Evolution of Photoperiodic Time Measurement in the Pitcher- Plant Mosquito, Wyeomyia smithii

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A wide variety of plants and animals use the length of day to synchronize theirdevelopment and reproduction with the warm days of summer and to synchronize their entry intohibernation in advance of the cold days of winter. At more northern latitudes, winter arrivesearlier when days are longer than at more southern latitudes. Consequently, northern organismsrespond to longer daylengths to switch from active development to hibernation in anticipation ofthe forthcoming winter. Many temperate plants and animals, including some of the mostproblematic agricultural pests in the United States, originated from more tropical latitudes ormigrated north after the last ice age. This migration required evolving the appropriate responsesto daylength before the immigrants could thrive in their new northern habitats. Thus, evolvedresponses to daylength to cue the seasonal programming of development, reproduction, andhibernation represent one of the most pervasive and important adaptations of temperateorganisms. The question then remains as to how they do it. What is the physiological basis for themeasuring of daylength within and between geographically distant populations? Are thephysiological processes that underlie genetic variation for day-length measuring withinpopulations the same processes that change over geographic distances; i.e., does understandinggenetic variation in the physiological clock within populations permit us to predict how thesepopulations might evolve in the future? The PIs have pursued these questions in temperatepopulations 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 northernpopulations measure day length with essentially a physiological hourglass. Yet, when unrelatedpopulations are crossed, their offspring show a reversion to the southern or ancestral switchingdaylength. 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 foradaptive evolution of biological time measurement, results of this study have implications forphysiological adaptation to changing environments in general. If vestigial physiologicalcapabilities reside covertly within populations, physiological adaptation to environmental changemay proceed far more rapidly than would be predicted from present-day, overt genetic variationfor that trait in a population. If these covert capabilities are relics from ancestral genes lyinglatent in extant populations, then response to selection may proceed faster towards ancestral thantowards new capabilities. Hence, predicting the rate of evolution in response to environmentalchange such as global warming may depend critically upon understanding the historical role ofmasked or covert genes in physiological adaptation.