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

To study how gamma-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 47cM surrounding the b locus. Next, losses of paternal alleles of these DNA markers were examined in each of the 51 gamma-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 20-35cM were the most frequently observed in 18 of the 51 mutants examined. The largest one extended more than 40cM. These results suggest that most of the gamma-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.