Measuring selective effects of modifier gene polymorphisms on the Bare locus of Drosophila subobscura

An attempt to quantify the effects of modifier gene polymorphisms on the operation of natural selection on a major locus has been carried out. The modifier system we have investigated is constituted by a set of polygenic modifier loci affecting the morphological expression of the Bare (Ba) bristle mutant of Drosophila subobscura. Ba is a dominant mutant that is lethal in homozygous condition and both the polygenic modifiers and Ba are located on the O chromosome of this species. Experimental populations were found with Ba/+ individuals and two different types of populations were started according to their modifier genetic background: populations with wild O chromosomes of either high or low modifier effect (cages H and L, respectively). Fitness estimates (total fitness, viability and fertility) for genotypes of the Ba locus were obtained under the two different modifier backgrounds. In the populations with high modifier background the total fitness of the Ba/+ heterozygote was very similar to that of the +/+ homozygote (fitness equal to 1). However, in cages with low modifier background a strong selection against the Ba/+ heterozygote was detected (average of total fitnesses over generations was 0.66 +/- 0.10), and fertility appears to be the fitness component responsible for this effect (mean fertility was 0.55 +/- 0.08). These findings demonstrate that modifier gene polymorphisms affecting the expression of the Ba mutant may be associated with large selective effects on the major locus.