VARIATION AMONG INBRED AND LINECROSS MICE IN RESPONSE TO FESCUE TOXICOSIS

Variation in response to fescue toxicosis was examined in inbred and l inecross mice. In Exp. 1, exposure to a 50% endophyte-infected tall fe scue diet (E+) reduced ADG of males from six inbred lines, but ADG of males from one line was modestly higher on E+. Lines differed (P < .01 ) for reproductive organ weight, but the diet x line interaction was n ot significant. In Exp. 2, an apparently susceptible (C57) and an appa rently resistant line (FVB) were mated to produce inbred and linecross offspring. The reduction in weight gain caused by the E+ diet did not differ significantly among the genetic groups. In Exp. 3, C57 and C57 backcrosses had smaller reductions in ADG during E+ vs control feedin g periods than FVB and FVB backcrosses (P < .10). In Exp. 4, the E+ di et reduced litter size of mates of C57 males by one pup, whereas litte r size of mates of FVB males was four pups larger (interaction P = .07 ). Neither diet, line, nor their interaction affected male reproductiv e organ weights or tissue proportions in testis cross-sections. In Exp . 5, the E+ diet did not affect weight gain of C57 or FVB males, but e ffects of the E+ diet on litter size of mates were similar to those in Exp. 4. Percentage of abnormal sperm was increased in C57 males on th e E+ diet but decreased in FVB males (Exp. 5). Differences among inbre d lines in susceptibility to fescue toxicosis may depend on severity o f the challenge and life cycle stage when the challenge is imposed.