Jean R. David

Historical biogeography of the Drosophila melanogaster species subgroup

Whereas there has been increasing interest in the eight members of the melanogaster species subgroup of Drosophila, no comprehensive survey exists of the biogeography and ecology of these species in the Afrotropical region.

Phenotypic and genetic variability of morphometrical traits in natural populations of Drosophila melanogaster and D simulans. I. Geographic variations

Summary - Geographical variability between natural populations of the 2 related cosmopolitan species Drosophila melanogaster and D simulans was investigated on a large number of populations (ie 55 and 25, respectively) for 6 morphometrical traits concerning weight, size, reproductive capacity and bristle numbers. For 21 populations, sympatric samples of the 2 species were available. For most traits, the mean values of D melanogaster are higher than those of D simulans, with the exception of the sternopleural bristle number, for which the species are similar. In D melanogaster, similar latitudinal variations exist along an African-European axis, in both hemispheres, and on the American continent. In D simulans, a latitudinal cline that is parallel to those observed in D melanogaster was observed suggesting that variability between populations is partially adaptive. In addition to these parallel variations, in which the mean values of all traits increase with latitude, inter-continental variations were also detected in D melanogaster when populations sampled at similar latitudes were compared (eg, West Indian and Far Eastern populations). Different demographic strategies (r or K) could explain such variations. Analysis of morphological distances (Mahalanobis generalized distance D between populations of the 2 species, showed that D melanogaster is much more diversified than D simulans. All the traits except the sternopleural bristle number are involved in these differences.

CHILL‐COMA TOLERANCE, A MAJOR CLIMATIC ADAPTATION AMONG DROSOPHILA SPECIES

Abstract.— Most drosophilid species can be classified either as temperate or tropical. Adults of species were submitted to a cold treatment (0°C) and then brought back to ambient temperature. They generally exhibited a chill coma and the time needed to recover was measured. We found in a set of 26 temperate species that recovery was rapid (average 1.8 min, range 0.15–4.9). In contrast, a long recovery time (average 56 min, range 24–120) was observed for 48 tropical species. A few species, like Drosophila melanogaster, are cosmopolitan and can proliferate under temperate and tropical climates. In 9 of 10 such species, slight genetic differences were found: a shorter recovery in temperate than in tropical populations. Comparing physiological data to phylogeny suggests that chill‐coma tolerance has been a recurrent adaptation that is selected for in cold climates but tends to disappear under a permanently warm environment. This major climatic adaptation, evidenced in drosophilids, seems to occur in other insect groups also.

VARIATIONS IN CUTICULAR HYDROCARBONS AMONG THE EIGHT SPECIES OF THE DROSOPHILA MELANOGASTER SUBGROUP

In addition to protecting against desiccation, some of the hydrocarbons of the waxy cuticle have previously been shown to be mating pheromones in Drosophila melanogaster and D. simulans. Therefore, cuticular hydrocarbons were compared among the eight species in the D. melanogaster subgroup. For the two cosmopolitan species and several geographic strains that were studied, all males are quite similar with very abundant monoenes. The major compound in most cases is 7‐tricosene. Only three exceptions were found: D. sechellia, and the Afrotropical strains of D. melanogaster and D. simulans.

PHYLOGENETIC CONSTRAINTS IN KEY FUNCTIONAL TRAITS BEHIND SPECIES’ CLIMATE NICHES: PATTERNS OF DESICCATION AND COLD RESISTANCE ACROSS 95 DROSOPHILA SPECIES

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92–95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate‐to‐strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids’ high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate‐to‐high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change.

Isofemale lines in Drosophila: an empirical approach to quantitative trait analysis in natural populations

