Abstract
An extremely large amount of detrimental load was found in the Osaka population of D. melanogaster, although the frequency of lethal-carrying second chromosomes (Q) was not different from the average of Japanese populations (Q=0.199 and D:L=2.04). Tests for male recombination and sterility tests for the P-M dysgenic system indicated that there is a putative transposon (tentatively named MY factor) which induces non-lethal deleterious mutations at a rate about 140 times higher than that of recessive lethal mutations per second chromosome in the M cytoplasm, and the frequency of individuals carrying the M cytoplasm was estimated to be ca. 20%. Thus, it was concluded that the extremely large detrimental load was induced by the MY factors.
