Comparative studies of courtship behaviour in sympatric sibling species of Zaprionus (Diptera - Drosophilidae)
Zaprionus tuberculatus and Z. sepsoides are two sibling species of drosophilid fly which are sympatric throughout tropical Africa. Their sexual behaviour has previously received little study. In this thesis, the courtship behaviours of the two species are described, and the descriptions compared with previously published descriptions by other authors. Three different laboratory-bred strains of Z. sepsoides, and two of Z. tuberculatus, were studied, and their male courtship "songs" (produced by male wing vibration) were compared. The main objects of the project were to study the mechanisms by which members of these two species identify the sex of other individuals (sexual dimorphism being slight); and the isolating mechanisms operating between the two species. Preliminary observations suggested that males could probably identify females by chemosensory cues. However, males would sometimes also court each other. Further experiments on the males' ability to distinguish sex-specific pheromones gave ambiguous results. There were no grounds to suspect that females could distinguish between the sexes by chemosensory means, and further experiments gave no evidence for such an ability. Preliminary observations indicated that that there was an effective pre-mating reproductive barrier between male Z. sepsoides and female Z. tuberculatus, but male Z. tuberculatus were sometimes able to copulate with female Z. sepsoides. These interspecific matings resulted in infertile eggs. Further experiments seemed to confirm that the isolating mechanism between the two species is asymmetric, the onus of species recognition lying largely on Z. sepsoides. Male Z. sepsoides apparently recognise a female's species by chemosensory means, while female Z. sepsoides recognize a male's species by his "Type-2" courtship "song". There was also some evidence that female Z. sepsoides could distinguish between males of the two species by olfactory means. Z. tuberculatus were not shown to be able to distinguish reliably between the two species. The relevance of these findings to current theories about mechanisms of speciation is discussed.