Characteristics of a genetic polymorphism for reproductive photoresponsiveness in the white-footed mouse (Peromyscus leucopus)

Wild populations of Peromyscus are often composed of individuals that vary greatly in their reproductive response to photoperiod. A population of white-footed mice (P. leucopus) from Michigan (43 degrees N) was subjected to mass selection in the laboratory both for and against reproductive photoresponsiveness for four generations. The first generation of selection yielded one line of mice in which about 80% of the individuals were classified as reproductively photoresponsive (i.e., with undeveloped reproductive tracts when reared in short days, 8L: 16D) and another in which only about 20% were reproductively photoresponsive. Some and perhaps most of this difference was accounted for by changes in degree of responsiveness to photoperiod rather than by alterations in the proportion of discrete responsive vs. unresponsive phenotypes. Alteration of critical day length was not a factor. Three more generations of selection failed to change the proportions noted above significantly. Although the genetic control of reproductive photoresponsiveness is undoubtedly complex, a single variable locus may be responsible for much of the heritable variation present in this population. These results also suggest that natural populations contain genetically determined phenotypes that are intermediate between absolutely photoresponsive and absolutely unresponsive. The factors that might promote maintenance of heterogeneity of reproductive photoresponsiveness in a wild population of rodents are considered.