Genetica 120: 151–163, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Comparative life histories and ecophysiology of Drosophilamelanogaster and D. simulans

Jean R. David1, Roland Allemand2, Pierre Capy1,∗, Mohamed Chakir3, Patricia Gibert1,Georges Petavy1 & Brigitte Moreteau1

1CNRS, UPR 9034, Populations, Genetique et Evolution, 91198 Gif sur Yvette Cedex, France; 2Universite Lyon-1, UMR CNRS 5558, 43 Bd du 11 Novembre, 69622 Villeurbanne, France; 3Universite Cadi Ayyad, Faculte desSciences et Techniques, BP 618, Marrakech, Morocco; ∗Author for correspondence (Phone: +33-1-69-82-37-09;Fax: +33-1-69-07-04-21; E-mail: capy@pge.cnrs-gif.fr)

Key words: alcohol and acetic acid tolerance, circadian rhythm, CO2 tolerance, egg production, extremetemperatures, flight capacity, male fertility, rate of development, starvation and desiccation, viability

Abstract

Numerous laboratory investigations have compared Drosophila melanogaster and D. simulans for various lifehistory traits and fitness related ecophysiological parameters. From presently available information, it is howeverdifficult to get a general comparative pattern describing the divergence of their ecological niches and understandingtheir demographic success. Two environmental factors seem however to have played a major role: temperatureand alcoholic resources. From an ecophysiological approach, D. simulans may be described as generally moresensitive to stresses; other results point to this species as more cold adapted than its sibling; in some cases, however,D. simulans may appear as better adapted to a warm environment. When investigated, ecophysiological traits showa lesser geographic variability in D. simulans than in D. melanogaster. Presently available information does notexplain the ecological prevalence of D. simulans in many places with a mild temperate or subtropical climate. Thisis presumably due to the fact that most comparisons have been done at a single, standard temperature of 25◦C.Comparative studies should be undertaken, spanning the thermal ranges of the two species, and the phenotypicplasticity of ecophysiological traits should now be considered.

Introduction

Drosophila melanogaster is a reference species for allkinds of genetic investigations, but it is also a wildliving species, which can be collected in all places,including oceanic islands, where climatic conditions(temperature and humidity) are compatible with its lifehistory characteristics. Indeed, this basically domesticspecies, often introduced and transported by man, isextremely differentiated into geographic races (David& Capy, 1988), many of them corresponding to shortor long range adaptations. D. melanogaster is thus areference for ecological genetic studies in Drosophila.

Its sibling D. simulans, which has diverged froma common ancestor about 2 Myr ago, exhibits numer-ous common features but also significant differences.

Both species are native to tropical Africa but are alsoable to proliferate under temperate climates. Both havebenefited from human transportations for colonizingnew places. There is however one difference sinceD. simulans is not found, so far, in all possible placeswith a convenient climate. For example in Caribbeanislands, it does not exist in Martinique and Guade-loupe, although it is found in the nearby island ofSt. Martin: human transportations are apparently notabsolutely efficient. Most surprisingly, D. simulansis still absent from continental favorable territories,which are in direct continuity with natural huge popu-lations. So D. simulans is absent from most continentalAsia and from West Africa. For the moment there is noplausible explanation for such a gap (see Lachaise &Silvain, 2004).

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Concerning their ecology, both species use sweetfermenting resources and can make huge popula-tions under different climates. In temperate and semi-tropical places, D. simulans is generally dominant,while D. melanogaster is more abundant in colderplaces: different thermal tolerances might play a sig-nificant role for explaining the abundance and distri-bution of the two species. Also a major, well-knowndifference concerns ethanol adaptation. D. melano-gaster, because of its high ethanol tolerance, is able touse artificial fermenting resources (e.g., grape residuesin wineries) while D. simulans cannot (see Alcoholand acetic acid tolerance).

Due to a recent common ancestry, both species areexpected to exhibit quite similar life history param-eters. This presumably explains the relative scarcityof precise investigations on D. simulans. As we shallsee in this review, there are many slight but signifi-cant differences which participate to the divergenceof the ecological niches. Most physiological compari-sons have been done at a single temperature (generally25◦C) while ecophysiological investigations spanninga broad thermal range are necessary for a better under-standing of species coexistence.

Several reviews have already compared the twospecies (Parsons, 1975, 1983; Parsons and Stanley,1981; David et al., 1983). In the present paper, we willshortly summarize the results of older investigationsand focus on more recently acquired information.

