REDUCED VARIATION AT CONCERTINA, A HETEROCHROMATIC LOCUS IN DROSOPHILA

In Drosophila melanogaster and closely related species, polymorphism h as been shown to be reduced at loci located in regions of low recombin ation on the X chromosome and on the fourth chromosome, which does not normally recombine. This positive correlation between nucleotide poly morphism level and recombination rate is not predicted by standard neu tral theory and therefore must result from natural selection and genet ic hitchhiking along the chromosomes. We report here the near-complete absence of variation at concertina (cta), a locus located in the beta -heterochromatic base of chromosome 2L, a region of strongly reduced r ecombination. A 1.2 kilobase region containing coding regions and intr ons was sequenced from each of nine lines of D. melanogaster and nine lines of D. simulans representing worldwide collections. Variation is significantly reduced in era in both species compared with other avail able loci on the same chromosome. Two analyses of background selection demonstrate that the reduction in variation at era, considered in com bination with other loci on chromosome 2L or alone, is consistent with the background selection model.