Differential responses to artificial selection on oviposition site preferences in Drosophila melanogaster and D. simulans

The preference–performance relationship in plant–insect interactions is a central theme in evolutionary ecology. Among many insects, eggs are vulnerable and larvae have limited mobility, making the choice of an appropriate oviposition site one of the most important decisions for a female. We investigated the evolution of oviposition preferences in Drosophila melanogaster Meigen and Drosophila simulans Sturtevant by artificially selecting for the preference for 2 natural resources, grape and quince. The main finding of our study is the differential responses of D. melanogaster and D. simulans. Although preferences evolved in the experimental populations of D. melanogaster, responses were not consistent with the selection regimes applied. In contrast, responses in D. simulans were consistent with expectations, demonstrating that this species has selectable genetic variation for the trait. Furthermore, crosses between D. simulans divergent lines showed that the genetic factors involved in grape preference appear to be largely recessive. In summary, our artificial selection study suggests that D. melanogaster and D. simulans possess different genetic architectures for this trait.     

ACCLIMATION,CROSS-GENERATION EFFECTS,AND THE RESPONSE TO SELECTION FOR INCREASED COLD RESISTANCE IN DROSOPHILA

The way populations respond to selection can be altered when populations are acclimated prior to selection. To examine this possibility, the responses of replicate lines of Drosophila melanogaster and D. simulans to selection for increased resistance to cold were compared. Flies were selected without hardening or after they had been hardened by holding them at 4°C for one hour. The selection response in both species was much greater when flies were not cold-hardened. Cold resistance in both sets of selected lines reached a plateau after a few generations. Surprisingly, continued selection for increased resistance resulted in decreasing levels of resistance. This decrease was no longer evident after selection had been relaxed for a generation, suggesting cross-generation effects. The magnitude of the cross-generation effects increased with additional generations of selection. Cross-generation effects were also detected for fitness components. Relaxing selection for a generation increased fecundity, weight, viability, and development time. Comparisons of relaxed lines and control lines indicated that only fecundity was influenced by selection. Both sets of selected lines had a lower fecundity than control lines. Crosses between control and selected lines and among replicate selected lines indicated that this decrease in fecundity was not associated with inbreeding. The direct and correlated responses to selection for cold resistance can therefore be influenced by acclimation and cross-generation effects.     

Comparative population genomics of latitudinal variation in Drosophila simulans and Drosophila melanogaster

Examples of clinal variation in phenotypes and genotypes across latitudinal transects have served as important models for understanding how spatially varying selection and demographic forces shape variation within species. Here, we examine the selective and demographic contributions to latitudinal variation through the largest comparative genomic study to date of Drosophila simulans and Drosophila melanogaster, with genomic sequence data from 382 individual fruit flies, collected across a spatial transect of 19 degrees latitude and at multiple time points over 2 years. Consistent with phenotypic studies, we find less clinal variation in D. simulans than D. melanogaster, particularly for the autosomes. Moreover, we find that clinally varying loci in D. simulans are less stable over multiple years than comparable clines in D. melanogaster. D. simulans shows a significantly weaker pattern of isolation by distance than D. melanogaster and we find evidence for a stronger contribution of migration to D. simulans population genetic structure. While population bottlenecks and migration can plausibly explain the differences in stability of clinal variation between the two species, we also observe a significant enrichment of shared clinal genes, suggesting that the selective forces associated with climate are acting on the same genes and phenotypes in D. simulans and D. melanogaster.     

Ecological differences and competitive interaction between Drosophila melanogaster and Drosophila simulans in small laboratory populations

Summary In interspecific competition studies, some cases of apparent change in competitive ability have been reported. But the change in competitive outcome could equally well be due to character displacement. As a preliminary to studies of the effects of association of D. melanogaster (yellow white mutant strain) and D. simulans (vermilion mutant strain), the nature and extent of ecological differences between them, and the nature of their competitive interaction was studied. Differences between the strains were shown for oviposition site preferences, and for larval and pupal distribution. In pure species cultures, simulans showed a greater preference than melanogaster for oviposition in the center of the medium surface. In mixed populations, simulans had an increased preference for this oviposition site, where melanogaster was at low frequency. D. simulans larvae utilized the lower half of the medium to a significantly greater extent than did melanogaster. At low density (5 pairs of parents) in pure species cultures, 68.7% of simulans pupae were on the medium surface. As parental numbers increased, this proportion decreased. The distribution of melanogaster pupae was quite different, with only 8 to 12% on the medium at all densities. But the remaining pupae tended to occur higher on the cylinder wall as parental numbers increased. The competitive interaction changed during the developmental period. At four and eight days after culture initiation, simulans appeared superior, while for total adult progeny production, melanogaster was slightly superior. These strans of the two species were not ecologically equivalent.     

