The degree and pattern of sexual isolation among closely related species can be utilized for predicting the direction of evolution and deriving a phylogeny. The Drosophila bipectinata species complex is a group of four morphologically very similar species, belonging to the ananassae subgroup of the melanogaster species group. Sexual isolation among the members of this complex was studied by employing the male-choice technique. To test the difference between the homogamic and heterogamic matings, chi-square values were calculated under the assumption of random mating and the degree of sexual isolation was measured by estimating isolation index. To test the difference in proportions of heterogamic matings between reciprocal crosses, normal deviate (z) was calculated. The direction of evolution among these species has been discussed in the light of models proposed by different workers. Our results suggest that while D. bipectinata, Drosophila parabipectinata and Drosophila malerkotliana are closely related to each other, D. pseudoananassae is distantly related to these three species. The species pairs D. bipectinata and D. parabipectinata and D. parabipectinata and D. malerkotliana show asymmetrical sexual isolation. Based on this, it is suggested that D. bipectinata and D. malerkotliana share a common ancestor where as D. parabipectinata has been derived from D. bipectinata. Their phylogenetic relationship has been discussed in the light of phylogenies suggested by earlier workers.
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 F 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 F larvae from both sets of crosses had similar configurations. In F 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.
Comparison of population dynamics with regard to chromosomal arrangements, protein and nucleotide polymorphism among closely related species can provide invaluable glimpses into the intricacies of speciation. In addition, recently diverged closely related species may also exhibit behavioural differences especially sexual behaviour which can be compared across the species and eventually may aid in understanding the process of speciation. To get the finer details of the so called 'speciation genes', evolutionary studies pertaining to hybrid incompatibilities are required. This involves detailed mapping of the speciation genes through introgression of a candidate gene from one species into the genetic background of another species. Evolutionary studies also include studies on the degree and pattern of sexual isolation among closely related species.The Drosophila bipectinata species complex comprises four species namely, D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae. In three members of this complex i.e. D. bipectinata, D. malerkotliana and D. pseudoananassae, subspecies have been identified. This complex with its species and their subspecies is a good model for speciation studies and has been extensively utilized for the purpose. This review attempts to compile all the important population genetical, behavioural and evolutionary studies that have been done in this complex so far. Eventually, we have also discussed what is still left to be done and the prospects of this complex being used in future to get answers relating to speciation. Keywords: Drosophila bipectinata IntroductionStudies on the intricacies of speciation involve varied approaches dealing mostly with population genetics, behaviour and evolution.Population genetics constitutes studies on changes in the genetic composition of a population, spanning investigations on polymorphism at the level of chromosomes (chromosomal arrangements), proteins and nucleotides. Comparing polymorphism levels across different populations of a species throws light on the pattern of polymorphism exhibited by the species. Further, comparing the pattern across different closely related species can definitely indicate the degree of their divergence and phylogenetic relationship.During the process of speciation, behavioural differentiation is expected as divergence in traits related to male courtship and female preference is necessary for reproductive isolation. Adaptations to their respective niches may also cause differences in certain behavioural aspects in different species. Also, alterations in the behavioural echelon may be a pleiotropic sequel of divergence at a certain locus/ loci not having any direct behavioural implication or simply a by-product of the divergence (although not often the case with behaviour). Therefore, studies on behavioural divergence amongst closely related species aids in unfolding the knots of the complex process of speciation. Behavioural studies mostly involve analysis of the courtship rituals of males and preferen...
Banerjee P. and Singh B. N. 2016 The Drosophila bipectinata species complex: degree of sterility and dystrophied ovaries in interspecific hybrid females. J. Genet. 95, [167][168][169][170]
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