PATERNAL TRANSMISSION OF X-LINKED PLACENTAL DYSPLASIA IN MOUSE INTERSPECIFIC HYBRIDS

It has previously been shown that abnormal placental development, i.e. , hyper- and hypoplasia, occurs in crosses and backcrosses between dif ferent mouse (Mus) species. These defects are caused mainly by abnorma l growth of the spongiotrophoblast. The precise genetic basis for thes e placental malformations has not been determined. However, a locus th at contributes to the abnormal development (Ihpd: interspecific hybrid placental dysplasia) has been mapped to the X chromosome. The X-chrom osomal location of Ihpd and its site of action, that is the spongiotro phoblast, mean that normally only the maternally inherited Ihpd locus is active even in female fetuses. However, by making use of the X-chro mosomal inversion In(X)1H, we have produced interspecific hybrid; X(p) 0, in which the active X chromosome was inherited from Mus macedonicus males. In contrast to XX female and XY male conceptuses from this cro ss, which have hypoplastic placentas, the X(p)0 female conceptuses hav e hyperplastic placentas. This finding supports the view that it is ex pression of the M. macedonicus Ihpd locus in the spongiotrophoblast th at leads to hyperplasia due to an abnormal interaction with M. musculu s autosomal loci.