Genomic analysis of γ-ray-induced germ-cell mutations at the b locus recovered from the medaka specific-locus test

To study how γ-ray-induced germ-cell mutations are fixed at the early embryonic stage of the next generation, genomic alterations in the b locus mutants (colorless melanophores) detected during development in the medaka specific-locus test (SLT) were analyzed. First, nine anonymous DNA markers linked to the b locus were cloned and mapped into the region extending about 47 cM surrounding the b locus. Next, losses of paternal alleles of these DNA markers were examined in each of the 51 γ-ray-induced b locus mutants obtained after irradiation of sperm or spermatids. In these mutants, 47 were dominant lethals, three were semi-viable and one was viable. All the mutants examined had large deletions surrounding the b locus. One viable mutant had an interstitial deletion, while all the semi-viable and dominant lethal ones appeared to have terminal deletions. Deletions extending about 20–35 cM were the most frequently observed in 18 of the 51 mutants examined. The largest one extended more than 40 cM. These results suggest that most of the γ-ray induced germ cell mutations recovered as total specific-locus mutants were accompanied by large genomic deletions, which eventually led the mutant embryos to dominant lethality.