Given the mosquito's affinity for warm temperatures and standing water, it is a front-runner to benefit from global warming. Now new research suggests that one type of mosquito has already undergone measurable genetic change in response to increasing temperatures over the last few decades. The findings appear in this week's edition of the Proceedings of the National Academy of Sciences.

William E. Bradshaw and Christina M. Holzapfel of the University of Oregon studied the pitcher-plant mosquito, Wyeomyia smithii, so-named because its development occurs entirely within the leaves of the purple pitcher plant. Collecting mosquitoes from sites around North America in 1972, 1988, 1993 and 1996, the researchers kept them under identical, controlled conditions in the laboratory for at least two generations and observed their behavior. (Keeping them in the lab allowed controlling genetic factors to be isolated from environmental influences, Bradshaw notes.) Specifically, the team examined alterations in the insects' photoperiodic response¿that is, their use of the length of day as an environmental clue to regulate seasonal behaviors such as dormancy, migration and reproduction. As global temperatures increase and growing seasons lengthen, Bradshaw explains, the appropriate time for the mosquito to enter dormancy comes later, on a day with less daylight. This requires a response to a shorter photoperiod.

The mosquitoes from 1996, Bradshaw and Holzapfel found, exhibited significantly shorter photoperiodic responses than did their 1972 counterparts. The researchers then compared results from 1988 and 1993 for three specific populations of insects from Florida, Maine and Ontario. Within just five years, the photoperiodic responses of all three had shortened. "This shift toward shorter critical photoperiods," the authors conclude, "is consistent with an adaptive response to longer growing seasons and, therefore, with the indirect effects of global warming on seasonality."

Because the photoperiodic response is known to be genetically based and highly heritable, the scientists assert that the pitcher-plant mosquito's genes are changing as a result of global warming. "[This species] represents an example of actual genetic differentiation of a seasonality trait that is consistent with an adaptive evolutionary (genetic) response to global warming," they write.

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