The effects of lethal selection on the EST-6 to PGM region of chromosome 3 in Drosophila melanogaster.

A powerful means of studying the effects of selection on chromosome segments in Drosophila melanogaster has been described by Clegg et. al. (1976, 1978). This method utilizes a recessive lethal, dominant visible allele whose selection dynamics can be accurately modelled to predict the fates of nonlethal alleles at linked loci. Results of these experiments indicate that strong epistatic interactions among loci occur that effect fitnesses associated with gametic types in the basal region of chromosome 3. We have used similar methods in studying a different segment of chromosome 3, that spanned by Est-6 (3-36.8) and Pgm (3-43.4), with the aim of determining whether the results of Clegg and his colleagues could be reproduced when a different region was studied. Our experiment showed that selection did operate on a region of the chromosome marked by Pgm, but that no evidence of selection at loci marked by Est-6 was apparent. Weak evidence for epistatic interactions among loci within the marked region was also found. Three possible explanations for the discrepancies between our experiments and those of Clegg et al. are suggested. First, the geographically homogeneous origin of our populations may preclude selectively significant changes as a result of recombination. Second, the results seen by Clegg et al. may have been unique to the regions they studied, which included the basal heterochromatin of the chromosomes. Finally, the three loci employed may not adequately mark the unit of selection, so that actual departures from predictions of selective neutrality may not have been apparent.