LENGTH VARIATION OF CAG CAA TRINUCLEOTIDE REPEATS IN NATURAL-POPULATIONS OF DROSOPHILA-MELANOGASTER AND ITS RELATION TO THE RECOMBINATION RATE/

Eleven genes distributed along the Drosophila melanogaster chromosome 2 and showing exonic tandem repeats of glutamine codons (GAG or CAA) w ere surveyed for length variation in a sample of four European and Afr ican populations. Only one gene was monomorphic. Eight genes were poly morphic in all populations, with a total number of alleles varying bet ween five and 12 for 120 chromosomes. The average heterozygozity per l ocus and population was 0.41. Selective neutrality in length variation could not be rejected under the assumptions of the infinite allele mo del. Significant population subdivision was found though no geographic al pattern emerged, all populations being equally different. Significa nt linkage disequilibrium was found in four out of seven cases where t he genetic distance between loci was <1 cM and was negligible when the distance was larger. There is evidence that these associations were e stablished after the populations separated. An unexpected result was t hat variation at each locus was independent of the coefficient of exch ange, although the latter ranged from zero to the relatively high valu e of 6.7%. This would indicate that background selection and selective hitchhiking, which are thought to affect levels of nucleotide substit ution polymorphism, have no effect on trinucleotide repeat variation.