Genetic and environmental influences on short-day responsiveness in Siberian hamsters (Phodopus sungorus)

Siberian hamsters are photoperiodic rodents that typically exhibit several physiological changes when exposed to a short-day photoperiod. However, dev elopment of the winter phenotype in short days is largely conditional on pr ior photoperiod history: Hamsters that have been reared in an exceptionally long day length (18 L) do not usually exhibit the winter phenotype after t ransfer to short days, whereas animals reared under "moderately" long days (16 L) are more variable in responsiveness to subsequent short-day exposure , with 20% to 30% generally failing to exhibit winter-type responses. Hamst ers reared exclusively in an "intermediate" day length (14 L) are almost un iformly responsive to short photoperiod. In the present study, the authors examine the influence of photoperiod history on short-day responsiveness in a breeding line of hamsters that has been subjected to artificial selectio n for resistance to the effects of short days. The results demonstrate that photoperiod history is an important determinant of short-day responsivenes s in both random-bred (UNS) hamsters and animals artificially selected and bred for nonresponsiveness to short photoperiod (PNR). The PNR hamsters hav e a reduced requirement for long-day exposure to evoke a state of unrespons iveness to short days. The results are discussed in relation to possible si gnificance for the origin of population and species differences in photoper iod responsiveness.