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Systematics, Evolution, and Biodiversity

10-Minute Paper

SysEB: Evolution On-Demand Presentations

Recursive adaptation in action: allochronic isolation and divergence of host-associated populations of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae) following its recent introduction to the western United States

On-Demand
On-Demand

 
An outstanding issue in the study of insect host races concerns the idea of ‘recursive adaptive divergence’, whereby adaptation can occur repeatedly across space and/or time, and the most recent adaptive episode is defined by one or more previously similar cases. The host plant shift of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae), from ancestral downy hawthorn to introduced, domesticated apple in the eastern USA has served as a model for investigating ecologically-driven host race formation in phytophagous insect specialists. Here, we report results from annual geography surveys of eclosion time demonstrating a similar ecological pattern among nascent host-associated populations of the fly recently introduced ~40 year ago from its native range in the east into the Pacific Northwestern (PNW) USA. Specifically, using data collected from 25 locations across 5 years, we show that apple-infesting fly populations in the PNW have rapidly and repeatedly shifted their adult eclosion life-history timing to infest two novel hawthorn hosts with different fruiting phenologies–a native (Crataegus douglasii) and an introduced (Crataegus monogyna) species—generating partial allochronic reproductive isolation in the process. The shifts in the PNW parallel the classic case of host race formation in the east, but have occurred bi-directionally to two hawthorn species with phenologies slightly earlier (black hawthorn) and significantly later (ornamental hawthorn) than apple. Our results imply that R. pomonella can possess and retain extensive standing variation (‘adaptive memory’) in diapause traits, even following introductions, to rapidly and temporally track novel phenological opportunities when they arise.