Inheritance of behavioural differences between two interfertile, sympatric species, Drosophila silvestris and D. heteroneura.

The Hawaiian fly species, Drosophila silvestris and D. heteroneura, are sympatric and interfertile but show strong behavioural isolation and major differences in male aggressive behaviour and the associated morphology. As a first step in elucidating the genetic control of the differences between these species, we examined the mating and aggressive behaviour of their reciprocal F1 hybrids. The latency to the first wing vibration and the latency to copulate did not differ significantly between the parental species. However, D. heteroneura females had a very low tendency to copulate with D. silvestris males, rarely mating during the observation period. The duration of copulation also differed significantly; same-species pairs of D. silvestris had copulations that lasted about 50% longer than those of same-species pairs of D. heteroneura. The hybrids were intermediate between the parental species for both the tendency to copulate with D. silvestris males and the duration of copulation, suggesting codominance or polygenic inheritance for those traits. The aggression traits that we scored were the leg posture and wing extension during early aggression, and the leg posture and head position during escalated aggression. The parental species showed clear differences for each of these traits. The F1 hybrids resembled one parent or the other, without showing intermediate values, suggesting single-gene dominance or threshold expression of many genes for those traits. None of the courtship or aggressive traits showed X-chromosomal effects, although the head shape of hybrids is influenced by genes on the X chromosome. It is difficult to reconcile the patterns of inheritance of aggressive behaviour and the lack of an X-chromosomal effect with the hypothesis that these traits are influenced by a coadapted gene complex.