Genetic variation in photoperiodism in Peromyscus leucopus: Geographic variation in AN alternative life-history strategy

Phenotypic variation in reproductive response to photoperiod is typical of many temperate-zone populations of rodents. This variation could be due either to genetic variation in photoresponsiveness or phenotypic plasticity within populations genetically homogeneous for photoresponsiveness. At least some of this variation has been shown to be genetic in single populations of three species of rodents and in one laboratory population of another species of rodent. Neither the latitudinal range over which this genetic variation occurs nor the frequency with which populations contain this kind of genetic variation is known. To test for genetic variation in reproductive photoresponsiveness in a population of Peromyscus from a mid-temperate latitude, a population of P. leucopus from Williamsburg, Virginia (37°16′N, 76°42′W), was subjected to artificial selection either for reproductive response or lack of reproductive response to photoperiod, with a control line that was not subject to selection. Reproductive photoresponsiveness was significantly heritable (narrow sense h² ranging from 0.54 to 0.74 in the first laboratory generation). Three generations of selective breeding yielded one line of mice with 80% of individuals responding strongly to photoperiod (regression of gonads under short day length, 8L:16D) and a nonresponsive line with only 16% strongly responsive individuals. There were significant responses to selection in each generation in one or both selected lines, while a control line changed significantly only in one sex in a single generation. These results are similar to those from a population from Michigan (43°N) but are different from a population from Georgia (34°N). These data are consistent with the hypothesis that genetic variation in photoresponsiveness is present in a mosaic pattern rather than a latitudinal cline. Presence of genetic variation in reproductive photoresponsiveness in two widely separated populations of P. leucopus provides support for the hypothesis that genetic variation in reproductive photoresponsiveness is likely to be a common life-history feature of populations of this species between 37 and 45°N, and probably further north as well.