HAPLOTYPE TEST REVEALS DEPARTURE FROM NEUTRALITY IN A SEGMENT OF THE WHITE GENE OF DROSOPHILA-MELANOGASTER

Restriction map studies previously revealed extensive linkage disequil ibria in the transcriptional unit of the white locus in natural Drosop hila melanogaster populations. To understand the causes of these diseq uilibria, we sequenced a 4722-bp region of the white gene from 15 line s of D. melanogaster and 1 line of Drosophila simulans. Statistical te sts applied to the entire 4722-bp region do not reject neutrality. In contrast, a test for high-frequency haplotypes (''Haplotype test'') re vealed an 834-bp segment, encompassing the 3' end of intron 1 to the 3 ' end of intron 2, in which the structure of variation deviates signif icantly from the predictions of a neutral equilibrium model. The varia nts in this 834-bp segment segregate as single haplotype blocks. We pr opose that these unusually large haplotype blocks are due to positive selection on polymorphisms within the white gene, including a replacem ent polymorphism, Arg --> Leu, within this segment.