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.
There are several barriers to preclude the gene flow between diverging populations. On the basis of their temporal nature, these can be broadly categorized into two forms: pre- and post-zygotic. Post-zygotic reproductive isolation can manifest in the form of reductions in hybrid fertility. Keeping this fact in view, in the present study, we studied sterility in hybrids of D. ananassae and D. pallidosa. Surprisingly a distinguishable pattern of infertility was found in the hybrids. This pattern, referred to as Haldane’s rule, is often observed in hybrids of recently diverged populations or species. Reduction in the fertility of hybrids provides the clue of incipient kind of post-zygotic reproductive isolation in these two sibling species. This is the first report of hybrid sterility in this species pair. However, hybrid sterility is not very prominent especially when compared to that of other species pairs with the similar divergence time. Thus, on the basis of our results, we conclude that either sexual isolation between these sibling species is sufficient and does not require the aid of post-zygotic isolation to preclude gene flow or rate of divergence between D. ananassae and D. pallidosa is very slow in comparison to other species pair or even races of some species.
Darwinian theory of evolution states that, evolution occurs through the natural selection. Therefore, demonstration of natural selection in nature is the central aim of many evolutionary studies and selection acts primarily at the phenotypic level because it is well known that phenotypic traits are the primary target of natural selection. While keeping this in view, we have studied certain morphometric traits in the sibling species pair, D. ananassae and D. pallidosa to test intra-and interspecific variations. The traits studied are wing length, thorax length, ratio of wing length and thorax length, sternopleural bristle number, ovariole number and sex-comb tooth number. In females of D. ananassae, significant strain differences were found for all the traits except ovariole number. In males, significant strain differences were found for all the traits. On the other hand, in D. pallidosa, significant strain differences were found for all the traits in both, males and females. The values of all the morphometric traits were significantly higher in females of both the species in comparison to males. The values of all the morphometric traits were higher in D. ananassae. However, the phenotypic variability, expressed in terms of coefficient of variation, was higher in D. pallidosa. Except for ratio of wing length and thorax length, CV was higher in the case of females in comparison to males. Size related traits are least variable while bristle numbers and reproductive traits are most variable. Except few, most of the traits are positively correlated with each other in both the species. Intra-and interspecific variations were found with respect to different morphometric traits. Although sibling species have been defined as morphologically identical, our results show that sibling species may show 2
During speciation different kinds of reproductive barriers originate to preclude gene flow between diverging populations. Reproductive isolation or barriers to gene flow can be categorized by the temporal nature of their effect: pre-zygotic barriers occur before fertilization and post-zygotic barriers occur after fertilization. In this study, we studied each components of reproductive isolation between D. ananassae and D. pallidosa, including both pre-zygotic and post-zygotic barriers. Because it might be possible that by dissecting these barriers one can get the answers of many unresolved questions related to the process of speciation of these two sibling species. We reported premating isolation because females of both D. ananassae and D. pallidosa were more discriminative for mating against the alien males rather than conspecific males, and this discrimination was much stronger in case of D. ananassae females for being the ancestral and cosmopolitan species. We also did not find any decrease in the production of progeny or viable offspring in comparison to conspecific males, indicating a lack of postmating prezygotic isolating barriers. Further, there is no complete lack of intrinsic post-zygotic isolation between these species or not complete presence of post-zygotic isolation, as both the hybrid sons were producing less number of progeny in comparison to all the crosses but it is near to significant but not significant and this is contrasting to the results of a previous study.
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