THE GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F₁ diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F₁ diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F₁ males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.     

Genetics and the environment in interspecific competition: a study using the sibling species Drosophila melanogaster and Drosophila simulans

The outcome of interspecific competition of two closely related species may depend upon genetic variation in the two species and the environment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanogaster and D. simulans, is usually investigated using longterm laboratory stocks that often have mutant markers that distinguish them. To examine competition in flies that genetically more closely resemble flies in nature, we utilized freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory conditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in competitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measures of productivity and developmental time did not explain the differences in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influenced competitive ability. When lines of flies were combined, the outcome of competition was generally consistent with competitive outcomes between pairs of lines. In several cases, the combination of lines performed better than the best of the constituent lines, suggesting that competitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific competition.     

OVIPOSITION SITE SELECTION BY DROSOPHILA MELANOGASTER AND DROSOPHILA SIMULANS

The effects of texture and larval residues in the medium on oviposition site selection (OSS) by Drosophila melanogaster and Drosophila simulans were studied. Drosophila melanogaster laid over 95% of its eggs in sieved medium (vs. unsieved medium); D. simulans laid all of its eggs in sieved medium. Surgical removal of antennal segments, and of fore-, mid-, or hindtarsi did not affect this result, indicating that sense organs involved in discriminating between sieved and unsieved medium are not confined to only one of the tested structures. In a “multiple choice” experiment, females were allowed to lay eggs in sieved medium of three types: unconditioned (fresh) medium, medium conditioned by D. melanogaster larvae (i.e., medium containing larval residues of D. melanogaster), and medium conditioned by D. simulans larvae. This choice experiment was performed with D. melanogaster and with D. simulans, using three densities of females (10, 20, and 40 per experimental unit). Both species laid more eggs in unconditioned medium than in either of the conditioned media, and density had no effect. D. melanogaster laid more eggs near the edges of food patches than in the center, whereas D. simulans showed no preference for edge or center. Under crowded conditions, both species survived at a higher rate in conditioned media (egg-to-adult survival) than in unconditioned medium, leading to the anomalous conclusion that females of these species seem not to maximize the survival of their offspring. This anomaly was partially resolved by the finding that medium already containing larvae gave lower survival rates than unoccupied medium.     

Seasonal variation in life history traits in two Drosophila species

Seasonal environmental heterogeneity is cyclic, persistent and geographically widespread. In species that reproduce multiple times annually, environmental changes across seasonal time may create different selection regimes that may shape the population ecology and life history adaptation in these species. Here, we investigate how two closely related species of Drosophila in a temperate orchard respond to environmental changes across seasonal time. Natural populations of Drosophila melanogaster and Drosophila simulans were sampled at four timepoints from June through November to assess seasonal change in fundamental aspects of population dynamics as well as life history traits. D. melanogaster exhibit pronounced change across seasonal time: early in the season, the population is inferred to be uniformly young and potentially represents the early generation following overwintering survivorship. D. melanogaster isofemale lines derived from the early population and reared in a common garden are characterized by high tolerance to a variety of stressors as well as a fast rate of development in the laboratory environment that declines across seasonal time. In contrast, wild D. simulans populations were inferred to be consistently heterogeneous in age distribution across seasonal collections; only starvation tolerance changed predictably over seasonal time in a parallel manner as in D. melanogaster. These results suggest fundamental differences in population and evolutionary dynamics between these two taxa associated with seasonal heterogeneity in environmental parameters and associated selection pressures.     

ADAPTATION AND SPECIALIZATION IN A TWO-RESOURCE ENVIRONMENT IN DROSOPHILA SPECIES

We assayed two components of performance (development time and survivorship), on food medium with and without ethanol, in laboratory populations of Drosophila simulans and D. melanogaster for which ethanol-medium was a novel food resource. These assays were done before and after 12 generations of rearing in either one-(regular medium only) or two-resource (regular medium and ethanol medium) environments. Initially, D. simulans was highly susceptible to ethanol, whereas D. melanogaster was relatively unaffected. After 12 generations in the two-resource environment, D. simulans showed significantly improved mean performance on ethanol medium; mean performance of D. melanogaster did not significantly change. Variation among families for both traits was higher on ethanol medium in D. simulans. Variation in D. melanogaster was not significantly affected by ethanol level, suggesting that resource quality was more important than novelty per se. In both species, the least variation was seen in populations after 12 generations in the two-resource environment. For development time in D. simulans, the decrease in variation was largely due to reduced variation within families, suggesting the evolution of canalization. Development time on the two media was not negatively correlated. In D. simulans, correlations measured before and after the experiment were not heterogeneous, suggesting that trade-offs in performance did not block diet expansion. In D. melanogaster, correlations became significantly less positive after 12 generations in the two-resource environment, supporting the view that correlations between performance on different resources may become less positive over time through selection.     

