Phenotypic evolution and the genetic architecture underlying photoperiodic time measurement

A wide variety of higher plants, vertebrates, and arthropods use the length of day to synchronize growth, development, reproduction, dormancy, and migration with the changing seasons. In the pitcher-plant mosquito, Wyeomyia smithii, critical photoperiod mediating the onset and maintenance of larval diapause has evolved about 10 standard deviations in mean critical photoperiod between the ancestral, Gulf Coast populations, and the derived, Canadian populations. We are seeking to understand how this evolution has been accomplished at both the genetic and the physiological levels. genetic level, average heterozygosity at protein-coding loci decreases with latitude of origin, while genetic variation for photoperiodic response increases with latitude of origin, particularly within the formerly glaciated regions of North America. Hybridization experiments reveal widespread genetic differences in critical photoperiod due to epistasis. We ascribe the increase in genetic variation in photoperiodic response, despite directional and stabilizing selection to the contrary, to the release of additive from epistatic variance during successive founder events in W. smithiiʹs northward dispersal following recession of the Laurentide Ice Sheet, and to the resulting genetic drift and reorganization of genetic architectures in descendent populations. physiological level, northern populations of W. smithii, as well as northern populations of spider mites, flies, moths, and beetles in both North America and Europe show a declining expression of the rhythmic component of photoperiodic response. sophila melanogaster, when the epistatic coupling between the period locus and photoperiodic response is disrupted, the critical photoperiod is shifted towards shorter daylengths; analogously in W. smithii, when epistatic interactions are disrupted in the recombining generations of hybrid populations, the critical photoperiod is shifted towards shorter daylengths. The implications here are (1) that in W. smithii, it is the epistatic modification of the photoperiodic response curve by the circadian clock that is being disrupted in the recombining generations and (2) that post-glacial range expansion into the North-Temperate Zone by arthropods in general may involve uncoupling of the circadian and photoperiodic clocks. e the tremendous advances that have been made in understanding circadian rhythmicity at the molecular level, virtually nothing is known about how or whether any of the downstream ‘clock-controlled genes’ connect with photoperiodic time measurement. Except in W. smithii, little is known about how this connection changes with seasonal adaptation of photoperiodic response through evolutionary time. It is our goal and desire that our top–down approach to the evolution of photoperiodic time measurement will meet and mesh with the bottom–up approach that is being developed so fruitfully for circadian clocks.