Evidence for genetic variation in the occurrence of the photoresponse of the Djungarian hamster, Phodopus sungorus.

The Djungarian hamster generally responds to a short-day photoperiod with a complex syndrome of physiological and behavioral changes; however, not all hamsters are photoresponsive. The phenotypic difference is, in part, genetically determined. Parent-offspring regression on a number of continuous and discontinuous measures indicated significant heritability for photoresponsiveness. Four generations of replicated bidirectional selection on a photoresponse index (PI) resulted in significant shifts in the percentage of responsive hamsters, although the average PI of responsive individuals was not significantly changed. Eight estimates of heritability ranged from 0.20 to 0.52. We hypothesize that the circadian system is responsible for the occurrence of the photoresponse, but that the extent of photoresponse is controlled by a separate functional system.