E Pla

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.

Cold stress tolerance in Drosophila: analysis of chill coma recovery in D. Melanogaster

Abstract Drosophila melanogaster adults, grown at 21°C, were distributed in groups of 50 after a light anaesthesia. Culture vials with flies were later submitted to a cold treatment at 0°C. All adults entered a chill coma; the recovery time was measured at ambient temperature. 2. Recovery time was strongly influenced by recovery temperature, with shorter values between 20-25°C. 3. Recovery time increased almost linearly with duration of cold treatment. 4. Recovery time was consistently larger for males than for females. It was highly variable among groups and increased with flies’ age. 5. Variability among flies of the same group was always very high, with CVs often over 25%. 6. Chill coma and its recovery seem to imply a modification of the nervous system, analogous in several aspects to what is observed with usual anaesthetics such as CO 2 .

PHENOTYPIC PLASTICITY AND REACTION NORMS IN TEMPERATE AND TROPICAL POPULATIONS OF DROSOPHILA MELANOGASTER: OVARIAN SIZE AND DEVELOPMENTAL TEMPERATURE

The plasticity of ovariole number relative to developmental temperature was studied in three populations of Drosophila melanogaster at both ends of the cline: a temperate French population and two equatorial Congolese. Ovary size was much greater in the French flies, in agreement with an already known latitudinal cline. Among isofemale lines, significant differences in genetic variability were observed between populations with a maximum variability at intermediate temperatures. Parameters of phenotypic variability (CV and FA) were not statistically different among lines or populations, but a significant increase at low temperature was demonstrated for both. The shapes of the response curves (i.e., the norm of reaction) were analyzed by adjusting the data to a quadratic equation. The parameters of the equation were highly variable among lines. On the other hand, the temperature for maximum value of ovarioles (TMV) was much less variable and exhibited only a slightly significant difference between temperate and tropical flies (22.2°C vs. 22.7°C). During its geographic extension toward colder places, D. melanogaster underwent a large, presumably adaptative, increase in ovariole number but very little change in the norm of reaction of that trait.

Alcohol tolerance, ADH activity, and ecological niche of Drosophila species

In vitro alcohol dehydrogenase (ADH) activity was measured in adults of species belonging to Drosophila and to the related genus Zaprionus. Data were analyzed according to the known breeding sites and the level of ethanol tolerance of these species. Alcohol dehydrogenase activity was assayed with both ethanol (E) and isopropanol (I). Our results show a very broad range of activities among the 71 species investigated, the ratio of the highest value observed (D. melanogaster) to the lowest (D. pruinosa) being 65:1. A general positive correlation was found between the level of ADH activity and the capacity to detoxify ethanol. Nevertheless, many species show exceptions to this rule. Contrary to a logical expectation, adaptation to high alcoholic resources, which has been a recurrent evolutionary event, was not mediated by a more efficient use of ethanol, that is, an increase of the E/I ratio. This ratio seems to be quite variable according to the phylogeny and is especially low in the subgenus Sophophora as well as in Zaprionus. Alcohol tolerance clearly is related to the larval habitat of the species and shows that adaptation to alcoholic resources has been a major evolutionary challenge in drosophilids. This adaptation is not related to phylogeny, having occurred independently several times during the evolution of the group. Finally, it should be borne in mind that, besides metabolization and detoxification, other physiological processes such as nervous‐system tolerance or ethanol excretion may be involved in ethanol tolerance, and such functions also should be investigated. Environmental ethanol, which is certainly a major ecological parameter for many drosophilids, has selected a diversity of physiological adaptations, all related to the Adh locus, but presumably much more complicated than was previously believed.

