DISTRIBUTION OF NATURAL LETHAL GENES ON THE SECOND CHROMOSOME OF DROSOPHILA MELANOGASTER
The Japanese Journal of Genetics
The distribution of lethal genes on the second chromosome in natural populations of D, melanogaster has been analyzed by Paik (1960) and Seto (1963) . These authors concluded that lethal genes were distributed randomly on the chromosome. However, the portion of the chromosome they examined were too short for asserting the distribution to be random for the entire length. On the other hand, the distribution of many chemically induced lethal genes on the second chromosome has been considered by
... en considered by Nafei and Auerbach (1964) to be not random. Oshima, Watanabe and Watanabe (1964) and Oshima (1965) observed localized distribution of lethal genes combined with a paracentric inversion in the middle region of the chromosome. In the present study, individual loci of seventy-six lethal genes, including persistent ones, were determined by crossing-over values from three dominant marker genes. Their loci on the genetic map were found to be distributed non-randomly, much similar to the cases of recessive visible mutants, described in Bridges and Brehme's book (1944). From these results, it seems safe to conclude that all the single lethal genes analyzed in this experiment, had appeared by mutation of different major genes. However, some clustering tendency was also to be recognized at the proximal part (55.0-57.0) of the right arm, where most lethal genes were associated with In(2L)B. This paper deals with the mechanisms of persistence of lethal genes in natural populations. Besides, the part played by heterotic inversions on the persistence of lethal genes will be considered. MATERIALS AND METHODS A total of 120 second chromosomes carrying recessive lethal genes (94: natural, 26: spontaneous) were used in this experiment. These lethal chromosomes have been maintained individually by Cy balanced system, and most of the lethal genes located on them were ascertained to be non-allelic with each other by allelism test. Natural lethals : Among ninety-four natural lethal chromosomes, eighteen were isolated from the Suyama-Juriki natural populations in 1962, and the remaining 76 were isolated from the Kofu-Katsunuma natural populations in 1963 and 1964.