Viability

Viability is a major component of preadult fitness, andit is usually defined as the proportion of laid eggswhich reach the adult stage. It is itself a combinationof three components which can be easily measuredunder laboratory conditions: (1) egg hatchability, thatis, the proportion of eggs which produce young lar-vae; (2) larval viability, the proportion of young larvaewhich pupate; (3) pupal viability, the proportion ofpupae which produce adults.

So far as we know it, there is no major differ-ence between the two species. In D. melanogaster,egg hatchability is very high at the beginning of fe-male life and then decreases progressively with aging(David, Biémont & Fouillet, 1974). Similar resultswere obtained in D. simulans (Allemand, 1982). Lar-val and pupal viability are not much variable accordingto mother’s age, but may vary in a large proportion

according to experimental conditions, such as foodquality and larval density.

Overall viability (eggs producing adults) is also ameans to measure temperature adaptation, and majordifferences may be found between species (Cohet &David, 1980). The response curves generally show asub-rectangular shape, with a stable value over mostof the thermal range and a sharp decrease at bothends. A precise comparison was recently done onEuropean D. melanogaster and D. simulans (Pétavyet al., 2001a), which showed that between 14 and28◦C, overall viability was high in both species, closeto 80%. On the cold temperature side, a linear decreasewas observed in both species, with a complete mortal-ity at 10◦C. On the high temperature side, a significantdifference was observed between the two species,D. melanogaster being slightly more tolerant to heatthan its sibling. More precisely, it is generally easyto breed D. melanogaster at 31◦C, while D. simulansexhibits a very low viability at that temperature.

Rate of development

A classical observation, made by all investigators, isthat the development of D. simulans is slightly morerapid than that of D. melanogaster. This difference isillustrated in Table 1, which compares European andAfrican strains of the two species grown at 25◦C. Onaverage, the development of D. melanogaster is 9%longer than that of its sibling. D. melanogaster is alsoknown by the fact that the smaller sex (the male) hasa longer development. This occurs in a similar way inD. simulans. Indeed sexual dimorphisms, expressed asthe female/male ratio, are almost identical (Table 1),on average 0.976.

As all metabolic processes, duration of develop-ment is strongly dependent on temperature, and thisrelationship is a major information for estimating gen-eration time in insects. A precise comparison of thetwo species was recently made, studying populationsfrom France and Spain (Pétavy et al., 2001a). Con-firming previous data on D. melanogaster (see Davidet al., 1983), an increase in growth temperature result-ed in a decrease of duration over most of the thermalrange in the two species, reaching a minimum a fewdegrees below the upper viability limit. As usualin such investigations, the rate of development (i.e.,the reciprocal of duration) is generally considered(Figure 1). The response curves of the two spe-cies, as well as those of most insects (Brière et al.,

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Table 1. Comparison of duration of development (h) of Frenchand Congolese strains of D. melanogaster and D. simulansgrown at 25◦C (after Allemand, 1982). Each value is the meanof five different experiments

France Congo

D. melanogaster

Female 219.58 ± 1.06 223.04 ± 2.39

Male 225.66 ± 1.18 227.38 ± 1.83

Female/male ratio 0.973 ± 0.003 0.981 ± 0.004

D. simulans

Female 200.04 ± 0.88 205.04 ± 1.13

Male 205.34 ± 1.14 210.26 ± 0.87

Female/male ratio 0.974 ± 0.002 0.975 ± 0.002

Figure 1. Variation of the rate of development (reciprocal of dur-ation) as a function of growth temperature in D. melanogasterand D. simulans. For each species, two strains from France andSpain were investigated and the results were so similar that a singleaverage curve is shown.

1999), exhibit a smooth sigmoid increase, then reacha maximum followed by a sharp decrease.

The analysis of such a complex curve is difficultand numerous mathematical adjustments have beenproposed. Pétavy et al. (2001a) suggested that thedata could be described by a sum of two exponentialfunctions. One function describes, in a classical way,the rise of metabolic rate due to increasing tempera-ture. The other function, which has a negative value,describes the deleterious effects of heat which are ob-served above a critical temperature, identified by theinflection point of the sigmoid part of the curve, andwhich may be interpreted as a physiological optimum.

Table 2. Comparison of characteristic temperatures (◦C) duringthe development of the two species; strains from Bordeaux andCordobaa

D. melanogaster D. simulans

Developmental zero 8.1–8.2 7.7–7.8

Optimum temperature 22.0–22.4 21.2–21.4

Temperature of maximum 28.9–29.0 28.0–28.2

rate

a Notice the lesser values in D. simulans, suggesting an adaptationto lower temperatures.

Figure 2. Influence of growth temperature on the relative durationof development in the two species. Ordinates show, for each pairof geographic populations (Bordeaux and Cordoba), the ratio ofduration in D. melanogaster to the duration in D. simulans.

