A wide variety of plants and animals use the length of day to synchronize their development and reproduction with the warm days of summer and to synchronize their entry into hibernation in advance of the cold days of winter. At more northern latitudes, winter arrives earlier when days are longer than at more southern latitudes. Consequently, northern organisms respond to longer daylengths to switch from active development to hibernation in anticipation of the forthcoming winter. Many temperate plants and animals, including some of the most problematic agricultural pests in the United States, originated from more tropical latitudes or migrated north after the last ice age. This migration required evolving the appropriate responses to daylength before the immigrants could thrive in their new northern habitats. Thus, evolved responses to daylength to cue the seasonal programming of development, reproduction, and hibernation represent one of the most pervasive and important adaptations of temperate organisms. The question then remains as to how they do it. What is the physiological basis for the measuring of daylength within and between geographically distant populations? Are the physiological processes that underlie genetic variation for day-length measuring within populations the same processes that change over geographic distances; i.e., does understanding genetic variation in the physiological clock within populations permit us to predict how these populations might evolve in the future? The PIs have pursued these questions in temperate populations of a formerly tropical mosquito. The PIs have found that the internal biological clock (circadian rhythm) plays a role in the day-length measuring process in southern populations; but, the expression of the circadian clock declines with increasing latitudes so that northern populations measure day length with essentially a physiological hourglass. Yet, when unrelated populations are crossed, their offspring show a reversion to the southern or ancestral switching daylength. This grant will help the PIs to determine whether reversion to the ancestral daylength also brings out the ancestral expression of the circadian clock. In addition to contributing to our understanding of the genetic and physiological basis for adaptive evolution of biological time measurement, results of this study have implications for physiological adaptation to changing environments in general. If vestigial physiological capabilities reside covertly within populations, physiological adaptation to environmental change may proceed far more rapidly than would be predicted from present-day, overt genetic variation for that trait in a population. If these covert capabilities are relics from ancestral genes lying latent in extant populations, then response to selection may proceed faster towards ancestral than towards new capabilities. Hence, predicting the rate of evolution in response to environmental change such as global warming may depend critically upon understanding the historical role of masked or covert genes in physiological adaptation.
