Antagonistic selection between adult thorax and wing size in field released Drosophila melanogaster independent of thermal conditions

Rapid seasonal changes in phenotypes in a wild Drosophila population

Seasonal environmental change is one of the most rapid and striking environmental variables. Although relatively rapid adaptation to environmental changes over several years or several decades has been described in many taxa, rapid responses to seasonal environments are delicate, and therefore, the detection of the evolutionary responses requires sensitive methods. In this study, we examined seasonal changes in phenotypes related to thermal tolerance and morphological traits of Drosophila lutescens collected at the spring and autumn periods from a single location. We first demonstrated that flies in the two seasonal periods were almost genetically identical using double-digest restriction site-associated DNA sequencing and analysis. Using an experimental design to eliminate the effect of possible confounding factors that influence phenotypes (i.e., maternal effects and the environmental conditions in which each phenotype was analyzed), we showed that the heat tolerance of D. lutescens was significantly higher in the autumn population than in the spring population. Furthermore, cold tolerance was slightly higher in the spring population than in the autumn one. Although wing length and thorax length did not change significantly between seasons, the ratio of wing length to thorax length changed significantly between them. These results suggest that seasonal environmental heterogeneity induces rapid phenotypic changes within a year. Finally, we discuss the possibility of rapid evolutionary responses to seasonal changes.

Flight-reproduction trade-offs are weak in a field cage experiment across multiple Drosophila species

Flight-reproduction trade-offs, such that more mobile individuals sacrifice reproductive output (e.g., fecundity) or incur fitness costs, are well-studied in a handful of wing-dimorphic model systems. However, these trade-offs have not been systematically assessed across reproduction-related traits and taxa in wing monomorphic species despite having broad implications for the ecology and evolution of pterygote insect species. Here we therefore determined the prevalence, magnitude and direction of flight-reproduction trade-offs on several fitness-related traits in a semi-field setting by comparing disperser and resident flies from repeated releases of five wild-caught, laboratory-reared Drosophila species, and explicitly controlling for a suite of potential confounding effects (maternal effects, recent thermal history) and potential morphological covariates (wing-loading, body mass). We found almost no systematic differences in reproductive output (egg production), reproductive fitness (offspring survival), or longevity between flying (disperser) and resident flies in our replicated releases, even if adjusting for potential morphological variation. After correction for false discovery rates, none of the five species showed evidence of a significant fitness trade-off associated with increased flight (sustained, simulated voluntary field dispersal). Our results therefore suggest that flight-reproduction trade-offs are not as common as might have been expected when assessed systematically across species and under the relatively standardized conditions and field setting employed here, at least not in the genus Drosophila. The magnitude and direction of potential dispersal- or flight-induced trade-offs, and the conditions that promote them, clearly require closer scrutiny. We argue that flight or dispersal is either genuinely cheaper than expected, or the costs manifest differently than those assessed here. Lost opportunities (i.e., time spent on mate-finding, mating or foraging) or nutrient-poor conditions could promote fitness costs to dispersal in our study system and that could be explored in future.

The evolution of larger size in high-altitude Drosophila melanogaster has a variable genetic architecture

Important uncertainties persist regarding the genetic architecture of adaptive trait evolution in natural populations, including the number of genetic variants involved, whether they are drawn from standing genetic variation, and whether directional selection drives them to complete fixation. Here, we take advantage of a unique natural population of Drosophila melanogaster from the Ethiopian highlands, which has evolved larger body size than any other known population of this species. We apply a bulk segregant quantitative trait locus mapping approach to 4 unique crosses between highland Ethiopian and lowland Zambian populations for both thorax length and wing length. Results indicated a persistently variable genetic basis for these evolved traits (with largely distinct sets of quantitative trait loci for each cross), and at least a moderately polygenic architecture with relatively strong effects present. We complemented these mapping experiments with population genetic analyses of quantitative trait locus regions and gene ontology enrichment analysis, generating strong hypotheses for specific genes and functional processes that may have contributed to these adaptive trait changes. Finally, we find that the genetic architectures indicated by our quantitative trait locus mapping results for size traits mirror those from similar experiments on other recently evolved traits in this species. Collectively, these studies suggest a recurring pattern of polygenic adaptation in this species, in which causative variants do not approach fixation and moderately strong effect loci are present.

