AN X-CHROMOSOME LINKED LOCUS CONTRIBUTES TO ABNORMAL PLACENTAL DEVELOPMENT IN MOUSE INTERSPECIFIC HYBRIDS

Interspecific hybridization between closely related species is commonl y associated with decreased fertility or viability of F-1 hybrids. Thu s, in mouse interspecific hybrids, several different hybrid sterility genes that impair gametogenesis of the male hybrids have been describe d. We describe a novel effect in hybrids between different mouse speci es that manifests itself in abnormal growth of the placenta. Opposite phenotypes, that is, placental hypotrophy versus hypertrophy, are obse rved in reciprocal crosses and backcrosses. The severity of the phenot ype, which is mainly caused by abnormal development of the spongiotrop hoblast, is influenced by the sex of the conceptus. In general, placen tal hypertrophy is associated with increased fetal growth. Hypotrophy of the placenta frequently leads to growth impairment or death of the fetus. One of the major genetic determinants of placental growth maps to the proximal part of the mouse X chromosome.