DEVELOPMENTAL STABILITY, FITNESS, AND TRAIT SIZE IN LABORATORY HYBRIDS BETWEEN EUROPEAN SUBSPECIES OF THE HOUSE MOUSE.

The effects of hybridization on developmental stability and size of tooth characters were investigated in intersubspecific crosses between random-bred wild strains of the house mouse (Mus musculus domesticus and M. m. musculus). Fluctuating asymmetry (FA) and trait size were compared within and between parental, F1 , backcross, and F2 hybrid groups. The relationship between FA and reproductive fitness within the F1 hybrids was also studied. The results indicated that both FA and character size levels differed significantly between the two subspecies. The F1 hybrids and the recombined groups (backcrosses and F2 hybrids) showed heterosis for both parameters. No significant differences in the FA of fertile and sterile F1 hybrid individuals were found. Comparison of the FA levels obtained in this study with those found in wild populations from the hybrid zone in Denmark showed that the levels of FA were lower in laboratory-bred samples than in the wild populations. This study provides further evidence that, in hybrids, the developmental processes underlying most of the morphological traits we studied benefit from a heterotic effect, despite the genomic incompatibilities between the two European house mice revealed by previous genetical and parasitological studies.