MULTIPLE SELECTION RESPONSES IN-HOUSE MICE BIDIRECTIONALLY SELECTED FOR THERMOREGULATORY NEST-BUILDING BEHAVIOR - CROSSES OF REPLICATE LINES

Replicate high-selected, control, and low-selected lines were crossed at generation 46 of bidirectional selection for thermoregulatory nest- building behavior. Previous analysis of the lines at their limits had revealed multiple responses to uniform selection, where each of the fo ur selected lines responded differently to reverse selection (Laffan, 1989). The reciprocal F-1 crosses showed significant heterosis for nes t-building behavior compared to the contemporaneous generations of the parental lines. This pattern of heterosis in all three crosses is con sistent with the finding that nest-building behavior in each of the fo ur replicate lines had a different genetic basis, in spite of the phen otypic similarity between the two replicate lines in the high and low direction of nesting. This heterosis effect and the larger number of y oung weaned in all three crosses compared to their respective contempo raneous generation of the parental lines also support earlier findings that larger nests are closely related to fitness.