Evolutionary changes of nonlinear reaction norms according to thermal adaptation: a comparison of two Drosophila species

While the adaptive significance of discontinuous reaction norms is generally accepted, the evolutionary interpretation of continuous response curves remains speculative, and the occurrence of internal constraints is often suggested as an explanation of experimental observations. In Drosophila melanogaster, various morphometrical traits exhibit convex reaction norms to growth temperature, with a maximum value within the developmental thermal range. We compared a cold-adapted species (D. subobscura) with a mid thermal range at 16 degrees C, to the warm-adapted D. melanogaster (mid thermal range at 22 degrees C) for three different morphometrical traits: wing and thorax length in both sexes and ovariole number in females. Maximum value temperatures were ordered in the same way for the three traits in both species: ovariole number > thorax length > wing length. Significant differences were also observed between the two species for the curvature parameter of the quadratic adjustment. The major observation was a significant lateral shift in the reaction norms: maximum values were observed at much lower temperatures in the cold-adapted species than in the warm-adapted one. The parallelism between mid thermal range variation and the position of the maximum value strongly suggests an adaptive displacement of the response curves. Natural selection may thus act not only on trait mean values but also on phenotypic plasticity and on the shape of reaction norms.

Phenotypic and genetic variability of morphometrical traits in natural populations of Drosophila melanogaster and D simulans. II. Within-population variability

Within-population variability was investigated in the 2 sibling species Drosophila mela!nogaster and D simulans at both phenotypic and genetic levels. Six quantitative traits were studied in 55 different populations of D melanogaster and 25 populations of D simulans encompassing most of the cosmopolitan range of the 2 species. The phenotypic variabilities of all the traits were compared using the coefficients of variation (CV). Differences among CV’s were broader than expected from their theoretical sampling distribution. Temperate populations were generally less variable than tropical ones. Moreover, in both species, the CVof the 3 size-related traits (fresh weight, wing length and thorax length) were correlated. Comparison of the 2 species showed that the average variabilities (mean values of Ct! were almost identical with the exception of ovariole number which is much less variable in D simulans (6% against 8%). At the genetic level, distributions of intraclass correlations did not show any departure from the expected sampling distributions, suggesting that all populations harbored a similar

ADAPTATION TO FERMENTING RESOURCES IN DROSOPHILA MELANOGASTER : ETHANOL AND ACETIC ACID TOLERANCES SHARE A COMMON GENETIC BASIS

Ethanol and acetic acid tolerances were compared in a French, highly tolerant population, and in a Congolese, very sensitive population. For both tolerances, chromosome substitutions demonstrated a major effect on chromosome 3, a lesser effect on chromosome 2, and no effect on chromosome 1, except in interactions. Directional selection experiments led to significant increases of tolerance to both toxics. Of greater interest, a strong correlated response was observed in each line: increased ethanol tolerance was accompanied by higher acetic acid tolerance and vice versa. A high genetic correlation (average value r = 0.77) was found between the two traits. These data suggest that alcohol dehydrogenase (ADH) activity does not play a major role in explaining the physiological differences known between Afrotropical and European populations. The metabolic flux permitting the detoxification of ethanol and acetic acid seems to be mainly controlled by acetyl‐coA synthetase (ACS) at least in adult flies. Acetic acid adaptation could be as important as ethanol adaptation in the ecology of Drosophila melanogaster and other Drosophila species.

Genetic basis of some morphological differences between temperate and equatorial populations of Drosophila melanogaster.

The genetic basis of three morphological traits (ovariole number, sternopleural bristle number and wing length) ofDrosophila melanogaster has been investigated in natural populations that show great differences in these traits, i. e. Bordeaux (France) and Loua (Congo). F1 and F2 crosses, and chromosome substitutions between these two populations, were analysed. Maternal and/or X chromosome effects were found for sternopleural bristle number and wing length. For all traits, significant effects from each of the three chromosomes were found, but in general only one or two chromosomes had a major effect. Moreover, in all cases significant interactions between chromosomes were observed, suggesting the existence of epistatic effects. Our results are discussed and compared to those obtained from the analysis of selected laboratory strains.

Ethanol and acetic-acid tolerances in Drosophila melanogaster: similar maternal effects in a cross between 2 geographic races

Ethanol and acetic-acid tolerances were studied in a cross between 2 geographic races of Drosophila melanogaster, ie a very sensitive population from equatorial Africa and a resistant French population. Average values in the Fl and F2 were similar and close to the mid-parent value. A clear maternal genotype effect was, however, observed for both traits between reciprocal Fis, and the difference disappeared in the F2. Further investigations demonstrated that for ethanol tolerance, the large difference between the parental strains was not entirely due to differences in their allelic frequencies at the Adh locus. The possible mechanisms of these physiological variations are discussed.