These two curves permit a precise estimate of char-acteristic temperatures, which are given in Table 2.Values for the developmental zero, the optimum tem-perature and the temperature of maximum rate arealways less in D. simulans. This suggests that thisspecie is more tolerant, or better adapted to cold thanD. melanogaster.

A general conclusion from the analysis of the re-action norms of duration of development as a functionof growth temperature is that we cannot define a stabledifference between the two species. Table 1 is validonly for a development at 25◦C. If we consider thewhole thermal range and compare the two speciesby calculating a ratio, we find that the relative dif-ference between the two species decreases monotoni-cally, in a curvilinear way, with increasing temperature(Figure 2). If we assume that a faster development isa selective advantage, we see that the advantage of

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D. simulans is maximum at low temperatures and thendecreases regularly, becomes null at about 27◦C andthen turns out into a disadvantage.

Female fecundity

Numerous investigations have analyzed egg produc-tion curves in D. melanogaster. In mated femalesunder good feeding conditions at 25◦C, the curves ex-hibit an overall triangular shape (David, Biémont &Fouillet, 1974). A sharp increase occurs during thefirst 4 days of life, reaching a maximum daily egg pro-duction, followed by a slow and almost linear decreaseup to the end of the laying period, at an age of a monthand half. In D. simulans, egg laying curves have a sim-ilar shape (Allemand, 1982). Whether the egg layingperiods in the two species have the same duration isnot well known and deserves further investigations. InD. melanogaster, at the age of maximum fecundity,the daily egg production is proportional to the ovariolenumber, each of them producing approximately twoeggs per day (David, 1970; Boulétreau-Merle et al.,1982). Allemand (1982) found that, in two recentlycollected strains of D. simulans, the maximum rateof egg production was only 1.2 per ovariole per day,although it was 1.6 in an old laboratory strain. Com-paring the rate of oogenesis in laboratory strains of twosibling species, R’Kha et al. (1997) found that eachovariole produced two eggs per day in D. simulans, butonly 1.1 in D. sechellia. These results suggest that eggproduction of D. simulans, measured under laboratoryconditions, could be better in strains kept for sometime in the laboratory than in recently collected pop-ulations, while such a phenomenon does not occur inD. melanogaster. More precise investigations shouldbe undertaken to address specifically this problem of apossible laboratory adaptation in the two species.

Male fertility

Numerous papers (e.g., Rice, 1998; Chippindale& Rice, 2001) have investigated male fertility inD. melanogaster, while similar data are lacking inD. simulans. That species, on the other hand, is knownto harbor a segregation–distortion system, in which Xand Y chromosomes are involved and which reducesthe viability of Y bearing sperm (see Jutier et al.,2004).

Figure 3. Comparison of male fertility in the two species grownat 21◦C. (A) Increase of progeny number over time when singlemales were kept with three females and transferred to fresh foodeach day. (B) Analysis of the individual progeny production curves(linear model). Intercept is the number of progeny produced by amale at ‘age 0’; slope describes the increase in progeny number overtime. Each point corresponds to a single male.

It has been known for a long time that malesof D. melanogaster grown at extreme temperatures,either heat or cold, are sterile due to a lack of func-tional sperm (David, Arens & Cohet, 1971; Cohet,1973) and that this phenomenon is reversible after areturn to a permissive temperature. Comparative stud-ies between the two species (Chakir et al., 2002) haverevealed a significant difference. D. simulans, with re-spect to male sterility, is more tolerant to cold thanD. melanogaster (male sterility average temperaturesare 12.2 and 13.2◦C in D. simulans and D. melano-gaster, respectively). A significant difference in op-posite direction is observed on the high temperatureside: sterility thresholds are 27.6 and 29.1◦C in D. sim-ulans and D. melanogaster, respectively. Such differ-ences, which have been observed in several Europeanand North African populations of the two species, sug-gest that D. simulans is better adapted to cold than itssibling.

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The same investigation (Chakir et al., 2002) re-vealed an unexpected difference, observed in controlflies reared at the optimum temperature of 21◦C. Inorder to analyze progeny production and male fer-tility, single males were mated with three femalesand transferred daily to fresh vials. These normalmales were immediately fertile but the number ofprogeny increased linearly over time in both species(Figure 3(A)). It was supposed that progeny produc-tion was related, in a complex way (including spermstorage and remating) with sperm production. Theprogeny number was however much less in D. sim-ulans than in D. melanogaster, and also the rate ofincrease over time was two times less (Figure 3(B)).These observations, which should be extended, sug-gest that D. simulans males produce much less spermthan D. melanogaster males.

