DNA variability and divergence at the notch locus in Drosophila melanogaster and D. simulans: a case of accelerated synonymous site divergence.

DNA diversity in two segments of the Notch locus was surveyed in four populations of Drosophila melanogaster and two of D. simulans. In both species we observed evidence of non-steady-state evolution. In D. simulans we observed a significant excess of intermediate frequency variants in a non-African population. In D. melanogaster we observed a disparity between levels of sequence polymorphism and divergence between one of the Notch regions sequenced and other neutral X chromosome loci. The striking feature of the data is the high level of synonymous site divergence at Notch, which is the highest reported to date. To more thoroughly investigate the pattern of synonymous site evolution between these species, we developed a method for calibrating preferred, unpreferred, and equal synonymous substitutions by the effective (potential) number of such changes. In D. simulans, we find that preferred changes per "site" are evolving significantly faster than unpreferred changes at Notch. In contrast we observe a significantly faster per site substitution rate of unpreferred changes in D. melanogaster at this locus. These results suggest that positive selection, and not simply relaxation of constraint on codon bias, has contributed to the higher levels of unpreferred divergence along the D. melanogaster lineage at Notch.