Sexualisolation inDrosophila melanogaster: A possible caseof incipient speciation

Itisgenerally believed that Drosophila mela- nogaster hasnoclosely related species withwhichitcan produce theviable andfertile hybrids that areessential forthe genetic analysis ofspeciation. Following therecent report of molecular differentiation between aZimbabwe, Africa, pop- ulation andtwoUnited States populations, weprovide evi- dencethatstrong sexual isolation exists between theD. melanogaster population inZimbabweandpopulations of other continents. Inthepresence ofmalesoftheir ownkind, females frommostisofemale lines ofZimbabwe wouldnot matewithmales fromelsewhere; thereciprocal mating isalso significantly reduced, buttoalesser degree. Thegenes for sexual behaviors areapparently polymorphic inZimbabwe andpostmating reproductive isolation between this andother populations hasnotyetevolved. Wholechromosome substi- tutions indicate significant genetic contributions tomale matingsuccess bybothmajorautosomes, whereas theX chromosome effect istooweaktomeasure. Inaddition, the relative mating success between hybrid andpureline males supports theinterpretation ofstrong female choice. These observations suggest thatweareseeing theearly stages of speciation inthisgroupandthatitisdriven bysexual selection. Thegenetic andmolecular tractability ofD.mela- nogaster offers great promise forthedetailed analysis ofthis apparent caseofincipient speciation.