The Evolutionary Genetics of Seasonal Development in the Pitcher-plant Mosquito, Wyeomyia smithii
At temperate latitudes, a great diversity of arthropods, fish, birds and mammals exhibit seasonal patterns of development and reproduction alternating with dormancy or migration. The major environmental cue used to switch between these alternative life styles is the length of day (photoperiod). As the length and timing of the favorable season vary with latitude and altitude, so also does the day length used to switch between active development and dormancy or migration. Hence, over geographical gradients, the adaptive modification of the switching day length constitutes the major evolutionary mechanism determining the seasonal cessation and resumption of development. My dissertation focuses on the physiological and genetic mechanisms by which insects measure the length of day and how these mechanisms have evolved, resulting in the appropriate timing of seasonal development over large latitudinal and altitudinal gradients.
There are two components of photoperiodic induction of dormancy or development in insects: the timer that measures actual day length and the counter that counts the number of long or short days resulting in a long- or short-day response.
A mounting number of physiological studies indicate that the photoperiodic timer regulating seasonal development of the mosquito, Wyeomyia smithii, has evolved independently of the circadian clock regulating daily activities. As part of my dissertation, I show that the photoperiodic counter does not covary either with the daylength used to switch between active development and dormancy or with the rhythmic output of the circadian clock over the geographic range of this mosquito. Our results mean that the photoperiodic timer, the photoperiodic counter, and the circadian clock have evolved independently of each other.
The genes responsible for photoperiodic time measurement and its adaptive modification are unknown in any animal. We are developing a cDNA microarray system for Wyeomyia smithii that will allow us to screen for genes that are differentially expressed in different photoperiodic environments during the stages of the life cycle where the physiological go/no-go “decision” between development and dormancy is made. Using the experimental power of geographic variation in the photoperiodic timer and counter, we shall then be able to determine how expression of these genes has been modified during the adaptive evolution of developmental timing in natural populations of W. smithii.
Identifying the physiological and genetic mechanisms controlling seasonal development is important in predicting how animals are likely to evolve during range expansion and climate warming. This information will become increasingly important as agriculturally-important animals are confronted with altered seasonal environments and as new vectors of human and animal diseases expand into the temperate zone.