Nucleotide variation at the yellow gene region is not reduced in Drosophila subobscura: a study in relation to chromosomal polymorphism.

In contrast to Drosophila melanogaster and Drosophila simulans, the yellow (y) gene region of Drosophila subobscura is not located in a region with a strong reduction in recombination. In addition, this gene maps very close to the breakpoints of different inversions that segregate as polymorphic in natural populations of D. subobscura. Therefore, levels of variation at the y gene region in this species relative to those found in D. melanogaster and D. simulans may be affected not only by the change in the recombinational environment, but also by the presence of inversion polymorphism. To further investigate these aspects, an approximately 5.4-kb region of the A (=X) chromosome including the y gene was sequenced in 25 lines of D. subobscura and in the closely related species Drosophila madeirensis and Drosophila guanche. The D. subobscura lines studied differed in their A-chromosomal arrangements, A(st), A(2), and A(1). Unlike in D. melanogaster and D. simulans, levels of variation at the y gene region of D. subobscura are not reduced relative to those found at other genomic regions in the same species (rp49, Acp70A, and Acph-1). This result supports the effect of the change in the recombinational environment of a particular gene on the level of neutral variation. In addition, nucleotide variation is affected by chromosomal polymorphism. A strong genetic differentiation is detected between the A(1) arrangement and either A(st) or A(2), but not between A(st) and A(2). This result is consistent with the location of the y gene relative to the breakpoints of inversions A(1) and A(2). In addition, the pattern of nucleotide polymorphism in A(st)+A(2) and A(1) seems to point out that variation at the y gene region within these chromosomal classes is in the phase transient to equilibrium. The estimated ages of these arrangements assuming a star genealogy indicate that their origin cannot predate the D. madeirensis split. Therefore, the present results are consistent with a chromosomal phylogeny where Am(1), which is an arrangement present in D. madeirensis but absent in current populations of D. subobscura, would be the ancestral arrangement.