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Parasitisms or infections with microorganisms are not rare phenomena in Drosophila. Genetical analyses of a virus responsible for COz sensitivity of flies (L'Heritier 1958) and a spirochete for "sex ratio" agent (Poulson and Sakaguchi 1961) have been performed. The present author found some killing agent for its carrier of Drosophila melanogaster, in the course of isolation of second chromosomes from natural populations in Hiroshima City in 1964, when the carriers were crossed with Cy/Pm flies, the number of F1 offspring decreased appreciably and the segregation of Curly and Plum zygotes was distorted significantly.The cause of the distortion was ascribed to a higher mortality of the Cy-zygotes. Hence, the killing agent was tentatively called Cy-killer (Minamori 1965a, b).Data presented in this paper show that the killing agent may be infective microorganism in the flies, and flies having different genotypes might have different survival rates as a consequence of disturbance by the agent. MATERIALS AND METHODSWhen Cy/Pm females were crossed with several wild males collected from a pickle factory on Tera Street, Hiroshima City in 1961, a segregation ratio between Cy-and Pin-zygotes in the offspring was distorted.Eighteen males (7.11 %) among 253 males collected in summer and five males (1.94 %) among 258 males collected in autumn showed the segregation distortion in their offspring. These males were assumed to carry a killing agent.Second chromosomes were isolated by a mating scheme represented in Figure 1, from these carriers or other non-carriers.On the other hand, these chromosomes were classified into three groups ; lethal (0 %), semilethal (less than 16.7 %) and quasinormal (more than 16.7 %) groups according to the viability of homozygotes in the F4 generation. In the present study, a symbol K is given for the carrier ahead the line number, e. g. K-89, and a symbol F is given for non-carrier, e. g. F-413.The nature manifested by the killing agent was tested clearly by mating the carrier with Cy/Pna or other strains having marker genes.Eight strains, which carry quasinormal second chromosomes homozygously and the killing agent, were used in this experiment.The nature could be measured by the grade of segregation distortion and its magnitude is represented by the frequency of non-Curly flies in the offspring. The number of progeny is also used as a measure for the magnitude of nature.

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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

Males and females heterozygous for the Cy chromosome and certain other non-Cy second chromosomes produced only Cy-bearing progeny when the heterozygotes carried an appreciable amount of the extrachromosomal element delta.