Functional and evolutionary implications of natural variation in clock genes.

Nearly all studies of natural variation within clock genes involve the period (per) locus, which was originally isolated in the fruit-fly. Intra- and interspecific work on per has focused mostly on a region of Thr-Gly or Ser-Gly repeats, which show rapid length and sequence evolution. The functional implications of nucleotide variation in this repetitive array have been characterised using behavioural, molecular, ecological, structural and statistical analyses. A population genetics approach to variation in per has also been useful in defining species histories within Drosophilids and, in some cases, in implicating selective processes in the evolution of the per gene. Interspecific analysis of per expression patterns reveals evolutionary alterations in this clock gene's regulation.