Adjacent chromosomal regions can evolve at very different rates: evolution of the Drosophila 68C glue gene cluster

The 68C puff is a highly transcribed region of the Drosophila melanogaster salivary gland polytene chromosomes. Three different classes of messenger RNA originate in a 5000-bp region in the puff; each class is translated to one of the salivary gland glue proteins sgs-3, sgs-7, or sgs-8. These messenger RNA classes are coordinately controlled, with each RNA appearing in the third larval instar and disappearing at the time of puparium formation. Their disappearance is initiated by the action of the steroid hormone ecdysterone. In the work reported here, we studied evolution of this hormone-regulated gene cluster in the melanogaster species subgroup of Drosophila. Genome blot hybridization experiments showed that five other species of this subgroup have DNA sequences that hybridize to D. melanogaster 68C sequences, and that these sequences are divided into a highly conserved region, which does not contain the glue genes, and an extraordinarily diverged region, which does. Molecular cloning of this DNA from D. simulans, D. erecta, D. yakuba, and D. teissieri confirmed the division of the region into a slowly and a rapidly evolving portion, and also showed that the rapidly evolving region of each species codes for third instar larval salivary gland RNAs homologous to the D. melanogaster glue mRNAs. The highly conserved region is at least 13,000 bp long, and is not known to code for any RNAs.