Evolution and population biology of theperiodgene

The clock gene period (per), originally identified inDrosophila, evolves relatively quickly within the insects; probably for this reason, no convincingperhomologues have been identified outside this class. However antibodies toDrosophilaPER have labelled neural pacemakers in other organisms, including mammals. Conserved regions, such as the PAS dimerization domain, reflect its functional importance, but the long Thr–Gly repeat encoded withinperis not conserved outside theDrosophila. The repeat appears to be a component of the temperature compensation system in the fly. This is reflected in the population structure of natural Thr–Gly length variants ofD. melanogaster. Patterns of nucleotide variation withinDrosophila perhave been used to examine the selective forces that have shaped the evolution of this gene.