Dr. Jantina Toxopeus
Department of Integrative Biology, University of Colorado Denver
Challenging the hallmarks of insect diapause: A case study in Rhagoletis pomonella
Most temperate insects overwinter in diapause, a dormant state characterized by developmental arrest, metabolic rate suppression, and enhanced stress tolerance. These three ‘hallmarks’ of diapause are thought to address several of the challenges associated with living in a temperate climate. However, there is considerable variability in the diapause phenotype across insect species, and these ‘hallmarks’ may not be universal. We tested whether the apple maggot fly (Rhagoletis pomonella) conformed to these hallmarks by examining: 1) cold tolerance (stress tolerance) of diapause and non-diapause pupae; 2) metabolic rates of diapause and non-diapause pupae during chilling; 3) the effect of low temperature on diapause duration. For all experiments, we used pupae from field-collected host fruits, transferred to 4 °C after 10 days in the lab at room temperature. Unlike many species, non-diapause and diapause R. pomonella pupae exhibited similar cold tolerance. Both groups froze at a similar temperature and >80 % of non-diapause and diapause pupae cooled (not frozen) to -18 °C for short time periods survived. As expected, diapause pupae suppressed their metabolic rate relative to non-diapause pupae at warm (25 °C) and cold (4 °C) temperatures. However, non-diapause pupae were able to flexibly achieve metabolic suppression approaching that of diapause pupae after several weeks at 4 °C. Finally, diapause development of R. pomonella partially conformed to the standard chilling threshold or chilling acceleration model: low temperatures were required for most pupae to complete diapause. Following c. 100 d of chilling, however, diapause development actually progressed faster at warm than cold temperatures, suggesting flexibility in the diapause developmental program. These results challenge the classic ‘hallmarks’ of diapause, and suggest that there may be multiple developmental and physiological adjustments to facilitate overwintering.