Mating propensity of Indian Drosophila melanogaster populations with D. simulans: a nonadaptive latitudinal cline

Crosses were investigated between a single strain of Drosophila simulans and 28 natural Indian populations of D. melanogaster. In each case, a mass culture and 10 isofemale lines of D. melanogaster were studied. Crosses were much easier between D. melanogaster females and D. simulans males than in the reciprocal case. Also, hybrid offspring were easier to obtain by using D. melanogaster flies (either males or females) from isofemale lines than from mass cultures. Finally, the crossability, estimated by progeny production, showed a clear-cut latitudinal cline: hybrids were easier to obtain with D. melanogaster populations from higher latitudes. As D. simulans does not occur in India, this cline does not reflect an evolutionary interaction between the two sibling species.