Intraspecific variation ofDrosophila buzzatii larval breeding success onOpuntia: A yeast-plant-insect relationship

The cactophilic species,Drosophila buzzatii, normally breeds in decaying pockets ofOpuntia cladodes, in which there is a complex interaction with the microbial flora, especially yeast species. Isofemale lines were used to estimate genetic variation among larvae reared on their natural feeding substrate. Four naturally occurring cactophilic yeast species isolated from the same Tunisian oasis as theDrosophila population were used. Two fitness components were studied for each line, viability and developmental time. Genetic variations amongD. buzzatii lines were observed for both traits. A significant yeast species x isofemale line interaction for viability was also evidenced, suggesting the occurrence of specialized genotypes for the utilization of breeding substrates. This genetic heterogeneity in the natural population may favor a better adaptation to the patchily distribution of yeasts.     

Divergence for tolerance to thermal-stress related traits in two Drosophila species of immigrans group

We investigated associations of the species specific divergence of thermal adaptations of Drosophila nasuta and Drosophila immigrans with seasonal as well as geographical changes in the relative abundance. In D. immigrans, cold resistance and egg-to-adult viability after a cold-shock have shown a positive cline with latitude, whereas no such clinal patterns were evident for heat resistance as well as egg-to-adult viability after heat stress. In contrast, basal heat tolerance and egg-to-adult viability after a heat-shock increased towards the equator i.e. from north to south in D. nasuta. Further, we also found species-specific divergence in seasonal climatic selection responses for heat as well as cold resistance i.e. warmer and higher humidity conditions of the rainy season have evidenced ∼2-fold higher heat tolerance of D. nasuta, but lower temperature as well as lower humidity conditions of the autumn significantly increased (∼3-fold) cold tolerance of D. immigrans. Geographical variations were significant for the absolute and relative hardening capacity for cold tolerance in D. immigrans, but for heat resistance of D. nasuta. Results of multiple regression analysis of changes in thermal tolerance level in latitudinal populations as a simultaneous function of climatic variables (T_ave and RH) have shown that average yearly relative humidity levels significantly affect heat tolerance of D. nasuta, but both T_ave and RH contribute to changes in the cold tolerance of D. immigrans. Thus, our results suggest that climatic selection responses for basal and hardened thermo-tolerance levels affect relative abundance of D. nasuta and D. immigrans across a latitudinal transect as well as seasons.     

Effect of developmental temperature stress on fluctuating asymmetry in certain morphological traits in Drosophila ananassae

In the present study, the effect of thermal stress on the variability and fluctuating asymmetry (FA) in different morphological traits, viz., thorax length (TL), sternopleural bristle number (SBN), wing length (WL), wing-to-thorax (W/T) ratio, sex comb tooth number (SCTN) and ovariole number (ON), was investigated in 10 isofemale lines of Drosophila ananassae. The phenotypic and genetic variability is higher in the flies reared at low (20 °C) and at high (30 °C) temperatures as compared to that of standard (25 °C) temperature. Further, the levels of FA of measured traits differed significantly among the three temperature regimes except SBN and SCTN in males and SBN and W/T ratio in females. Moreover, the magnitude of positional fluctuating asymmetry is similar in males reared at three different developmental temperatures for SBN and SCTN but it varies significantly for SBN in females. However, when FA across all the traits was combined into a composite index (CFA), significant differences were found for both temperature regimes and sexes. Males showed higher CFA at 30 °C whereas in females it was higher at 20 °C. The results suggest that temperature increases the levels of variability and FA but the effect seems to be trait and sex specific in D. ananassae.     

Phenotypic plasticity of abdomen pigmentation in two geographic populations of Drosophila melanogaster: male–female comparison and sexual dimorphism

In Drosophila melanogaster male, the last abdominal tergites (A5–A6) are completely dark due to a strong internal constraint while, in female, all abdominal tergites (A2–A7) are phenotypically variable and highly plastic. Male A2–A4 are quite similar to those of female, but their plasticity was never investigated. In this paper, we compared the phenotypic plasticity of A2–A4 in both sexes in order to know if the major dimorphism (SD) expressed in male A5–A6 also extended toward the more anterior segments. We also compared two geographic populations living under very different climates in order to know if adaptive differences, previously observed in females also existed in males. With an isofemale line design, pigmentation variation according to growth temperature was investigated in the two populations from France and India. Male and female data were compared and sexual dimorphism (SD) analyzed in various ways. Reaction norms were quite similar in both sexes for A2 and A3, but clearly different for A4. Considering the total pigmentation (A2 + A3 + A4) males were darker than females at low temperatures and either identical to them (France) or lighter (India) above 25°C. SD (male–female difference) was genetically variable among lines and significantly different among segments. Reaction norms of SD exhibited an overall decrease with temperature and also a significant difference among populations, suggesting a local adaptation of SD to thermal conditions. The three plastic segments in male (A2–A4) seem to react adaptively to the thermal environment more efficiently than the same segments in female, in agreement with the thermal budget hypothesis. To our knowledge, it is the first time that a SD trait exhibits an adaptive difference between geographic populations.     

Divergent abdominal bristle patterns in two distantly related drosophilids: antero-posterior variations and sexual dimorphism in a modular trait

The number of neurosensory bristles on abdominal sternites of Drosophila is a most investigated trait for quantitative genetic studies. However, the developmental pattern expressed on successive segments in both sexes has remained so far a neglected field. We explored three aspects of this general problem with an isofemale line design: comparing two distantly related species, Drosophila melanogaster and Zaprionus indianus, investigating bristle number variation along the antero-posterior axis, and analysing the sexual dimorphism. Antero-posterior variations could be analysed from segment A2 to A7 in females, and A2-A5 in males. In D. melanogaster, males and females showed parallel changes with a consistently lower number in males. In Z. indianus females the number was quite stable along the abdomen, while in males an important antero-posterior increase was found. The sexual dimorphism was further analysed by considering the female-male correlation and the female/male ratio. The results suggest that sternite bristle number is determined by several developmental genetic systems. One is acting along the antero-posterior axis and may be associated to a gradient, since the genetic correlation decreases when more distant segments are compared. Another is acting in the same way on most segments of both sexes, since the female-male genetic correlation is similar between homologous and non-homologous segments. Finally, genes with specific sex effects are acting on A7 in females of both species, and on A5 in Z. indianus males. The overall architecture of female and male abdomen seems to be constrained by the development of reproductive organs. A large difference between species suggests, however, that the sexual dimorphism of abdominal bristle number is not evolutionarily constrained.