Medfly Phenotypic Plasticity as A Prerequisite for Invasiveness and Adaptation

The hypothesis of this study was that different plant hosts of the medfly Ceratitis capitata may cause variability as a prerequisite for its invasiveness. The main objective was to determine population variability based on medfly wing shape in three favorable medfly host plants (peach, fig and mandarin) from different agroecological growing areas with different pest management practices, and to evaluate phenotypic plasticity as a basis for future expansion into new areas and new hosts. Using geometric morphometric methods across 14 specific landmarks on the medfly wings, 10 populations were tested from infested peach, fig and mandarin fruits, as well as laboratory-grown sterile populations. The studies led to the following main findings: (1) all of the medfly populations that were studied exhibited sexual dimorphism in wing shape; (2) the hosts in which the medfly develops influence wing shape and condition its variability; (3) there is significant variability between laboratory mass-reared sterile and wild individuals in male and female populations; (4) a high phenotypic plasticity of medfly populations was observed along the study sites. Even the low but clearly detected variability between different agroecological conditions and localized variability indicate genotypic stability and high phenotypic plasticity, which can be considered as a prerequisite for medfly invasiveness and dispersal to new areas.

Effects of Thelytokous Parthenogenesis-Inducing Wolbachia on the Fitness of Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae) in Superparasitised and Single-Parasitised Hosts

Thelytokous Wolbachia-infected Trichogramma species have long been considered as biological control agents against lepidopteran pests in agriculture and forestry. Wolbachia has been suggested to increase the probability of the superparasitism of Trichogramma, but the fate of infected offspring in the superparasitised host is still unknown. The present study aimed to evaluate the fitness of thelytokous Wolbachia-infected (TDW) and bisexual Wolbachia-free (TD) Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae) lines in superparasitised or single-parasitised hosts. The results showed that irrespective of whether Trichogramma wasps were developed from superparasitised or single-parasitised hosts, the TDW line was characterized by reduced fitness, including lower fecundity, shorter longevity, and smaller body size of F1 offspring, and lower emergence rate of F2 offspring than the TD line. This was not true for the survival rate and developmental time of F1 offspring. Additionally, the fitness parameters of T. dendrolimi that developed from superparasitised hosts were lower compared with that of T. dendrolimi that developed from single-parasitised hosts. Interestingly, Wolbachia-infected females had higher dispersal capacity than bisexual females when they developed from superparasitised hosts. The results indicated that Wolbachia negatively affects fitness of T. dendrolimi, but enhance dispersal capacity of T. dendrolimi females in superparasitism condition. Further studies need to be carried out to select the best line that will allow Wolbachia and their host Trichogramma to be better adapted to one another.

Body and Wing Allometries Reveal Flight-Fecundity Tradeoff in Response to Larval Provisioning in Osmia lignaria (Hymenoptera: Megachilidae)

Variation in body size has important implications for physical performance and fitness. For insects, adult size and morphology are determined by larval growth and metamorphosis. Female blue orchard bees, Osmia lignaria, (Say) provision a finite quantity of food to their offspring. In this study, we asked how provision-dependent variation in size changes adult morphology. We performed a diet manipulation in which some larvae were starved in the final instar and some were given unlimited food. We examined the consequences on adult morphology in two ways. First, allometric relationships between major body regions (head, thorax, abdomen) and total body mass were measured to determine relative growth of these structures. Second, morphometrics that are critical for flight (wing area, wing loading, and extra flight power index) were quantified. Head and thorax mass had hyperallometric relationships with body size, indicating these parts become disproportionately large in adults when larvae are given copious provisions. However, abdominal mass and wing area increased hypoallometrically with body size. Thus, large adults had disproportionately lighter abdomens and smaller wing areas than smaller adults. Though both males and females followed these general patterns, allometric patterns were affected by sex. For flight metrics, small adults had reduced wing loading and an increased extra flight power index. These results suggest that diet quantity alters development in ways that affect the morphometric trait relationships in adult O. lignaria and may lead to functional differences in performance.