Founding isofemale lines from wild collected females is a basic tool for investigating the genetic architecture of Drosophila natural populations. The method permits the analysis of quantitative traits under laboratory conditions, with a much broader scope than the mere evidence of a significant genetic heterogeneity among lines. Genetic variability is generally demonstrated by a significant coefficient of intraclass correlation, but several experimental precautions are needed and explained here. The relationship between classical (additive) heritability and intraclass correlation is not straightforward, presumably because the genetic bottlenecks due to the initiation of the lines unravel a significant, nonadditive genetic variance due to dominance and epistatic effects. It is thus suggested to consider intraclass correlation as a specific genetic parameter that enables comparisons between different traits, different populations or different environments. The use of isofemale lines is, however, not restricted to the calculation of an intraclass correlation. It can be used to estimate genetic correlations among traits or environments. The method is also convenient for the analysis of phenotypic plasticity in relation to an environmental gradient. A precise description of the response curves (the reaction norms) is possible, distinguishing trait parameters and plasticity parameters. A fairly general conclusion is that, for a given trait, mean value and plasticity are genetically independent. It is also possible to analyze traits, which, like sexual dimorphism, must be measured on different individuals, and even to demonstrate their genetic variability. In many cases, further empirical and theoretical analyses are possible and needed. It is argued that, in the future, isofemale lines will have an increasing significance among the various techniques appropriate to the analysis of quantitative evolutionary genetics in a diversity of species.

THE REPRODUCTIVE RELATIONSHIPS OF DROSOPHILA SECHELLIA WITH D. MAURITIANA, D. SIMULANS, AND D. MELANOGASTER FROM THE AFROTROPICAL REGION

Hybridization tests among the four sibling species of the Drosophila melanogaster complex were made to determine the reproductive status of the recently discovered D. sechellia (which is endemic to a few islands and islets of the Seychelles archipelago) with regard to its three close relatives, D. mauritiana (endemic to Mauritius) and Afrotropical strains of the two cosmopolitan species D. melanogaster and D. simulans. Interstrain variation in the ability to hybridize with other species was also analyzed for D. melanogaster and D. simulans. D. mauritiana and D. simulans appear to be more weakly isolated from each other than either species is from D. sechellia. A striking unilateral mating success is observed in the cross of D. sechellia with D. simulans. The most extreme isolation is between D. melanogaster and its three siblings. Variation in the ability of strains to hybridize is observed in heterospecific crosses between D. simulans and either D. melanogaster or D. mauritiana.

Biological role of alcohol dehydrogenase in the tolerance of Drosophila melanogaster to aliphatic alcohols: Utilization of an ADH-null mutant

The toxicity of the first eight primary alcohols and of four secondary alcohols was compared in a wild-type strain (having active ADH) and an ADH-negative mutant. Differences between lc50 measured in the two strains allowed an evaluation of the biological activity of the enzyme. In vitro, ADH is mainly active on secondary alcohols, while in vivo its main role is the detoxification and metabolism of ethanol. These observations suggest that originally ADH was involved in unknown metabolic pathways and that its utilization in ethanol metabolism could be a recent event.

DESICCATION AND STARVATION TOLERANCE OF ADULT DROSOPHILA: OPPOSITE LATITUDINAL CLINES IN NATURAL POPULATIONS OF THREE DIFFERENT SPECIES

Desiccation and starvation tolerance were measured along latitudinal transects in three Drosophilid species (Drosophila ananassae, D. melanogaster, and Zaprionus indianus) of the Indian subcontinent. In each case, significant latitudinal clines were observed; desiccation tolerance increased with latitude while starvation tolerance decreased. Such field observations suggest that desiccation and starvation tolerance are fitness related traits that are independently selected in nature and genetically independent. It was, however, difficult to relate these genetic changes with precise climatic variables, except winter temperature. The overall negative correlation between the two traits, which was evidenced in natural populations, contrasts with a positive correlation generally observed in various laboratory selection experiments and that also seems to exist between different species. These observations point to the difficulty of interpreting correlations among fitness‐related traits when different evolutionary levels are compared, and also different sets of data, that is, field versus laboratory studies.

ALLOMETRIC AND NONALLOMETRIC COMPONENTS OF DROSOPHILA WING SHAPE RESPOND DIFFERENTLY TO DEVELOPMENTAL TEMPERATURE

Abstract Phenotypic plasticity of wing size and shape of Drosophila simulans was analyzed across the entire range of viable developmental temperatures with Procrustes geometric morphometric method. In agreement with previous studies, size clearly decreases when temperature increases. Wing shape variation was decomposed into its allometric (24%) and nonallometric (76%) components, and both were shown to involve landmarks located throughout the entire wing blade. The allometric component basically revealed a progressive, monotonous variation along the temperature. Surprisingly, nonallometric shape changes were highly similar at both extremes of the thermal range, suggesting that stress, rather than temperature per se, is the key developmental factor affecting wing shape.