Abstract1. A female choice experiment between two stocks of the sibling species Drosophila melanogaster and D. simulans revealed a complete sexual isolation between the two species. 2. The courtship behaviour of males and females of the two species has been recorded in single pair matings by one observer using a microcomputer. It can be classified into the same 16 elements without stressing the species-typical aspects of the performances. 3. A detailed comparison of four courtship parameters per behavioural element, (I) percentage of total number of courtship acts, (II) percentage of total courtship time, (III) mean boutlength, and (IV) mean frequency per minute, showed a substantial quantitative differentiation of both sexes between the two closely-related species. 4. Drosophila simulans males and females are both less active in performing their behaviours than D. melanogaster. Scissoring is the major type of wingdisplay in the courtship of D. simulans males, and vibration is more common in the courtship of D. melanogaster males. Drosophila simulans females show lower frequencies per minute of all elements, except walking, extruding, and flicking. 5. Courtship duration in D. melanogaster males is controlled by the elements: orientation, following, and attempted copulation, as was shown by correlation analysis. In D. simulans males, courtship duration is mainly controlled by the elements licking and attempted copulation. However, with respect to the correlation coefficients of individual male behaviours the two species do not differ significantly. Therefore, we could not single out decisive and distinctive elements for enhancement of female's receptivity as a discriminating feature of male sexual behaviour of the two species. 6. First-order sequential analysis of intra-male dyadic transitions between adjacent behaviour elements of D. melanogaster and D. simulans shows quantitative differences primarily in transitions with either vibration or scissoring as the preceding or succeeding acts. Differences between the two female species are mainly limited to transitions in which the rejection-movements decamping and kicking are involved. 7. Analysis of inter-individual dyadic sequences gives the communicative value of male elements relative to the female elements and vice versa. In the communicative interactions with females, scissoring is the major type of wingdisplay in D. simulans males. It accomplishes the same role either in responding to the female or in stimulating the female as vibration does in D. melanogaster males. The elements standing, preening, and extruding are the central behaviours of females in both species in elucidating the male elements orientation, major type of wingdisplay, licking, and attempted copulation. However, the degree to which these male elements are initiated by the female's elements differs quantitatively between the two species. These quantitative differences emphasizes the differentiation in patterns of interaction between sexes between the two species.
Genetics of sexual isolation in male hybrids of Drosophila simulans and D. mauritiana