Tolerance to extreme temperatures

Extreme temperatures may be defined as temperatureslaying outside the normal thermal range (12–31◦C)which permits a complete development from egg toadult. Most investigations have considered the adultstage, and various methods may be used to analyzethe response to a thermal stress:mortality and the de-termination of a lethal time 50 (LT 50) (e.g., Watson& Hoffmann, 1996); knockdown time (adults fallingdown in a vertical tube kept at a stressing tempera-ture) (e.g., Jenkins & Hoffmann, 1994); recovery timefrom a temperature induced coma (e.g., David et al.,1998). For the larvae and pupae, daily thermoperiodicregimes may be used (e.g., Pétavy et al., 2001a).

The tolerance of adults to heat stress generally in-volves temperatures above 36◦C. The knockdown timeat 37◦C was found to be 1.7 min for D. simulans and7.2 min for D. melanogaster (Jenkins & Hoffmann,1994). Such a knockdown implies that treated adultsenter a narcosis stage or heat coma. Investigationson mortality also point to the same conclusion, thatis, a higher sensitivity of D. simulans to heat stress.For example, a treatment of 14 min at 39◦C wasused to induce about 50% mortality in D. simulans,while 28 min were necessary to get a similar result inD. melanogaster (Hoffmann & Watson, 1993).

For analyzing adult cold tolerance, temperaturesaround 0◦C are generally used. It has been knownfor a long time (Parsons, 1983) that survival timeof D. simulans at −1◦C was much less than that ofD. melanogaster. More recent experiments, however,

using as a stressing temperature of −2◦C, failed toevidence such a clear cut difference between spe-cies, and both exhibited a much higher cold tolerancewhen acclimated at 13◦C for a week (Hoffmann &Watson, 1993). In other experiments, the two spe-cies were submitted to selection for increasing survivaltime at −2◦C (Watson & Hoffmann, 1996). ControlD. simulans adults survived slightly less than those ofD. melanogaster, and a lesser response to selectionwas also observed. Recovery time from chill comainduced by a treatment of 16 h at 0◦C has been com-pared in tropical and temperate populations of the twospecies (Gibert et al., 2001). In both species, tropicalflies recovered more slowly than temperate ones, andD. simulans adults wake up less rapidly than those ofD. melanogaster. Again this criterion points to a lessercold tolerance in D. simulans.

Drosophila larvae, treated at extreme temperatures(e.g., 0 or 40◦C) for a few hours would probablysurvive, and an LT 50 could be calculated. We arenot aware, up to now, of any investigations usingsuch extreme conditions upon larvae. The effects ofdaily alternating temperatures, with a lower and higherphase of 12 h each, are however well documented inthe two species (Pétavy et al., 2001a, b, 2002). Forexample, a significant viability (20%) was observedusing a thermal regime of 5–15◦C, while at a constanttemperature of 10◦C, viability is null. In other words,the deleterious effects of cold (5◦C) can be somehowrepaired by spending 12 h a day at 15◦C. In a similarway, adult production was possible with a 10–32◦Calternating regime, and in that case D. melanogasterwas clearly more tolerant. Phenotypic variations ofadults (body size and body pigmentation) were alsoanalyzed. As a rule, phenotypic values under alter-nating regimes were less than those obtained at anequivalent constant temperature. Also the phenotypicvariability was increased by alternating regimes, andin a way proportional to the magnitude of the stress.There were however no clear cut differences betweenthe two species.

Starvation and desiccation tolerance in adults

Both starvation tolerance (survival without food inhumid conditions) and desiccation tolerance (survivalwithout food in dry conditions) are considered as sig-nificant parameters for explaining species distributionand ecology (David et al., 1983). Presumably the bestargument for relating these traits to fitness has been the

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observation of regular but opposite latitudinal clinesamong populations of D. melanogaster and other spe-cies in the Indian subcontinent (Karan et al., 1997;Karan & Parkash, 1998), although those observa-tions were not confirmed on Australian populations(Hoffmann et al., 2001).

Starvation tolerance, as measured in the labora-tory, is related to the amount of reserves, mostly lipids(David, Cohet & Fouillet, 1975a, b) and to ambienttemperature (van Herrewege & David, 1997; Karan& David, 2000). The interpretation of the clines isbased on the fact that, for surviving 24 h without food,an adult will need a double quantity of reserves at25 than at 15◦C. Comparisons between D. melano-gaster and D. simulans are less numerous. Studies oftemperate populations have generally found that, inflies deprived of food but provided with water sur-vival time was longer in D. melanogaster (Yamamoto& Ohba, 1984b; Cohan & Hoffmann, 1989; vanHerrewege & David, 1997). This difference was how-ever not confirmed when comparing Afrotropical pop-ulations (van Herrewege & David, 1997). Starvationtolerance is known to respond rapidly to directionalselection (Chippindale, Chu & Rose, 1996). It mayalso change rapidly under laboratory culture condi-tions and this could explain some negative resultsin D. melanogaster (Robinson, Zwaan & Partridge,2000). More extensive investigations, consideringonly freshly sampled natural populations, are neededfor a precise comparison of the two species and thesearch for latitudinal clines or local adaptations.

