INHERITANCE OF BEHAVIORAL-DIFFERENCES BETWEEN 2 INTERFERTILE, SYMPATRIC SPECIES, DROSOPHILA-SILVESTRIS AND DROSOPHILA-HETERONEURA

The Hawaiian fly species, Drosophila silvestris and D. heteroneura, ar e sympatric and interfertile but show strong behavioural isolation and major differences in male aggressive behaviour and the associated mor phology. As a first step in elucidating the genetic control of the dif ferences between these species, we examined the mating and aggressive behaviour of their reciprocal F-1 hybrids. The latency to the first wi ng vibration and the latency to copulate did not differ significantly between the parental species. However, D. heteroneura females had a ve ry low tendency to copulate with D. silvestris males, rarely mating du ring the observation period. The duration of copulation also differed significantly: same-species pairs of D. silvestris had copulations tha t lasted about 50% longer than those of same-species pairs of D. heter oneura. The hybrids were intermediate between the parental species for both the tendency to copulate with D. silvestris males and the durati on of copulation, suggesting codominance or polygenic inheritance for those traits. The aggression traits that we scored were the leg postur e 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 F, hybrids resembled one parent or the other, without showing intermediate values, suggesti ng single-gene dominance or threshold expression of many genes for tho se traits. None of the courtship or aggressive traits showed X-chromos omal effects, although the head shape of hybrids is influenced by gene s on the X chromosome. It is difficult to reconcile the patterns of in heritance of aggressive behaviour and the lack of an X-chromosomal eff ect with the hypothesis that these traits are influenced by a coadapte d gene complex.