Lack of correlation between placenta and offspring size in mouse interspecific crosses

The placenta plays a pivotal role in fetal growth control and is considered a major site of genetic conflict between maternal and paternal genomes wit hin the conceptus and, in addition, the genome of the mother. Accordingly, placental development is a strictly con trolled process, and both placental and fetal weights do not vary much in intraspecific crosses of laboratory mice (Mus musculus). In mouse interspecific crosses and backcrosses [(M. mu sculus x M. spretus) x M. musculus], tremendous variation of placental, but not of fetal weight was observed. We have studied trophoblast cell type di stribution and differentiation, and their effect on the associated placenta s and fetuses in such backcrosses. Differentiation of spongious trophoblast , but not size of materno-fetal interface, correlated with fetal weight. Gi ant fetuses were observed only if less than one third of the spongiotrophob last was formed by glycogen cells. Thus, placental efficiency was inversely related to the amount of glycogen cells. This influence of a trophoblast-d erived cell type on fetal growth was not anticipated. We conclude that: (1) glycogen cells are able to negatively modulate fetal growth by an as yet u nidentified mechanism; (2) correlation between fetal and placental weights is weak or absent in interspecific hybrids; (3) impaired control over place ntal and embryonic development in hybrids may contribute to post-mating iso lation of species.