Brigitte Moreteau

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.

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 .

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.

Volatile components of ripe fruits of Morinda citrifolia and their effects on Drosophila

Abstract The only larval resource of the specialist species, Drosophila sechellia , is ripe fruits of Morinda citrifolia . The chemical composition of this fruit, which is very toxic to most Drosophila species, was investigated and 51 compounds were abundant enough to be identified by GC-MS. The ripe fruit is characterized by a large amount of carboxylic acids, especially octanoic and hexanoic acids. The biological effects of the ripe fruit and its main acids were investigated with behavioural studies. Octanoic acid is responsible for the general toxicity of the fruit to most Drosophila species; D. sechellia is the only species which is resistant to this acid. Hexanoic acid has a unique effect, causing reversible coma but no mortality. Decanoic acid is inactive. A mixture of these three acids in proportions similar to those found in the fruit, mimics the effects of ripe fruits of M. citrifolia .

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.

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.

Phenotypic plasticity and developmental temperature in Drosophila: Analysis and significance of reaction norms of morphometrical traits

Abstract In ectotherm species like Drosophila, morphometrical quantitative traits exhibit a large amount of phenotypic plasticity as a consequence of developmental temperature. Generally the response curves called reaction norms, are non-linear and exhibit either single maxima or minima, or a sigmoid shape. Up to now, such complex shapes were mainly considered as non-adaptive, reflecting interactions between internal developmental constraints and the environment. We show that the general, all purpose method of polynomial adjustment provides a convenient way for the description of the norms. From the polynomial parameters, characteristic values with an obvious biological significance (e.g. the temperature of a maximum size), can be calculated and used for comparing populations or species. Experimental data on body pigmentation and body size show that the shapes of the reaction norms are genetically variable and suggest adaptive responses to the environment. The possible occurrence of specific regulatory genes acting on norm shape will be a matter for further investigations.

Comparative analysis of morphological traits among Drosophila melanogaster and D. simulans: genetic variability, clines and phenotypic plasticity

The two sibling cosmopolitan species, Drosophila melanogaster and D. simulans, are able to proliferate under very different climatic conditions. This has resulted in local adaptations, which are often arranged in latitudinal clines. Such clines are documented for body weight, wing and thorax length, sternopleural and abdominal bristle number, ovariole number and thoracic pigmentation. The overall magnitude of geographical differentiation is, however, much less in D. simulans than in D. melanogaster, and latitudinal clines are less pronounced.The fact that natural populations live under different climates raises the problem of interaction between temperature and phenotype. The reaction norms of morphometrical traits have been investigated as a function of growth temperature. The shapes of the response curves vary according to the investigated trait. They are generally curvilinear and can be described by calculating characteristic values after polynomial adjustments. For a given trait, the reaction norms of the two species are similar in their shape, although some significant differences may be observed.Within each species, significant differences are also observed between geographic populations: reaction norms are not parallel and the divergence is better marked when more distant populations (e.g., temperate and tropical) are compared. It thus appears that besides mean trait value, phenotypic plasticity is also a target of natural selection.A specific analysis of wing shape variation according to growth temperature was also undertaken. Reaction norms with different shapes may be observed in various parts of the wing: the major effect is found between the basis and the tip of the wing, but in a similar way in the two species. By contrast, some ratios, called wing indices by taxonomists, may exhibit completely different reaction norms in the two species.For a single developmental temperature (25°C) the phenotypic variability of morphometrical traits is generally similar in the two species, and also the genetic variability, estimated by the intraclass correlation. A difference exists, however, for the ovariole number which is less variable in D. simulans. Variance parameters may vary according to growth temperature, and a detailed analysis was made on wing dimensions. An increase of environmental variability at extreme, heat or cold temperatures, has been found in both species. Opposite trends were, however, observed for the genetic variability: a maximum heritability in D. simulans at middle temperatures, corresponding to a minimum heritability in D. melanogaster. Whether such a difference exists for other traits and in other populations deserves further investigations.In conclusion, morphometrical analyses reveal a large amount of significant differences which may be related to speciation and to the divergence of ecological niches. Within each species, numerous geographic variations are also observed which, in most cases, reflect some kinds of climatic adaptation.

Male sterility at extreme temperatures: a significant but neglected phenomenon for understanding Drosophila climatic adaptations

The thermal range for viability is quite variable among Drosophila species and it has long been known that these variations are correlated with geographic distribution: temperate species are on average more cold tolerant but more heat sensitive than tropical species. At both ends of their viability range, sterile males have been observed in all species investigated so far. This symmetrical phenomenon restricts the temperature limits within which permanent cultures can be kept in the laboratory. Thermal heat sterility thresholds are very variable across species from 23 °C in heat sensitive species up to 31 °C in heat tolerant species. In Drosophila melanogaster, genetic variations are observed among geographic populations. Tropical populations are more tolerant to heat induced sterility and recover more rapidly than temperate ones. A genetic analysis revealed that about 50% of the difference observed between natural populations was due to the Y chromosome. Natural populations have not reached a selection limit, however: thermal tolerance was still increased by keeping strains at a high temperature, close to the sterility threshold. On the low temperature side, a symmetrical reverse phenomenon seems to exist: temperate populations are more tolerant to cold than tropical ones. Compared to Mammals, drosophilids exhibit two major differences: first, male sterility occurs not only at high temperature, but also at a low temperature; second, sterility thresholds are not evolutionarily constrained, but highly variable. Altogether, significant and sometimes major genetic variations have been observed between species, between geographic races of the same species, and even between strains kept in the laboratory under different thermal regimes. In each case, it is easily argued that the observed variations correspond to adaptations to climatic conditions, and that male sterility is a significant component of fitness and a target of natural selection.

Heat induced male sterility in Drosophila melanogaster: adaptive genetic variations among geographic populations and role of the Y chromosome.

SUMMARY We analyzed genetic variation among geographically diverse populations of Drosophila and showed that tropical flies are more tolerant than temperate ones to heat-induced male sterility, as assessed by the presence of both motile sperm and progeny production. In tropical populations, the temperature inducing 50% sterility (median threshold) is 1°C above the value for temperate populations (30.4 vs. 29.4°C). When transferred to a mild permissive temperature (21°C), males recover fertility. Recovery time is proportional to pre-adult culture temperature. At these temperatures, recovery time is greater for temperate than for tropical populations. Crosses between a temperate and a tropical strain (F1, F2 and successive backcrosses) revealed that the Y chromosome was responsible for much of the geographic variation. Sterile males exhibited diverse abnormalities in the shape and position of sperm nuclei. However, impairment of the spermatid elongation seems to be the major factor responsible for sperm inviability. Heat-induced male sterility seems to be quite a general phenomenon in Drosophilid species and variation of threshold temperatures may be important for explaining their geographic distributions.