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 hav e been identified in natural Drosophila melanogaster populations: Mtn( .3), Mtn(1), and Dp(Mtn(1)), where the latter designates a tandem dupl ication of Mtn(1). In Drosophila simulans, only Mtn(.3)-type alleles h ave been found. It has been suggested that Mtn(.3) is the ancestral al lele and demonstrated that a presumed two-step transition from Mtn(.3) to Mtn(1) to Dp(Mtn(1)) is accompanied by an approximate 5-fold incre ase in RNA levels. We analyzed the evolutionary genetics of the Mtn lo cus 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(.3): (i) it is identical with Mtn(.3 ) at the amino acid level, but differs from Mtn(1) in its terminal cod on; (ii) its 3' UTR contains a characteristic extra DNA segment of abo ut 50 bp which is present in Mtn(.3), but lacking in Mtn(1); (iii) dup lications of Mtn were not found in a worldwide sample of 110 wild D. a nanassae chromosomes. However, the intron of the Mtn gene in D. ananas sae is only 69 bp long, whereas the length of the Mtn(.3) and Mtn(1) i ntrons is 265 bp; and it lacks a polypyrimidine stretch upstream of th e 3' splice site in contrast to the much greater pyrimidine-richness f ound in the Mtn(.3) and Mtn(1) introns. A short intron (67 bp) was als o identified in a D. pseudoobscura Mtn allele, suggesting that the sho rt 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 recen tly proposed classification of D. melanogaster introns into two groups : short introns (<90 bp) which tend to lack polypyrimidine stretches, and longer ones which have strong 3' splice signals similar to mammali an introns. A database search revealed that this length dimorphism is an evolutionarily conserved feature of Drosophila introns; transitions from one size class to the other appear to be rare between closely re lated species (e.g., within the melanogaster subgroup).