Elevated polymorphism and divergence in the class C scavenger receptors of Drosophila melanogaster and D. simulans

Scavenger receptor proteins are involved in the cellular internalization of a broad variety of foreign material, including pathogenic bacteria during phagocytosis. I find here that nonsynonymous divergence in three class C scavenger receptors (Sr-C's) between Drosophila melanogaster and D. simulans and between each of these species and D. yakuba is approximately four times the typical genome average. These genes also exhibit unusually high levels of segregating nonsynonymous polymorphism in D. melanogaster and D. simulans populations. A fourth Sr-C is comparatively conserved. McDonald-Kreitman tests reveal a significant excess of replacement fixations between D. melanogaster and D. simulans in the Sr-C's, but tests of polymorphic site frequency spectra do not support models of directional selection. It is possible that the molecular functions of SR-C proteins are sufficiently robust to allow exceptionally high amino acid substitution rates without compromising organismal fitness. Alternatively, SR-Cs may evolve under diversifying selection, perhaps as a result of pressure from pathogens. Interestingly, Sr-CIII and Sr-CIV are polymorphic for premature stop codons. Sr-CIV is also polymorphic for an in-frame 101-codon deletion and for the absence of one intron.