Desiccation tolerance has been more extensivelystudied in the two species. A general conclusion fromAustralian or Japanese populations is that D. simulansis much more sensitive to desiccation than its sib-ling (McKenzie & Parsons, 1974a; Prince & Parsons,1977; Parsons & Stanley, 1981; Yamamoto & Ohba,1984b; Hoffmann & Parsons, 1991, 1993). Therewas also a tendency for the populations from dryerlocalities to survive longer in both species. Some eco-logical observations seem also to confirm the greatersensitivity of D. simulans to dryness. For examplein a Tunisian oasis, banana traps generally collect-ed more D. melanogaster. D. simulans however wasmuch more abundant on rainy days (unpublished ob-servations). The fact that such changes could be ob-served over successive days suggests a behavioral, nota demographic difference. More recent comparisonsbetween the two species (van Herrewege & David,1997) failed however to evidence a species differ-ence. At 25◦C, French and African strains of the two

species were compared, and all exhibited the samesurvival time of 18 h. Again, the danger of comparinglaboratory strains may be emphasized.

Alcohol and acetic acid tolerance

A huge literature exists in D. melanogaster concerningits capacity to tolerate alcohol and its polymorphismat the Adh (alcohol dehydrogenase) locus, and evena simplified overview of present knowledge would befar over the scope of this paper. With respect to thespecies comparison, we may point out that D. melano-gaster is generally tolerant to alcohol and polymorphicat the Adh locus, and that both traits exhibit latitudinalclines (David & Bocquet, 1975; van Delden, 1982;Parsons & Stanley, 1981; Parsons, 1983). D. simu-lans, on the other hand, is monomorphic at the Adhlocus, more sensitive to alcohol and does not show anyclinal variation in alcohol tolerance (David & Bocquet,1975). The low tolerance to ethanol of D. simulanspresumably explains its incapacity to use man madeartificial fermentations (McKenzie & Parsons, 1974b)in temperate countries. Laboratory experiments haveshown, however that D. simulans could respond to dir-ectional selection and increase its tolerance to ethanol,estimated either by measuring adult mortality (Davidet al., 1977) or knockdown time (Cohan & Hoffmann,1989).

The significance of the cline for alcohol tolerancein D. melanogaster is far from clear. It is not explainedby the parallel increase in frequency of the more ac-tive Adh F allele (Chakir et al., 1993) but more likelyby other mechanisms. The lack of clinal variation forethanol tolerance in D. simulans (David & Bocquet,1975) is by itself surprising since other traits exhibitclines (Capy, Pla & David, 1993; Gibert et al., 2004),and more interestingly, several other species, generallymonomorphic at the Adh locus, exhibit an increase oftolerance with increasing latitude. Such a trend hasbeen observed in D. virilis (David & Kitagawa, 1982),in D. hydei (Chakir et al., 1995), in several species ofthe Indian subcontinent (Parkash, Neena & Shamina,1993; Parkash & Vandna, 1995) and in D. lebanon-ensis (unpublished observation). Among geographicpopulations of D. melanogaster, and also among dif-ferent species, ethanol tolerance is correlated to aceticacid tolerance (Chakir et al., 1993, 1996). The factthat selection for increasing acetic acid tolerance inD. melanogaster induced a correlated response in eth-anol tolerance suggested that ADH activity was not

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very important, and that the enzyme converting ace-tate into acetyl-Co-A (ACS) was more important incontrolling the metabolic flux (Chakir et al., 1993,1995). Field observations in southern France (Gravot,2000) have shown that the yeast species (Saccharo-myces cerevisiae) which is responsible for alcoholicfermentation in wineries, is almost completely absentin outdoor natural resources, including grapes. Dro-sophila larvae are using other yeast species which areable to convert directly sugars into acetic acid. It isthus possible that, in nature, acetic acid might bea stronger and more general environmental selectivefactor than alcohol. This does not explain, howeverwhy, so far as we know it, D. simulans does not showany clinal change in acetic acid tolerance.

