Fluctuating Asymmetry in Hybrids of Sibling Species, Drosophila ananassae and Drosophila pallidosa, Is Trait and Sex Specific

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.

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.

Effects of genetic factors and infection status on wing morphology of Triatoma dimidiata species complex in the Yucatán peninsula, Mexico

Triatoma dimidiata is one of the main vectors of Chagas disease, and it has been shown to be a species complex. In the Yucatán peninsula, Mexico, vector populations are non-domiciliated, and the transmission of Trypanosoma cruzi thus critically relies on vector dispersal. This leads us to study the morphologic variations in T. dimidiata wings with respect to genetic factors (sex and genotype at the ITS-2 locus), geographic location, and T. cruzi-infection status. Females were found to have larger and more symmetrical wings than males. Wing shape was influenced by ITS-2 genotypes, although differences are unlikely sufficient to allow taxonomic discrimination of the sibling species. Hybrids were shown to have similar fluctuating asymmetries in wing size and shape as parental species, but the level of asymmetry in shape varied slightly between villages. The two later findings are consistent with a high level of gene flow between parental species, and the high dispersal potential of these non-domiciliated vectors. More surprisingly, individuals infected with T. cruzi were found to have larger wings than non-infected ones. This effect, which was consistently observed across sexes, genotypes and villages, is likely to be due to a direct impact of T. cruzi on insect development. Sex and infection status are thus likely to be key factors influencing vector dispersal with important impacts on disease transmission, since dispersal directly controls the domestic abundance of these vectors. These aspects should be investigated further to fully capture the ecology and evolution of Chagas disease transmission by non-domiciliated vectors.

Heterozygosity maintains developmental stability of sternopleural bristles in Drosophila subobscura interpopulation hybrids

Interpopulation hybridization can lead to outbreeding depression within affected populations due to breakdown of coadapted gene complexes or heterosis in hybrid populations. One of the principal methods commonly used to estimate the level of developmental instability (DI) is fluctuating asymmetry (FA). We used three genetically differentiated Drosophila subobscura populations according to inversion polymorphism analysis and measured the variability of sternopleural bristle number and change in FA across generations P, F1, and F2 between intra- and interpopulation hybrids of D. subobscura. The mean variability of sternopleural bristle number in intra- and interpopulation hybrids of D. subobscura across generations cannot determine whether the changes at the level of developmental homeostasis are due exclusively to genomic coadaptation or to heterozygosity. Phenotypic variance (V(p)) and FA of sternopleural bristle number was higher in interpopulation than in intrapopulation hybrids across generations. F1 hybrids were more developmentally stable compared to each parental population in both intra- and interpopulation hybrids. The most probable mechanism providing developmental homeostasis is heterozygote or hybrid superiority, also called overdominace. However, V(p) was higher and FA lower in the F2 generation when compared to F1, due mainly to crossing-over in the formation of F2.