Reproduction in a population of chimeric mice: Relationship of chromosomal sex to functional germ cells and proportions of chimeric components in several tissues

When preimplantation stage mouse embryos are aggregated to form chimeras, 50% should have a single (XX/XX, XY/XY) and 50% a mixed (XX/XY) chromosomal sex. The XX/XY chimeras develop as males or females (with a preponderance of males) rather than as hermaphrodites. The purpose of this report is to provide additional and comprehensive information on sex ratios, progeny tests, karyotype analyses, and estimates of chimeric proportions in tissues from a population of chimeras derived from two inbred mouse strains. BALB/cBy(J) ⇆ C57BL/6J chimeras were produced following aggregation of 8-12 celled embryos. Of 30 chimeras produced, 18 were males and 12 were females. From karyotype analyses (and breeding analyses of single chromosomal sex chimeras), we concluded that 7 were of single chromosomal sex (4 males, 3 females), 12 were of mixed chromosomal sex (8 males, 4 females), and 11 were of uncertain chromosomal composition (6 males, 5 females). Many in this latter category had a low proportion (or absence) of one genotype in their tissues. Our results support the contention that XX/XY females must be predominantly of the female genotype; otherwise, XX/XYs develop as males. Progeny tests revealed that 22 of 29 chimeras produced germ cells of only one genotype. No apparent germ cell reversals were discovered in the sex ratios of progeny of the XX/XY chimeras. Great fluctuations were observed in the frequencies of genotypes in successive litters of single chromosomal sex chimeras. Analyses of chimerism in white blood cells, coat colors, and the ovaries and testes revealed great variability in the proportions of the two genotypic components among and within animals.