BIG FLIES, SMALL REPEATS - THE THR-GLY REGION OF THE PERIOD GENE IN DIPTERA

The region of the clock gene period (per) that encodes a repetitive tr act of threonine-glycine (Thr-Gly) pairs has been compared between Dip teran species both within and outside the Drosophilidae. All the non-D rosophilidae sequences in this region are short and present a remarkab ly stable picture compared to the Drosophilidae, in which the region i s much larger and extremely variable, both in size and composition. Th e accelerated evolution in the repetitive region of the Drosophilidae appears to be mainly due to an expansion of two ancestral repeats, one encoding a Thr-Gly dipeptide and the other a pentapeptide rich in ser ine, glycine, and asparagine or threonine. In some drosophilids the ex pansion involves a duplication of the pentapeptide sequence, but in Dr osophila pseudoobscura both the dipeptide and the pentapeptide repeats are present in larger numbers. In the nondrosophilids, however, the p entapeptide sequence is represented by one copy and the dipeptide by t wo copies. These observations fulfill some of the predictions of recen t theoretical models that have simulated the evolution of repetitive s equences.