Incipient sexual isolation in the nasuta-albomicans complex of Drosophila: mating preference in male-, female- and multiple-choice mating experiments.

Interracial divergence is an important facet of speciation. The nasuta-albomicans complex of Drosophila with sixteen morphologically identical, karyotypically different but cross-fertile races is an excellent system to study a few dimensions of raciation. Drosophila nasuta nasuta, Drosophila nasuta albomicans, Cytorace 1, Cytorace 2, Cytorace 3 and Cytorace 4 of this subgroup have been subjected to male-, female- and multiple-choice mating experiments. Out of 8456 crosses conducted, 7185 had successful matings. The overall impression is that mating is far from random amongst these six closely related races of the nasuta-albomicans complex. The males of D. n. albomicans, Cytorace 1 and Cytorace 4 in male-choice, the females of Cytorace 1 and Cytorace 2 in female-choice, and the males and females of D. n. nasuta, D. n. albomicans, Cytorace 1 and Cytorace 4 against the males and females of Cytorace 2 in multiple-choice experiments, had significantly more homogamic matings than expected. Thus in this study of evolutionary experimentation on raciation under laboratory conditions, we have documented the initiation of preference for conspecific matings among closely related and independently evolving members of the nasuta-albomicans complex of Drosophila.