Divergence at neutral and non-neutral loci in Drosophila buzzatii populations and their hybrids

Intra- and interspecific comparison of protein polymorphism to establish genetic differentiation in two sympatric species of Drosophila: D. bipectinata and D. malerkotliana

Drosophila bipectinata species complex includes four monophyletic closely related sympatric species with perfect reproductive isolation in natural conditions. Among these, two species, i.e., D. bipectinata and D. malerkotliana are known to be more prevalent in their occurrence all along their distribution area of Oriental-Australian regions. Genetic characteristics of these two species have been studied by earlier researchers mainly by focusing on their chromosomal polymorphism to gain knowledge pertaining to their evolutionary trend. The present work is one step farther, as we considered isozyme variants of three polymorphic enzymes to envisage genetic differentiation among the different natural populations of these two species, along with, a comparative perusal of their certain population genetic parameters. Natural populations of these two species were sampled from eight places aligned on the north-south axis of India and were subjected to isozyme analysis. The collection of both species and the geographical locations were kept same to avoid the effect of temporal or seasonal factors on the distribution of their allelic frequency. The results of this study clearly indicate that there is intraspecific genetic structuring among the populations of both the species with a distinct steady increase in the level of heterozygosity from north to south. The evolutionary tree in the form of dendrogram depicts two genetically differentiated groups of natural populations, one for north and other for South India. Most essential aspect that has been noticed through this study is the evolutionary trend of these two sympatric species being largely similar all along the north-south axis of India.

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.

Q(ST) < F(ST) As a signature of canalization

A key aim of evolutionary biology - inferring the action of natural selection on wild species - can be achieved by comparing neutral genetic differentiation between populations (F(ST)) with quantitative genetic variation (Q(ST)). Each of the three possible outcomes of comparisons of Q(ST) and F(ST) (Q(ST) > F(ST), Q(ST) = F(ST), Q(ST) < F(ST)) is associated with an inference (diversifying selection, genetic drift, uniform selection, respectively). However, published empirical and theoretical studies have focused on the Q(ST) > F(ST) outcome. We believe that this reflects the absence of a straightforward biological interpretation of the Q(ST) < F(ST) pattern. We here report recent evidence of this neglected evolutionary pattern, provide guidelines to its interpretation as either a canalization phenomenon or a consequence of uniform selection and discuss the significant importance this issue will have for the area of evolutionary biology.