Genetic analysis of Drosophila sechellia specialization: oviposition behavior toward the major aliphatic acids of its host plant.

Oviposition behavior of the four species in the Drosophila melanogaster complex (D. melanogaster, D. simulans, D. mauritiana, D. sechellia) was investigated versus natural morinda fruit (the normal resource of D. sechellia) and the two major aliphatic acids of this fruit (hexanoic acid, C6, and octanoic acid, C8). Two different experimental techniques were compared. When control and experimental food were set on the same egg laying plate, three species (D. sechellia, D. mauritiana, D. melanogaster) exhibited a significant preference for morinda; with aliphatic acids, only D. sechellia manifested a preference. With separate oviposition sites, a preference was found in D. sechellia for morinda and acids, and a general avoidance behavior in the three other species. Genetic analysis of the behavioral response toward C6 and C8 was done with the two plates technique on D. sechellia, D. simulans, F1 hybrids and backcrosses. Significant behavioral differences were observed with major effects due to genotype, concentration and their interaction. Hybrid behaviors were intermediate between those of their parents. In several cases, a qualitative reversal from preference to avoidance was observed with increasing concentration. In F1 flies, a dominance reversal was observed with increasing C8 concentration. Different reaction thresholds in different receptors might explain such observations.