Somatic and germ cell chimerism in chimeric mice between strains of high litter size and regular litter size

CFO is an inbred strain of mice showing a high litter size. The high fertility of CFO is due mainly to a very low embryonic death rate during uterine development. C57BL and 129 strains are characterized by regular litter size. Crosses were made and CFO----C57BL and CFO----129 chimeras were produced. In CFO----C57BL mice, coat color of the C57BL predominated over that of CFO; internal chimerism except for that of the gonads was observed to be in the same proportion as the two genotypes, but the genotypic component of the gonads was almost entirely CFO. Germ cells undergoing gametogenesis in the CFO----C57BL mice were almost all derived from the CFO genotype, and the chimeric females showed high fertility just as did the CFO females. In the CFO----129 mice no obvious skewing toward one genotype was observed in the coat color, but the germ cells undergoing oogenesis in the two types of chimeric females were recognizable as nearly all of the 129 genotype but with the females showing the same high fertility as do the CFO females. This fact suggests that the genotypes of germ cells in the ovary or in developing embryos do not influence fertility, but rather that the litter size is controlled mainly by the uterine environment.