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 tractof threonine-glycine (Thr-Gly) pairs has been compared between Dipteranspecies both within and outside the Drosophilidae. All the non-Drosophilidae sequences in this region are short and present a remarkablystable picture compared to the Drosophilidae, in which the region is muchlarger and extremely variable, both in size and composition. Theaccelerated evolution in the repetitive region of the Drosophilidae appearsto be mainly due to an expansion of two ancestral repeats, one encoding aThr-Gly dipeptide and the other a pentapeptide rich in serine, glycine, andasparagine or threonine. In some drosophilids the expansion involves aduplication of the pentapeptide sequence, but in Drosophila pseudoobscuraboth the dipeptide and the pentapeptide repeats are present in largernumbers. In the nondrosophilids, however, the pentapeptide sequence isrepresented by one copy and the dipeptide by two copies. These observationsfulfill some of the predictions of recent theoretical models that havesimulated the evolution of repetitive sequences.