INTRASPECIFIC AND INTERSPECIFIC VARIATION AT THE Y-AC-SC REGION OF DROSOPHILA-SIMULANS AND DROSOPHILA-MELANOGASTER

A 2.2-kb region including the ac gene of Drosophila simulans has been sequenced. Interspecific divergence between Drosphila melanogaster and D. simulans was estimated as 0.0695 and 0.0558 for silent and for all sites, respectively. Estimated silent site divergence for the ac regi on is comparable to that estimated for other regions of the genome bet ween these species, indicating that silent sites of the ac region are not under significantly stronger functional constraint. Intraspecific variation in both species was also investigated. Restriction-site and length polymorphism in the ac region of D. simulans has been investiga ted for 103 X chromosome lines sampled from three natural populations in Spain using eight four-cutter restriction enzymes. Neither restrict ion-site nor length variation was detected in the three populations su rveyed. In D. melanogaster restriction-site and length polymorphism in all major transcription units of the y-ac-sc region (23.1-kb region) has been studied using four four-cutter restriction enzymes for 245 X chromosome lines sampled from 10 natural populations (seven from Europ e, two from North America and one from Japan). Fourteen restriction-si te and 28 length polymorphisms were detected. There was some indicatio n of population subdivision for North American vs. European samples of D. melanogaster. The frequency spectrum of restriction-site polymorph isms in European populations was skewed toward rarer frequencies than predicted by the neutral theory. Comparison of silent site variation a t this telomeric region with that in the Adh 5'-flanking region showed a reduced level of heterozygosity in the y-ac-sc region. Since inters pecific silent divergence is not reduced in the y-ac-sc region as comp ared to other regions, the reduction in standing levels of variation a t this telomeric locus in both D. simulans and D. melanogaster is most easily explained by a hitchhiking effect of linked selected substitut ions.