Materials and Methods
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Maternal effects were negligible in these data; in other words, the results were similar whether the mother was of one parental strain or another. For example, isolation induced fighting was present in 50% and 56% of the WAG/Rij and DA F1 hybrids when the mothers were WAG/Rij and DA, respectively. Similarly, isolation-induced fighting was present in 64% and 58% of the Fischer and DA Fl hybrids when the mothers were Fischer and DA, respectively. There were no maternal effects observed in olfactory investigation, scent-marking, or shock-induced fighting as well.
Competitive fighting results were intermediate in the Fl hybrid generation and remained intermediate (or somewhat lower) in subsequent backcross generations. The data are illustrated in Figure 3. Unfortunately, the results of testing of the third backcross generations (4WD and 4FD) were lost. Data from the seventh and eighth backcross generations have been reported elsewhere (Adams, 1979). Since each succeeding backcross generation became closer and closer in genotype to the parental strain which normally does not show competitive fighting, these intermediate levels of fighting are all the more remarkable. They suggest, as will be discussed below, that only one or a few tightly-linked genetic loci are involved in the competitive fighting differences between the parental strains.
Heterosis is also indicated for other behaviors. We found it in the present study in the measures of olfactory investigation and scent-marking. Although we did not test the hybrids for resistance to handling, one would expect heterosis from the literature: the process of inbreeding reduces the defensiveness of wild mice, including their escape, vocalization and biting to handling (Connor. 1975) presumably because inbreeding removes heterotic gene combinations. We did not find heterosis in the measures of shock-induced fighting or competitive fighting, but since relevant studies have not been done in other species, it is not possible to say if this is a general and wide-spread phenomenon.
In most of the behavioral measures that we tested, there were apparently a number of genetic factors involved in the differences obtained. For this reason, apparently, we found that the levels of behavior in backcross generations continued to shift towards the parental strain on which the backcrosses were made, instead of remaining at the same level as that of the animals from the previous backcross generation who were selected because they showed a high level of response. This is usually the case in studies of the behavior genetics of aggression in mice.
In one case, that of competitive fighting, there was apparently only one genetic locus (or several loci tightly linked). In this case, it was possible to maintain a high level of competitive fighting over many generations of backcrossing against parental stock that did not show competitive fighting, so long as the hybrid parents were selected for showing the behavior. Although such a finding has not previously been made in rats, there are similar findings in mice. For example, Whitney (1973) has reported that a single dominant gene is responsible for the increased tendency of vocalization in response to handling in an inbred mouse strain.
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