Academic journal article Genetics

Quantitative Trait Loci Associated with Photoperiodic Response and Stage of Diapause in the Pitcher-Plant Mosquito, Wyeomyia Smithii

Academic journal article Genetics

Quantitative Trait Loci Associated with Photoperiodic Response and Stage of Diapause in the Pitcher-Plant Mosquito, Wyeomyia Smithii

Article excerpt

ABSTRACT

A wide variety of temperate animals rely on length of day (photoperiodism) to anticipate and prepare for changing seasons by regulating the timing of development, reproduction, dormancy, and migration. Although the molecular basis of circadian rhythms regulating daily activities is well defined, the molecular basis for the photoperiodic regulation of seasonal activities is largely unknown. We use geographic variation in the photoperiodic control of diapause in the pitcher-plant mosquito Wyeomyia smithii to create the first QTL map of photoperiodism in any animal. For critical photoperiod (CPP), we detect QTL that are unique, a QTL that is sex linked, QTL that overlap with QTL for stage of diapause (SOD), and a QTL that interacts epistatically with the circadian rhythm gene, timeless. Results presented here confirm earlier studies concluding that CPP is under directional selection over the climatic gradient of North America and that the evolution of CPP is genetically correlated with SOD. Despite epistasis between timeless and a QTL for CPP, timeless is not located within any detectable QTL, indicating that it plays an ancillary role in the evolution of photoperiodism in W. smithii. Finally, we highlight one region of the genome that includes loci contributing to CPP, SOD, and hormonal regulation of development.

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THE annual change in day length at temperate latitudes provides a highly reliable indicator of future seasonal events and a wide variety of organisms use day length (photoperiod) to time their development, reproduction, dormancy, and migration. Concordance between individual photoperiodic response and local climate is an essential component of fitness for animals living in the temperate zone (BRADSHAW et al. 2004), and modification of photoperiodic response is an important adaptation of animals during range expansion (DANILEVSKII 1965; COOKE 1977; TAUBER et al. 1986; DANKS 1987; LOUNIBOS et al. 2003) or when confronted with rapid climate change (BRADSHAW and HOLZAPFEL 2001a, 2006). Photoperiodism provides an ecologically relevant, highly heritable trait whose adaptive significance in a temperate seasonal environment is not questioned. Length of the favorable or growing season in North America decreases with increasing latitude (BRADSHAW 1976). Consequently, the optimal time to enter diapause advances to an earlier day in the year and the day length used to switch from active development to diapause [hereafter, the critical photope-riod (CPP)] is positively correlated with latitude and altitude among a wide variety of temperate arthropods (DANILEVSKII 1965; TAYLOR and SPALDING 1986; DANKS 1987). The consistent genetic change in photoperiodic response over geographic and climatic gradients provides one of the most robust examples of repeated adaptive evolution in nature.

Although much progress has been made in identifying the genetic components of the circadian clock regulating daily activities, the molecular basis of the photoperiodic timer regulating seasonal activities is conspicuously absent from the literature. For insects, the model system for circadian rhythmicity has been Drosophila melanogaster; the Canton-S strain of D. mela-nogaster is photoperiodic for ovarian diapause but only over a very narrow range of temperatures, and its dia-pausing status must be determined destructively from dissection of the ovaries. Hence, while "D. melanogaster, with its unrivalled genetic background has provided a foundation of uncovering the molecular basis of the circadian mechanism, . . . it is probably less useful as a model for photoperiodism. Further studies should examine species with a much more robust [photoperiodic] response" (SAUNDERS 2002, p. 481). Toward the goal of understanding the molecular basis of photoperiodism and its adaptive evolution, we developed QTL maps for CPP and stage of diapause in the pitcher-plant mosquito Wyeomyia smithii, using a southern (31°N) and a northern (57°N) population recently collected from nature. …

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