Developmental stability in house mice heterozygous for single Robertsonianfusions

Fluctuating asymmetry (FA) of tooth traits has been reported to be increase d in Down syndrome patients as well as hybrids between chromosomal races of the house mouse differing in several Robertsonian (Rb) fusions, Developmen tal stability, assessed by FA, is thus thought to be impaired by spontaneou s chromosomal abnormality or by chromosomal heterozygosity. Although the ef fect of a single fusion on developmental stability could theoretically be e xpected, it has never been documented. Crosses involving two chromosomal ra ces of the house mouse diverging for one Rb fusion were performed to assess developmental stability in parental homozygous races as well as in their h ybrids. Moreover, the occurrence of a spontaneous chromosomal mutation (WAR T type-b) allowed us to study the instantaneous effect of such a translocat ion on developmental stability. No difference in fluctuating asymmetry leve ls was detected among the groups considered in this study. This result sugg ested that a single stable or spontaneous balanced structural rearrangement did not inherently disturb developmental stability. In addition, the diffe rential effect on developmental stability of one versus many heterozygous R b fusions highlights the role of their quantitative accumulation in the dis ruption of coadaptation in chromosomal hybrids.