Hsp70 duplication in the Drosophila melanogaster species group: How and when did two become five?

To determine how the modem copy number (5) of hsp70 genes in Drosophila mel anogaster evolved, we localized the duplication events that created the gen es in the phylogeny of the melanogaster group, examined D. melanogaster gen omic sequence to investigate the mechanisms of duplication, and analyzed th e hsp70 gene sequences of Drosophila orena and Drosophila mauritiana. The i nitial two-to-four hsp70 duplication occurred 10-15 MYA, according to fixed in situ hybridization to polytene chromosomes, before the origin and diver gence of the melanogaster and five: other species subgroups of the, melanog aster group. Analysis of more than 30 kb of flanking sequence surrounding t he hsp70 gene clusters suggested that this duplication was Likely a retrotr ansposition. For the melanogaster subgroup, Southern hybridization and an h sp70 restriction map confirmed the conserved number (4) and arrangement of hsp70 genes in the seven species other than D. melanogaster. Drosophila mel anogaster is unique; tandem duplication and gene conversion at the derived cluster yielded a fifth hsp70 gene. The four D. orena hsp70 genes are highl y similar and concertedly evolving. In contrast, the D. mauritiana hsp70 ge nes are divergent, and many alleles. are nonfunctional. The proliferation, concerted evolution, and maintenance of functionality in the D. melanogaste r hsp70 genes is consistent with the, action of natural selection in this s pecies.