Heterochromatin as a factor affecting X-inactivation in interspecific female vole hybrids ( Microtidae, Rodentia )

Female interspecific vole hybrids were examined for the expression of the G6PD and GALA genes on the X chromosomes. When one of the parents was a species with a heterochromatin block on the X, and the other parent was M. arvalis , without a heterochromatin block on the X, preferential expression of the genes of the M. arvalis X was consistently observed. When both parental species had heterochromatin on the X, the parental forms of G6PD and GALA were in about equal proportions in the hybrid females. The results of the cytological identification of the active and inactive X on the metaphase spreads in the hybrid females are in agreement with the biochemical results. It is suggested that the observed phenomenon may be due to a nonrandom inactivation of the X chromosome containing a heterochromatin block in crosses involving M. arvalis and by a random inactivation in those with both parents having heterochromatin blocks on the X chromosomes. These results support our previous suggestion that heterochromatin has an effect on X inactivation in female interspecific vole hybrids.