Concerted evolution within a trypsin gene cluster in Drosophila

Abstract

A cluster of four trypsin genes has previously been localized to cytological position 47D-F of the Drosophila melanogaster genome. One of these genes had been sequenced, and the presence of the other three genes was identified by cross-hybridization. Here, we present the DNA sequence of the entire genomic region encoding these four trypsin genes. In addition to the four previously inferred genes, we have identified a fifth trypsin-coding sequence located within this gene cluster. This new gene shows a high degree of sequence divergence (more than 30%) from the other four genes, although it retains all of the functional motifs that are characteristic of trypsin- coding sequences. In order to trace the molecular evolution of this gene cluster, we isolated and sequenced the homologous 7-kb region from the closely related species Drosophila erecta. A comparison of the DNA sequences between the two species provides strong evidence for the concerted evolution of some members of this gene family. Two genes within the cluster are evolving in concert, while a third gene appears to be evolving independently. The remaining two genes show an intermediate pattern of evolution. We propose a simple model, involving chromosome looping and gene conversion, to explain the relatively complex patterns of molecular evolution within this gene cluster.