular, the ∼3‐week earlier mean fruiting phenology of apples in eastern North America appears to select for a slower rate of metabolism or deeper pupal diapause in apple than hawthorn flies. Second, host‐related fitness trade‐offs for R. pomonella may not be due to disruptive selection affecting any one specific life‐history stage. Rather, it is the sum total of directional selection pressures acting across different life‐stages that generates divergent selection on apple and hawthorn flies. For example, selection favors the alleles Me 100, Acon‐2 95 and Mpi 37 (or linked genes) in the larval stage in both host races. However, these same alleles are disfavored in the pupal stage to follow, where they correlate with early adult eclosion, and by inference premature diapause termination. Because apple trees fruit an average of ∼3 weeks earlier than hawthorn trees, this counter‐balancing selection is stronger on apple‐fly pupae. The net result is that the balance of selective forces is different between apple and hawthorn flies, helping to maintain the genetic integrity of the host races in sympatry in the face of gene flow. Finally, natural R. pomonella populations harbor a good deal of genetic variation for development‐related traits. This variation allows fly populations to rapidly respond to temporal vagaries in local environmental conditions across years, as well as to broad‐scale geographic differences that exist across the range of the species. Perhaps most importantly, this variation gives R. pomonella the flexibility to explore and adapt to novel plants. Taken together, our results underscore how difficult it can be to document host plant‐related fitness trade‐offs for phytophagous insects due to the need to consider details of the entire life‐cycle of a phytophagous insect. Our findings also show how reproductive isolation can arise as a by‐product of host‐associated adaptation in insects, a central theme for models of sympatric speciation via host shifts.