Studies of esterase 6 inDrosophila melanogaster: XI. Modification of esterase 6 activity by unlinked genes

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Abstract
SUMMARY: The often remarkable similarity in structural gene products among related species has led to the hypothesis that species differences may reside largely in changes at regulatory gene loci. This hypothesis assumes that groups capable of speciating have allelic variation at regulatory loci in their natural populations. We have undertaken an analysis of the mode of regulation of theesterase 6(Est 6) locus inDrosophila melanogasterto determine the nature and extent of regulatory gene variation in natural populations. Analyses of esterase 6 (EST 6) activity among strains carrying the same thermostability variants reveal that significant, specific-activity differences are present. Reciprocal crosses between lines having high and low EST 6 activity show that loci other than theEst 6structural gene influence EST 6 activity. Analyses of male hybrids from crosses betweenD. melanogasterandsimulansindicate that theXchromosome of these flies affects the expression of theEst 6locus, resulting in unequal levels of enzyme activity from the two alleles. The effect is sex and tissue specific. Female hybrids carrying theXchromosomes of both species exhibit equal expression of the twoEst 6alleles. We have determined whether natural populations are polymorphic forXchromosomes which affect EST 6 activity by extracting singleXchromosomes from wild-collected males and placing these chromosomes in identical genetic backgrounds. Stocks which are otherwise genetically identical but carry independently derivedXchromosomes show significant differences in the activity of EST 6. These data suggest that regulatory loci may be commonly polymorphic in natural populations.