Molecular evolution of the metallothionein gene Mtn in the melanogaster species group: results from Drosophila ananassae

Three distinctly different alleles of the metallothionein gene Mtn have been identified in natural Drosophila melanogaster populations: Mtn3, Mtn1, and Dp(Mtn1), where the latter designates a tandem duplication of Mtn1. In Drosophila simulans, only Mtn3-type alleles have been found. It has been suggested that Mtn3 is the ancestral allele and demonstrated that a presumed two-step transition from Mtn3 to Mtn1 to Dp(Mtn1) is accompanied by an approximate 5-fold increase in RNA levels. We analyzed the evolutionary genetics of the Mtn locus of Drosophila ananassae, a distant relative of D. melanogaster and D. simulans within the melanogaster species group. The Mtn gene of D. ananassae is most similar to Mtn (i) it is identical with Mtn at the amino acid level, but differs from Mtn1 in its terminal codon; (ii) its 3' UTR contains a characteristic extra DNA segment of about 50 bp which is present in Mtn, but lacking in Mtn1: (iii) duplications of Mtn were not found in a worldwide sample of 110 wild D. ananassae chromosomes. However, the intron of the Mtn gene in D. ananassae is only 69 bp long, whereas the length of the Mtn and Mtn1 introns is 265 bp; and it lacks a polyprimidine stretch upstream of the 3' splice site in contrast to the much greater pyrimidine-richness found in the Mtn3 and Mtn1 introns. A short intron (67 bp) was also identified in a D. pseudoobscura Mtn allele, suggesting that the short intron is the ancestral form and that the transition from the short to the long intron occurred within the melanogaster species group. We discuss the significance of this observation with regard to the recently proposed classification of D. melanogaster introns into two groups: short introns (