The extrachromosomal element denoted by delta was discovered first in bwD/bwD strain owing to its nature of producing a segregation distortion in the mating of females of this strain with certain Cy heterozygous males (Minamori 1969). The distortion was interpreted to be due to differential mortality of zygotes; namely the bwD heterozygotes for a special second chromosome, denoted as sensitive chromosome, may be killed by delta more frequently than Cy/bwD heterozygotes. There were two kinds of sensitive chromosomes. Some sensitive chromosomes (e. g. bwD, S-5, S-7, S-9 chromosome; Minamori 1969) allowed the multiplication of delta more in homozygous condition and less in heterozygous condition, hence, they were assumed to carry a semidominant gene(s) which allows the multiplication. These chromosome lines have stably retained delta since they were discovered in 1965. The segregation distortion was always observed in the reciprocal matings between any two of these sensitive lines. On the other hand, in other sensitive lines (e. g. Pm chromosome, Minamori 1969; S-Cy chromosome; un