Abstract
1. Segregation distorter genes (SD) were found in Kofu and Katsunuma natural populations of Drosophila melanogaster.
2. The nature of SD gene extracted from Kofu and Katsunuma populations was almost the same as in Madison; segregation distortion occurred only in SD heterozygous males and it did not occur either in SD homozygous males or in females.
3. SD gene was mostly found in association with a right arm inversion, In(2R)C, and some SD chromosomes simultaneously carried the same persistent lethal gene which was frequently found and maintained in Kofu and Katsunuma natural populations.
4. The distance between the persistent lethal gene and SD gene is so short that its retention in the population may be effectively due to its association with SD. Heterotic gene complexes including the lethal and SD genes were discussed as a persistent mechanism.
5. The sensitivity of the second chromosomes to SD action varied from complete sensitiveness (k=1.0) to resistance (k=0.5). The mean k value of many chromosomes isolated from Kofu and Katsunuma populations was 0.77.
