BETTENCOURT, B.R.*; FEDER, M.E.: Concerted evolution and concerted degeneration at the hsp70 genes and pseudogenes of the Drosophila melanogaster species subgroup.

To determine how the hsp70 genes (4-5 copies total, at two chromosomal loci) coevolved with the diversification of the D. melanogaster species subgroup, we sequenced multiple alleles of each gene in representative species. The hsp70 genes display strikingly different patterns of molecular evolution among the eight species of the subgroup. In D. simulans and melanogaster, frequent gene conversion homogenizes all the hsp70 coding sequences, diversifies flanking sequences, and interacts with both purifying and balancing selection to drive rapid concerted evolution. By contrast, hsp70 pseudogenes are common in D. mauritiana: null alleles of three of the four hsp70 genes are at high frequency. These pseudogenes also participate in gene conversion; copies at different loci share the same degenerative mutations. The hsp70 genes of D. orena, basal member of the subgroup, evolve in a fashion similar to those of D. simulans. While Hsp70 expression level, thermotolerance, and thermal niche breadth are strongly coupled to the hsp70 genes in D. melanogaster, the functional copy number and relative “health” of the hsp70 genome is only a fair predictor of these traits in other species. If, when, and how natural selection acts on the hsp70 genes varies in a lineage- and locus-specific fashion.