LINKAGE DISEQUILIBRIUM IN THE WHITE LOCUS REGION OF DROSOPHILA-MELANOGASTER

Linkage disequilibrium between molecular polymorphisms in a 10 kb regi on in the white locus of Drosophila melanogaster, revealed with a batt ery of four-cutter restriction enzymes, was investigated in 266 lines sampled from seven natural populations around the world. A total of 73 (35 restriction site, 37 insertion/deletion and 1 inversion) polymorp hisms were detected, of which 55 non-unique polymorphisms were analyse d for linkage disequilibrium. Clustering of significant linkage disequ ilibrium was observed in the transcriptional unit of the white locus a s in Miyashita & Langley (1988). It was shown that about two thirds of the 2-locus combinations showing significant linkage disequilibrium h ave similar degree and direction of association over different populat ions. Despite lower divergence in allelic frequencies of molecular pol ymorphisms among populations, an increase in the proportion of 2-locus pairs showing significant linkage disequilibrium is observed in the t ranscriptional unit. Large values of Ohta's D measure ratio (1982a, b) cluster in the transcriptional unit, and correspond to significant li nkage disequilibria. Although the exact molecular mechanism is not cle ar, these results suggest that epistatic selection is responsible for significant linkage disequilibrium in the transcriptional unit of this locus.