Restriction-map variation in the Notch region of Drosophila melanogaster.

A worldwide sample of 37 X chromosomes of Drosophila melanogaster was analyzed with four restriction endonucleases for a 60-kb region of the Notch locus. Any two randomly chosen homologous chromosomes were heterozygous at one in 143 nucleotides (theta = 0.007). The chromosomes that were sampled contained no more than one insertion/deletion. The four insertions and one deletion observed in the 37 chromosomes sampled were located 3' to the Notch transcript; one insertion was represented twice in the sample. The amount of linkage disequilibrium in the Notch region appears to be lower than that of the alcohol dehydrogenase locus in D. melanogaster. The few instances of linkage disequilibrium observed could be due to geographic differentiation of African populations. The genetic variation estimates in the Notch region were comparable with those of the alcohol dehydrogenase region in D. melanogaster, suggesting that molecular genetic variation on the X chromosome is not dramatically reduced by selection against slightly deleterious alleles.