Alcohol tolerance: An ecological parameter in the relative success of Drosophila melanogaster and Drosophila simulans

Summary Laboratory experiments have shown D. melanogaster adults to be more tolerant to alcohol in the environment than D. simulans, with the females being more tolerant than the males of their species. Larval development on alcohol supplemented media also demonstrated an increased tolerance by D. melanogaster although the effect was not as clear cut as for the adult survival. Oviposition choice experiments demonstrated a marked rejection of alcohol impregnated laying sites by D. simulans when compared to standard medium sites. D. melanogaster showed a slight preference for alcohol supplemented sites.Collections in the maturation cellar of a vineyard produced, with the exception of a single D. simulans fly, entirely D. melanogaster adults while larvae and pupae from the cellar were also all D. melanogaster. Away from the alcohol resource, outside the cellar, both species were collected with D. simulans being the more common. However, the outside distribution of the two species was affected by alcohol fumes during vintage, as was the distribution of the sexes of D. melanogaster, with the more tolerant species or sex being closer to the source. The field results were thus in agreement with the laboratory predictions that D. melanogaster is better able to utilize an alcohol resource than D. simulans.     

The effects of acclimation and rearing conditions on the response of tropical and temperate populations ofDrosophila melanogaster andD. simulans to a temperature gradient (Diptera: Drosophilidae)

The effects of rearing and acclimation on the response of adultDrosophila to temperature were investigated in a gradient.D. melanogaster flies preferred a higher mean temperature and were distributed over a wider range of temperatures thanD. simulans flies. Acclimating adults at different temperatures for a week did not influence the response of either species. Adults reared at 28°C as immatures had a lower mean preference than those reared at cooler temperatures, suggesting that flies compensated for the effects of rearing conditions. Adults from tropical and temperate populations ofD. melanogaster andD. simulans did not differ in the mean temperature they preferred in a gradient, suggesting little genetic divergence for this trait within species. The species differences and environmental responses may be related to changes in optimal physiological conditions for the flies.     

Variations of male cuticular hydrocarbons with geoclimatic variables: an adaptative mechanism in Drosophila melanogaster?

7-tricosene (7T) and 7-pentacosene (7P) are the major components of cuticular hydrocarbons in Drosophila simulans and D. melanogaster males. A chemical study of 16 isofemale lines of D. melanogaster sampled at the first and eighth generations in laboratory conditions showed the stability of chromatographical profiles. Then a large scale study of male 7T/7P polymorphism was performed with 85 populations of D. melanogaster and 29 of D. simulans collected all over the world. There were significant correlations of the values of the balanced ratio (7T − 7P)/(7T + 7P) with geo-climatic parameters, such as latitude, longitude, mean temperature, temperature range and vapour pressure. Parallel variations were also reported for the homologous linear alkanes (23 and 25 Carbon atoms) but not for the longer branched alkanes (27 and 29 Carbon atoms). No correlation was significant for the D. simulans populations studied. In this species a similar polymorphism of 7T/7P was found but restricted to a few populations from West Equatorial Africa. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of species frequency,temperature regime and previous development condition on relative competitive ability between Drosophila melanogaster and Drosophila simulans

Some fitness components of Drosophila melanogaster and D. simulans were measured in control and inter-specific competition tests. The effects derived from different relative frequencies of the competitors were examined under a factorial scheme with two temperatures, 21 °C or room temperature, and with adults developed in mixed- or pure-species cultures. D. melanogaster appeared as a strong competitor and outnumbered D. simulans in all the culture conditions. This was because intraspecific competition was stronger than inter-specific competition for D. melanogaster whereas the reverse occurred for D. simulans. In competition, the productivity of both species generally appeared as frequency-dependent, although density-dependent productivity seems to be a more accurate explanation. D. simulans was very sensitive to variations of laboratory conditions. Room temperature and previous development with D. melanogaster were more favorable for D. simulans than 21 °C and previous development in pure cultures. These factors did not substantially affect D. melanogaster, which showed a greater ability of adaptation to laboratory conditions than its sibling D. simulans.     

