Additive Effects of Multiple SEGREGATION DISTORTER ( SD) Chromosomes on Sperm Dysfunction in DROSOPHILA MELANOGASTER.

A portion of the Segregation distorter (SD) chromosome, including both the Sd and E(SD) loci, has been moved by insertional translocation from SD Roma into Y(L ). This Dp(2;Y)SD chromosome shows a negligible reduction in its ability to cause dysfunction of Rsp( s)-bearing sperm when compared to the parent SD chromosome, suggesting that SD can still act effectively, even when removed from its normal second chromosome milieu, and that its activity level does not depend on pairing with a normal autosomal homologue. Male genotypes have been constructed using this Dp(2;Y)SD along with a standard SD chromosome (either SD Roma or R( SD-36)-1(bw)) and a third chromosome suppressor of SD (TM6) in all possible three-way combinations. The observed level of SD-mediated dysfunction in each case is most compatible with a model that assumes that all SD elements act additively (in terms of M, the probit transformation of the probability of sperm dysfunction), rather than multiplicatively. The additive action of SD elements contrasts with the independent response to SD activity exhibited by multiple Rsp(s) copies.