Effects of inbreeding on skeletal development of Rattus villosissimus

In a species of Australian native rat, Rattus villosissimus, which experiences extreme population fluctuations and possibly episodes of local inbreeding in the wild, generations of inbreeding in a laboratory colony led to altered skull shape and increased fluctuating asymmetry in some skeletal characters. Although inbreeding was closely associated with the number of generations in captivity, the effect of inbreeding after controlling statistically for the generation number was to decrease skull length and width. The effect of generations in captivity after controlling for inbreeding was to increase skull length and width. The joint effect of these confounded determinants was to produce rats with increasingly broad and short skulls through generations of inbreeding. Developmental anomalies of the incisors and feet appeared in the population. The average effects of inbreeding on fluctuating asymmetry were not strong, as most inbred and noninbred rats were not detectably asymmetric. Asymmetry appeared to be a threshold phenomenon, as significantly asymmetric animals were more prevalent among the more highly inbred rats of later generations. We detected no trend for a lessening impact of inbreeding after many generations, as would have been expected if selection had been purging the population of its genetic load. Individual heterozygosity across five polymorphic allozyme loci was correlated with size and asymmetry metrics, but only in so far as could be accounted for by the correlation of sample heterozygosity with the level of inbreeding and the effect of inbreeding on size and asymmetry.