The Drosophila bipectinata species complex: degree of sterility and dystrophied ovaries in interspecific hybrid females

Drosophila pallidosa: whether a separate species or a light form of D. ananassae

Drosophila pallidosa belongs to the D. ananassae complex, which includes a total of 10 species. Earlier D. pallidosa was known as light form of D. ananassae but later it was described as a new species, sibling of D. ananassae. Both these terms, light form and sibling species were used by Futch. This makes the taxonomic status of D. pallidosa confusing. In this review we have tried to understand the actual status of this sibling species pair. Considering the similarities and dissimilarities, we suggest that D. pallidosa does not have the full status of a species, rather it is in the process of speciation, statu-nascendi. Our suggestion is strengthened by the fact that male genitalia are identical in both the cases and they lack postmating reproductive isolation since hybrids between them are normal and fully fertile.

The Drosophila bipectinata species complex: phylogenetic relationship among different members based on chromosomal variations

Making interspecific hybridizations, where possible remains an unparalleled option for studying the intricacies of speciation. In the Drosophila bipectinata species complex comprising of four species, namely D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae, interspecific hybrids can be obtained in the laboratory, thus bequeathing an ideal opportunity for studying speciation and phylogeny. With the view of investigating the degree of divergence between each species pair, we planned to study the polytene chromosomes of the F₁ hybrids, as it would mirror the level of compatibility between the genomes of the parental species. Two sets of crosses were made, one involving homozygous strains of all four species from India and the other including homozygous strains from different places across the globe. Polytene chromosomes of F₁ larvae from both sets of crosses had similar configurations. In F₁ larvae from crosses involving D. bipectinata, D. parabipectinata and D. malerkotliana, complex configurations (depicting overlapping inversions) could be detected in different arms. However, they were fairly synapsed, indicating that the differences are only at the level of gene arrangements. The polytene chromosomes of larvae obtained by crossing D. pseudoananassae with the other three species were very thin with gross asynapsis in all the arms, demonstrating that the genome of D. pseudoananassae is widely diverged from rest of the species. The overlapping inversions (reflected in complex configuration), are inferred in the light of earlier chromosomal studies performed in this complex.

Interspecific hybridization does not affect the level of fluctuating asymmetry (FA) in the Drosophila bipectinata species complex

The Drosophila bipectinata species complex comprises of four very closely related species namely D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae. It was found that irrespective of the evolutionary divergence among the species, FA which is reflective of the developmental precision remains nearly same in four species. During the present study, the level of FA in different morphological traits was studied in interspecific hybrids and compared with that of parental species with the view that it would throw light on the degree of divergence between the parental species. If they have not diverged much, the interspecific hybrids may have a similar FA level, incompatibilities between their genomes being negligible. On the other hand, if there is substantial divergence, the level of FA may be higher due to incompatibility between the genomes of the parental species. The morphological traits taken were sternopleural bristle number and wing length in both males and females and ovariole number and sex-comb tooth number in females and males respectively. However, except in a few cases, we could not detect any significant differences in the level of FA in hybrids as compared to pure species. On the other hand, a number of abnormalities like poor viability, dystrophied ovaries, asymmetrical eyes etc., could be detected in hybrids from crosses involving D. pseudoananassae as one of the parents. Therefore, we conclude that specific developmental pathways are more susceptible to developmental disturbances due to genomic incompatibilities than the large complex system bringing about developmental stability.