Experimental population-genetics of meiotic drive systems .2: Accumulation of genetic modifiers of Segregation Distorter (SD) in laboratory populations

The accumulation of modifiers of the meiotic-drive locus Segregation; Distorter (SD) in Drosophila melanogaster was monitored by measuring the; changes in the mean and variance of drive strength (in terms of “make” value); that occur in laboratory populations when SD and SD+ chromosomes are in; direct competition. The particular SD lines used are T(Y;Z),SD translocations; showing pseudo-Y drive. Four sets of population cages were analyzed. Two sets; were monitored for changes in SD fitness and drive strength (presumed to be; positively correlated) and analyzed for the presence of autosomal dominant; or X-linked modifiers after long periods of time. The remaining two sets were; made up of cages either made isogenic or variable for background genetic; material, and these were used to test whether the rate of accumulation of; modifiers was dependent on initial genetic variability.-Contrary to previous; studies in which most suppression of SD action could apparently be attributed; to a few dominantly acting modifiers of large effect, the conclusion here is; that laboratory populations that are initially free of such major dominant loci; evolve to suppress SD action by accumulating polygenic, recessive modifiers,; each of small effect, and that much of the required genetic variability can be; generated a!e novo by mutation. Possible explanations for these seemingly incompatible results and the evolutionary implications for SD are considered.