Sexual Isolation Between Drosophila Melanogaster, D. Simulans and D. Mauritiana: Sex and Species Specific Discrimination

The sexual isolation among the related species Drosophila melanogaster, D. simulans and D. mauritiana is asymmetrical. While D. mauritiana males mate well with both D. melanogaster and D. simulans females, females of D. mauritiana discriminate strongly against males of these two species. Similarly, D. simulans males mate with D. melanogaster females but the reciprocal cross is difficult. Interspecific crosses between several populations of the three species were performed to determine if (i) males and females of the same species share a common sexual isolation genetic system, and (ii) males (or females) use the same genetic system to discriminate against females (or males) of the other two species. Results indicate that although differences in male and female isolation depend on the populations tested, the isolation behaviour between a pair of species is highly correlated despite the variations. However, the rank order of the isolation level along the populations was not correlated in both sexes, which suggests that different genes act in male and female sexual isolation. Neither for males nor for females, the isolation behaviour of one species was paralleled in the other two species, which indicates that the genetic systems involved in this trait are species-pair specific. The implications of these results are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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. A significant sexual dimorphism exists in three species: D. melanogaster, D. erecta, and D. sechellia. Greater variation was observed in females than in males. D. erecta is singular in the production of long-chain molecules (31–33 carbons). Only three species (D. melanogaster, D. erecta, and D. sechellia) produce diene in significant amounts. Such products, especially 7,11-heptacosadiene, are known to act as aphrodisiacs for D. melanogaster males. In the five other species, females show only quantitative differences from males, generally with 7-tricosene as the most abundant compound. This compound is an aphrodisiac for D. simulans males. Some species such as D. yakuba, D. teissieri, D. orena, D. mauritiana, and the Seychelles strain of D. simulans are almost identical in the chemical composition of cuticular hydrocarbons. In contrast, important variations are observed between geographic populations of D. melanogaster and D. simulans.     

MITOCHONDRIAL–NUCLEAR EPISTASIS AFFECTS FITNESS WITHIN SPECIES BUT DOES NOT CONTRIBUTE TO FIXED INCOMPATIBILITIES BETWEEN SPECIES OF DROSOPHILA

Efficient mitochondrial function requires physical interactions between the proteins encoded by the mitochondrial and nuclear genomes. Coevolution between these genomes may result in the accumulation of incompatibilities between divergent lineages. We test whether mitochondrial–nuclear incompatibilities have accumulated within the Drosophila melanogaster species subgroup by combining divergent mitochondrial and nuclear lineages and quantifying the effects on relative fitness. Precise placement of nine mtDNAs from D. melanogaster, D. simulans, and D. mauritiana into two D. melanogaster nuclear genetic backgrounds reveals significant mitochondrial–nuclear epistasis affecting fitness in females. Combining the mitochondrial genomes with three different D. melanogaster X chromosomes reveals significant epistasis for male fitness between X‐linked and mitochondrial variation. However, we find no evidence that the more than 500 fixed differences between the mitochondrial genomes of D. melanogaster and the D. simulans species complex are incompatible with the D. melanogaster nuclear genome. Rather, the interactions of largest effect occur between mitochondrial and nuclear polymorphisms that segregate within species of the D. melanogaster species subgroup. We propose that a low mitochondrial substitution rate, resulting from a low mutation rate and/or efficient purifying selection, precludes the accumulation of mitochondrial–nuclear incompatibilities among these Drosophila species.     

Structural and genetic studies of the proliferation disrupter genes of Drosophila simulans and D. melanogaster

To examine whether structural and functional differences exist in the proliferation disrupter (prod) genes between Drosophila simulans and D. melanogaster, we analyzed and compared both genes. The exon–intron structure of the genes was found to be the same – three exons were interrupted by two introns, although a previous report suggested that only one intron existed in D. melanogaster. The prod genes of D. simulans and D. melanogaster both turn out to encode 346 amino acids, not 301 as previously reported for D. melanogaster. The numbers of nucleotide substitutions in the prod genes was 0.0747 ±  per synonymous site and 0.0116 ± 0.0039 per replacement site, both comparable to those previously known for homologous genes between D. simulans and D. melanogaster. Genetic analysis demonstrated that D. simulans PROD can compensate for a deficiency of D. melanogaster PROD in hybrids. The PRODs of D. simulans and D. melanogaster presumably share the same function and a conserved working mechanism. The prod gene showed no significant interaction with the lethality of the male hybrid between these species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans

We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.     

Molecular analysis of P element behavior in Drosophila simulans transformants

Summary In this report we describe the successful transformation of Drosophila simulans with an autonomous P element from Drosophila melanogaster without the use of a selectable marker. This result demonstrates that there is no species barrier for P element transposition. Utilizing gel blotting and in situ hybridization techniques, we have monitored the behavior of newly-introduced P elements in several D. simulans transformed lines over twelve generations. In most instances, an overall increase in the number of P elements was observed. An examination of the frequency of P-element-bearing individuals in one line revealed the rapid spread of P elements through the population. Analysis of well-characterized sublines confirmed that P elements increase in number by transposition to new genomic sites. The formation of degenerate elements occurred in at least one case. These observations suggest that P elements may behave similarly in D. melanogaster and D. simulans.