Flight capacity and wing beat frequency

D. simulans is now well known (see Gibert et al.,2004) for having relatively shorter wings thanD. melanogaster, a phenomenon which is easily evi-denced by calculating the wing length/thorax lengthratio. It is also known that both wing and thorax lengthare plastic traits and the shapes of the reaction norms,as a function of growth temperature, are similar in bothspecies, corresponding to concave response curves(Morin et al., 1996; Gibert et al., 2004). Wing/thoraxratio is also a plastic trait and quite unexpectedly hasa monotonically decreasing reaction norm. It was alsoshown (Morin et al., 1999) that tropical populationsof both species have a lesser wing/thorax ratio thantemperate species. In summary, adaptation of geo-graphic populations to warm environments results ina lesser ratio which is genetically determined. For agiven genotype, development at a higher temperaturealso results in a lesser ratio. This parallelism betweenplasticity and latitudinal clines is a classical argumentfor assuming the adaptive significance of a trait.

The wing/thorax ratio itself has presumably no di-rect relationship with fitness, but is related to anotherphenotype, wing loading, which is a likely target ofnatural selection (Stalker, 1980). This problem wasextensively analyzed in D. melanogaster and D. sim-ulans by Pétavy et al. (1997). A comparison of fliesof the same strain grown at different temperatures re-vealed a linear, negative relationship between wingloading and the ratio. At any temperature, however,wing loading was much higher in D. simulans. A lin-ear, positive relationship was also evidenced betweenwing loading and wing beat frequency of tethered flies.

In other words, due to its smaller wings, D. simulansis obliged to have a higher beat frequency.

The pros and cons of such a marked differencebetween the two species are not clearly understood andfor the moment, we may only speculate (see Pétavyet al., 1997). From general considerations on flightdynamics in insects, it is likely that a higher beat fre-quency permits a greater speed, but a greater speedis accompanied by a higher metabolic rate and thus ashorter possible flight duration. Several indirect argu-ments suggest that D. simulans has a higher dispersalcapacity than D. melanogaster. We do not know, how-ever, whether this capacity is related to wing loadingand flight speed.

Miscellaneous environmental factors(light, CO2)

Light is a major environmental factor for drosophilids.A classical observation is that D. simulans is more tol-erant to higher light intensities, while D. melanogasteris more tolerant to dark conditions, and even completedarkness (Parsons, 1975; Kawanishi & Watanabe,1978). Behavioral preferences may correlate withphysiological differences, and especially progeny pro-duction. In D. melanogaster, it was shown (Allemand,Cohet & Savolainen, 1976) that egg production washigher under complete darkness than under permanentlight or a photoperiod LD 12-12. Similar investi-gations on D. simulans (Allemand, 1982) evidencedsome differences among strains, but also an overalltendency to produce more progeny under completedarkness. In other words, there was no clear differencebetween the two species.

A well-established and consistent behavioral dif-ference between the two species is the tendency forD. melanogaster to concentrate into human construc-tions, but for D. simulans to avoid entering suchbuildings (Fuyama & Watada, 1981; Rouault & David,1982; Yamamoto, Fuyama & Watada, 1985; and un-published observations in various parts of the world).Reasons for this discriminating behavior are not clearbut might include light intensity, a direct percep-tion of what is a building, and also alcoholic vaporswhen D. melanogaster concentrates in wine cellars(McKenzie & Parsons, 1972).

Under photoperiodic conditions (LD 12:12),D. melanogaster exhibits an oviposition rhythm witha maximum laying at the beginning of the scoto-phase. This rhythm, when analyzed in numerous geo-

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Figure 4. Egg production under a photoperiodic regime (LD 12:12) in the two species. (A) Hourly percentage of eggs produced by twoequatorial strains (Gabon) of the two species. Darkness, indicated by a dark bar, starts at 8 p.m. and stops at 8 a.m. (B) Variation of thecircadian rhythm according to latitude of origin in the two species. Ordinate shows the percentage of eggs laid during the first two hours of thescotophase. Notice the opposite clines in the two species. r: correlation, b: regression as a function of latitude. Significance levels: ∗∗P < 0.01;∗∗∗P < 0.001.

graphic populations, revealed a clear cut genetic cline(Allemand & David, 1976). More precisely the mag-nitude of the peak of egg laying was maximum in highlatitude populations and decreased linearly toward theEquator. It was argued that populations living at theEquator did not experience major changes in the nat-ural photoperiod, and were thus quite insensitive tothe day–night transition. Similar investigations havebeen undertaken in D. simulans and results are shownFigure 4. Equatorial populations do not exhibit a clear

oviposition peak at the beginning of the scotophase,in major contrast with D. melanogaster (Figure 4(A)).When populations of higher latitudes are studied, thereis a slight tendency to get a more marked ovipositionpeak. The reverse cline found in D. simulans clearlycontrasts the two species (Figure 4(B)), and points tothe difficulty of general adaptive interpretations.

