Extrachromosomal element delta in Drosophila melanogaster. 8. Inseparable association with sensitive second chromosome.

The presence of delta is always accompanied by a sensitive second chromosome in Drosophila melanogaster or vice versa. The separability of delta and the chromosome line was investigated in experiments designed to eliminate delta or to suppress its multiplication for a number of generations. Cy/S(b)-5 males which transmit a minute amount of delta b (retained by chromosome S(b), kills S(b)/S(b) and S(b)/S(r) zygotes) to their progeny were backcrossed for 61 generations to Cy/Pm females which carried no delta. After backcrossing 465 Cy/S(b)-5 males were individually examined for retention of delta, and all were found to retain delta b in its latent state. A homozygous strain for the S(r)-20B chromosome which retained a minute amount of delta r (retained by chromosome S(r), kills S(r)/S(r) zygotes) was obtained. 1,252 Cy/S(r)-20B females derived from the homozygous strains at the 21st generation were tested individually for delta retention. All females tested were found to retain delta r in its latent state. The delta retention in S(b)-5 chromosome lines isolated from S(r)-Cy/S(b)-5 heterozygous strains which carried delta r but not delta b was examined. The descendant Cy/S(b)-5 lines from the heterozygous strains had accumulated their specific delta, delta b. These findings are consistent with those obtained in an earlier study, and lead to the conclusion that delta is associated inseparably with each specific sensitive chromosome.