DIVERGENCE OF THE YELLOW GENE BETWEEN DROSOPHILA-MELANOGASTER AND D-SUBOBSCURA - RECOMBINATION RATE, CODON BIAS AND SYNONYMOUS SUBSTITUTIONS

The yellow (y) gene maps near the telomere of the X chromosome in Dros ophila melanogasta but not in D. subobscura. Thus the strong reduction in the recombination rate associated with telomeric regions is not ex pected in D. subobscura. To study the divergence of a gene whose recom bination rate differs between two species, the y gene of D. subobscura was sequenced. Sequence comparison between D. melanogaster and D. sub obscura revealed several elements conserved in noncoding regions that may correspond spend to putative cis-acting regulatory sequences. Dive rgence in the y gene coding region between D. subobscura and D. melano gaster was compared with that found in other genes sequenced in both s pecies. Both, yellow and scute exhibit an unusually high number of syn onymous substitutions per site (p(s)). Also for these genes, the exten t of codon bias differs between both species, being much higher in D. subobscura than in D. melanogaster. This pattern of divergence is cons istent with the hitchhiking and background selection models that predi ct an increase in the fixation rate of slightly deleterious mutations and a decrease in the rate of fixation of slightly advantageous mutati ons in regions with low recombination rates such as in the y-sc gene r egion of D. melanogaster.