Sweet resources, which are the natural habitatof the two species, undergo a fermentation whichproduces large amounts of CO2. This production is

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especially noticeable when the volume of the re-sources is important, for example, grape residues inwine cellars during vintage time. D. melanogaster isknown to make huge populations in cellars, whileD. simulans is practically absent from such habi-tats (McKenzie & Parsons, 1974b; Parsons, 1983).The traditional interpretation is that D. melanogasteris able to use such alcoholic resources because ofits high ethanol tolerance. But, for an optimal useof these resources, a better tolerance to high levelsof CO2 in the atmosphere could also be expected.Experimental results (Boulétreau, Fouillet & Sillans,1984; Boulétreau-Merle & Sillans, 1996) revealedsignificant effects due to an atmospheric increase inCO2 concentration. More precisely, low concentra-tions (up to 5%) decreased viability, increased theduration of development, with similar effects on bothspecies. Higher concentrations (10–20%) evidencedhowever a difference between the two species, D. sim-ulans being more sensitive to CO2 than its sibling(Boulétreau, Fouillet & Sillans, 1984). For example,at a concentration of 20%, viability decreased by 43%in D. simulans, but by only 11% in D. melanogaster.Interestingly, a similar high concentration of nitrogenhad no effect.

Discussion and conclusions

As previously stated, D. melanogaster has been farmore investigated than its sibling and is thus bet-ter known. So, for any comparison, we may alwaysconsider D. simulans and compare it to its referencesibling. The survey of the extant comparative literatureshows that, for almost all traits, a significant differenceis evidenced, which is obviously a contribution to thedivergence of the ecological niches. Instead of pro-ducing a mere catalogue of differences, investigatorsgenerally tried to find some coherent interpretations,for example, which species is more sensitive to stress,or more sensitive to cold. As shown below, experi-mental results are very difficult to incorporate into asingle general interpretation.

D. simulans as a species more sensitiveto environmental stresses

This is a general interpretation of Australian investi-gators (Parsons, 1975, 1983; Parsons & Stanley, 1981;Hoffmann & Parsons, 1991). In comparative studies,D. simulans adults have revealed a shorter survival to

a cold or heat treatment, or to desiccation. The muchlesser tolerance to ethanol and acetic acid, and alsoto high concentrations of CO2, which are discussed inAlcohol and acetic acid tolerance and Miscellaneousenvironmental factor (light, CO2), point to the sameconclusion. For starvation tolerance, however, there isapparently a less clear cut difference (Starvation anddesiccation tolerance in adults) and also for the ef-fects of darkness (Miscellaneous environmental factor(light, CO2)). Numerous other relevant environmentalstresses could be considered, in future investigations,for example, the tolerance to various pesticides andpollutants, the sensitivity to larval crowding or tointerspecific competition.

D. simulans is far less differentiatedinto geographic populations

This is the major and most general conclusion ofmorphometrical analyses (see Gibert et al., 2004).Indeed, the two species generally exhibit similar lat-itudinal clines, but the slopes of the clines are alwaysmuch less in D. simulans. Physiological comparisons,when available, accentuate this difference: with re-spect to ethanol and acetic acid tolerance, there areclear cut clines in D. melanogaster, but nothing inD. simulans. For the oviposition rhythm there areindeed opposite clines, positive or negative, in thetwo species. Since most clines are considered to bea consequence of a temperature adaptation, and sinceboth species, native to tropical Africa (Lachaise et al.,1988) have progressively colonized colder temperatecountries, we could assume that D. simulans used ahigher plastic response instead of a genetic variabil-ity. Comparative studies, however, have shown that,for morphometrical traits at least, the two speciesare very plastic, but generally exhibit similar levelsof plasticity (Gibert et al., 2004). So, for the mo-ment, the lesser differentiation of D. simulans intogeographic races remains a kind of mystery. In spite ofits apparent lesser level of genetic adaptation, D. sim-ulans is in many places much more abundant thanD. melanogaster. This is especially true in many coun-tries with a warm temperate (e.g. Mediterranean) orsubtropical climate (Louis, 1983; Lachaise & Silvain,2004).

