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Our understanding of the genetic basis of host specialization in insects is limited to basic information on the number and location of genetic factors underlying changes in conspicuous phenotypes. We know nothing about general patterns of molecular evolution that may accompany host specialization but are not traceable to a single prominent phenotypic change. Here, I describe changes in the entire repertoire of 136 olfactory receptor (Or) and gustatory receptor (Gr) genes of the recently specialized vinegar fly Drosophila sechellia. I find that D. sechellia is losing Or and Gr genes nearly 10 times faster than its generalist sibling Drosophila simulans. Moreover, those D. sechellia receptors that remain intact have fixed amino acid replacement mutations at a higher rate relative to silent mutations than have their D. simulans orthologs. Comparison of these patterns with those observed in a random sample of genes indicates that the changes at Or and Gr loci are likely to reflect positive selection and/or relaxed constraint associated with the altered ecological niche of this fly.
Carolyn S. McBride (Fri,) studied this question.