Segregation-distorter (symbol SD) was first found in a natural population of D. melanogaster in Madison, Wisconsin and a number of studies on this element have been carried out (Sandier, Hiraizumi and Sandier 1959; Sandier and Hiraizumi 1959, 1960a, b ; Hiraizumi, Sandier and Crow 1960; Sandier and Hiraizumi 1961a, b ; Hiraizumi and Nakazima 1967). Since the SD system is unique and rather complicated, the several basic conclusions relevant to the present study will be briefly given below. 1. SD

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... is a locus on the second chromosome close to the centromere. When SD/ SD+ heterozygous males are mated to the standard females, SD is recovered in the next generation in great excess over its theoretical frequency of 50 %, often 90 % or more. 2. Segregation-distortion occurs only in SD heterozygous males, but neither in SD homozygous males nor in females. 3. Segregation-distortion is probably due to the preferential movement of the SD chromosome to the functional pole in the first anaphase of male meiosis (Peacock and Erickson 1965). 4. At, or near, the tip of the right arm of chromosome 2, there is a stabilizer of SD (=St(SD)J. In the presence of St(SD), the SD action is stable and the segregation ratio is consistently high from male to male; in the absence of St(SD), distorting effect of SD becomes somewhat variable resulting in a relatively weak distortion. 5. The SD region consists of two elements. One of these is the SD locus itself. Closely linked to, and to the right of SD, there is an activator of SD =AC(SD) i, which, in coupling with SD, is necessary for SD to operate. 6. When a male carries Segregation-distorter, the second and the sex chromosome do not assort randomly. SD tends to go to the functional pole together with the Y chromosome. Ac(SD) and St(SD) tend to reduce the overall male proportion in the Fl 1) 2) Genetics paper No.