D. simulans as a cold adapted, heatsensitive species

This seems to be the main conclusion of the malesterility thresholds recently analyzed (Chakir et al.,

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2002). D. simulans can be grown permanently at 12◦Cwhile the limit for D. melanogaster is 13◦C. Cor-responding values on the high temperature side are27 and 29◦C for D. simulans and D. melanogaster,respectively. Studies on Viability and Rate of devel-opment point to the same conclusion. For example,D. simulans has a lower developmental zero, a loweroptimum temperature, a lower temperature for itsmaximum developmental rate, a lower viability at veryhigh temperatures. D. simulans develops faster thanD. melanogaster at low temperatures, but the reverseis true above 25◦C (Figure 2). Phenotypic plasticityof size-related traits show that the maximum size isobserved at a lower temperature in D. simulans thanin D. melanogaster (Gibert et al., 2004). Such a phe-nomenon, also found when comparing more distantspecies, suggests a better adaptation to cold in D. sim-ulans. Finally, behavioral studies have shown that thepreferred temperature in a thermopreferendum gradi-ent was slightly lower in D. simulans (20.5◦C) thanin D. melnaogaster (21.3◦C) (Yamamoto & Ohba,1984a).

D. simulans as a warm adapted, coldsensitive species

In opposition with the previously described features,some observations suggest that D. simulans might dobetter in a warm environment. For example the max-imum ovarian size, which is presumably related tofemale fitness, is found at a higher temperature inD. simulans than in D. melanogaster (Gibert et al.,2004). In both species, a higher developmental tem-perature results in a decrease of the wing/thorax ratioor, in other words, in an increase of the wing load-ing. There are arguments (see Pétavy et al., 1997;Morin et al., 1999) suggesting that a higher wingloading is an adaptation to fly in a warm environ-ment. A regular observation (Morin et al., 1996, 1999;Pétavy et al., 2001b) is that, for any growth temper-ature, D. simulans exhibits a lesser wing/thorax ratio(a higher loading) than its sibling. Finally the recoverytime from a cold shock at 0◦C is longer in D. simu-lans (Gibert et al., 2001): in that respect, D. simulansappears to be ‘more tropical’ than its sibling.

D. simulans has a lesser proliferative capacity

D. simulans and D. melanogaster are both colonizingspecies with a high rate of natural increase (r param-eter): this results in a high demographic success and

the building of huge populations when conditions (cli-mate and resources) are favorable. D. simulans seemshowever to have a lesser rate of increase: the slightadvantage provided by a faster development is in-sufficient to compensate a lesser egg production, asshown by a lower ovariole number and possibly also,a lesser rate of oogenesis. Also the lesser productionof sperm in D. simulans (Chakir et al., 2002) mightbe an argument pointing to the same direction, al-though the male gametes production is generally notconsidered when calculating the intrinsic rate of in-crease (r). It is thus not surprising that in populationcages, D. melanogaster generally eliminated D. simu-lans in competition experiments (Tantawy & Soliman,1967; Montchamp-Moreau, 1983). This seems to bethe rule at 25◦C, but not at lesser temperatures. In-deed, at temperatures lower than 20◦C, D. simulansoften does better than its sibling (Tantawy & Soliman,1967; Montchamp-Moreau, 1983), and a competitionat 12◦C would eliminate D. melanogaster in one gen-eration due to male sterility. In field studies in southernFrance (Gravot, 2000), it has been found that, in sum-mer, D. melanogaster outnumbered D. simulans butthat the proportion of D. simulans increased with de-creasing temperature, and could make more than 80%of the total catch in autumn, suggesting a better fitnessat lower temperatures. A similar observation (D. sim-ulans becoming more abundant than D. melanogasterin autumn) was also found in Australia (McKenzie &Parsons, 1974b; Nielsen & Hoffmann, 1985). Obvi-ously, the proliferative capacity (r parameter) of thetwo species should be investigated experimentally atdifferent temperatures and especially below 25◦C.

Conclusions and perspectives

Ecophysiological comparisons have evidenced manysignificant differences between the two sibling spe-cies. However, most of the differences (e.g., a lessertolerance to environmental stresses) point to somekind of physiological inferiority of D. simulans, andthus do not explain the prevalence of that speciesunder mild temperate climates. The likely interpre-tation is that most comparisons have been done usinga single standard temperature of 25◦C. In temperatecountries, the average temperature of summer monthsis generally less than 25◦C and investigations mustbe undertaken at lower temperatures. With respect toD. melanogaster, D. simulans has the disadvantage tobe unable to use man-made, alcohol-rich resources,such as grape residues during vintage time. This is

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due to its low ethanol tolerance (see Alcohol andacetic acid tolerance) but also to its strange behaviorof avoiding to enter human constructions (Rouault &David, 1982). In open habitats, such as orchards dur-ing a cool season, D. simulans is generally prevalentand this still remains to be understood and explained.The likely explanation presumably resides in the plas-ticity of physiological traits. Phenotypic plasticity isnow well described for morphometrical traits. A sim-ilar analysis remains to be done for most physiologicaltraits and fitness parameters.

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