Modification of survival rate of mouse embryos developing in heterozygous females for ovum mutant gene

The DDK syndrome (polar infertility) is caused by an incompatibility system due to the ovum mutant (Om) locus. For brevity, the following gene symbols are used in the present report: DDK allele, Om; C57BL/6Cr allele, +. In th is investigation, we first attempted to introduce the Om allele of DDK stra in into the genetic background of C57BL/6Cr strain. The attempt resulted in the production of no young at the third generation of successive backcross es, Secondly, mating experiments were performed with heterozygous (Om/+) fe males having background genes of C57BL/6Cr and DDK strains in the ratios 1: 1(B1D), 3: 1(B3D), 7:1(B7D), and 15:1(B15D). The survival rate of the embry os as judged by the percentage number of live fetuses/number of corpora lut ea at Day 12 of pregnancy was 41.3 +/- 3.2%, 27.3 +/- 3.2%, 16.4 +/- 3.3%, and 11.3 +/- 3.2% (mean +/- SEM) in the B1D, B3D, B7D, and B15D females, re spectively, when they were mated with C57BL/6Cr males. Furthermore, the inc reased embryonic mortality in the heterozygous (Om/+) females with more bac kground genes of C57BL/6Cr strain was found to be due to a failure in blast ocyst formation, as in the DDK syndrome. The parallelism between the propor tion of C57BL/6Cr background genes and embryonic mortality has led to a hyp othesis proposing the participation of a modifier gene, namely that a mecha nism similar to allelic exclusion may be working in the synthesis of cytopl asmic factor of eggs and that only the Om allele is activated during oogene sis to produce DDK-type cytoplasmic factor in heterozygous (Om/+) females h aving a modifier gene in the homozygous state.