Zoological Studies

Differences in Morphological Traits between Two Sibling Species, Drosophila ananassae and D. pallidosa

Chavali Vishalakshi and Bashisth Narayan Singh (2008) Capturing the mechanisms of speciation that appear in the early stages of reproductive isolation has been of recent interest to evolutionary biologists. In the present study, we investigated interspecific variations in several morphological traits, the degree of crossability, productivity, and the sex ratio in 2 sibling species, Drosophila ananassae and D. pallidosa, and their hybrids. The present species pair is unique due to the presence of strong sexual isolation and the absence of postmating barriers such as hybrid inviability or sterility. The 2 sibling species significantly differed in the following morphological traits: thorax length, sternopleural bristle number, wing length, wing-tothorax ratio, sex comb tooth number, and ovariole number in males and females. Interspecific hybrids also significantly differed from their parental species in all morphological traits. Further, the degree of crossability and productivity was greater with conspecific matings than with heterospecific matings. Moreover, we found no sex ratio distortion in interspecific hybrids suggesting that there are fewer genetic incompatibilities between these 2 sibling species. These results are interpreted in terms of the evolutionary divergence between D. ananassae and its sibling species, D. pallidosa.