Does local adaptation along a latitudinal cline shape plastic responses to combined thermal and nutritional stress?

Thermal and nutritional stress are commonly experienced by animals. This will become increasingly so with climate change. Whether populations can plastically respond to such changes will determine their survival. Plasticity can vary among populations depending on the extent of environmental heterogeneity. However, theory conflicts as to whether environmental heterogeneity should increase or decrease plasticity. Using three locally adapted populations of Drosophila melanogaster sampled from a latitudinal gradient, we investigated whether plastic responses to combinations of nutrition and temperature increase or decrease with latitude for four traits: egg-adult viability, egg-adult development time, and two body size traits. Employing nutritional geometry, we reared larvae on 25 diets varying in protein and carbohydrate content at two temperatures: 18 and 25°C. Plasticity varied among traits and across the three populations. Viability was highly canalized in all three populations. The tropical population showed the least plasticity for development time, the sub-tropical showed the highest plasticity for wing area, and the temperate population showed the highest plasticity for femur length. We found no evidence of latitudinal plasticity gradients in either direction. Our data highlight that differences in thermal variation and resource predictability experienced by populations along a latitudinal cline are not sufficient to predict their plasticity.

Effect of altitude on wing metric variation of Aedes aegypti (Diptera: Culicidae) in a region of the Colombian Central Andes

In mosquitoes of medical importance, wing shape and size can vary with altitude, an aspect that can influence dispersion and, consequently, their vector capacity. Using geometric morphometry analysis, Aedes aegypti wing size and shape variation of males and females was studied in four altitudes in the second-smallest department in Colombia: 1,200 m (Tebaida), 1,400 m (Armenia), 1,500 m (Calarcá), and 1,700 m (Filandia). Wing shape in males (P < 0.001) and females (P < 0.001) was significantly different through the altitudinal gradient; in turn, wing size in males followed the altitudinal gradient males (R² = 0.04946, P = 0.0002), females (R² = 0.0011, P = 0.46). Wing allometry for males (P < 0.001) and females (P < 0.001) was significant. Likewise, the shape and size of the wings of males (P < 0.001) and females (P < 0.001) had significant fluctuating asymmetry. It is concluded that, in a small scale with an altitudinal variation of 500 meters, it is detected that the size and shape of the wings varied in A. aegypti, main vector the agents that cause dengue, chikungunya, and Zika. The fluctuating asymmetry is present in the individuals studied and could be associated with environmental effects caused by vector control campaigns present in some sampling locations.

Marking Drosophila suzukii (Diptera: Drosophilidae) with Fluorescent Dusts

The marking of Drosophila suzukii can be an important instrument for studying the ecology and behaviour of this economically important fruit pest, aiding the development of new Integrated Pest Management (IPM) tools or strategies. There is, however, a need for a cost-effective methodology that provides an easily detectable and stable mark. Whereas fluorescent pigment powders are often used in entomological research, the pigments (series, dyes), application techniques, or doses need to be evaluated for each studied species in terms of their efficacy and possible adverse effects on the performance of the insect. The effectiveness of different application techniques and dyes (RadGlo® TP-series) and their effect on the survival of adult D. suzukii were investigated in the laboratory. Furthermore, the influence of the marking on the behaviour of the flies was examined in laboratory trap assays (olfaction) and a field recapture study (general orientation). The persistence and detectability of the marks was evaluated both on living flies (for different application techniques) and dead flies under trapping/storage conditions. The use of fluorescent powders to mark D. suzukii flies yielded a clearly detectable and highly persistent mark, without any adverse effects on the survival and behaviour of the flies.

Exploring thermal flight responses as predictors of flight ability and geographic range size in Drosophila

Thermal flight performance curves (TFPCs) may be a useful proxy for determining dispersal on daily timescales in winged insect species. Few studies have assessed TFPCs across a range of species under standard conditions despite that they may be useful in predicting variation in performance, abundance or geographic range shifts with forecast climate variability. Indeed, the factors determining realized dispersal within and among flying insect species are generally poorly understood. To better understand how flight performance may be correlated with geographic range extent and potential latitudinal climate variability, we estimated the thermal performance curves of flight ability in 11 Drosophilidae species (in 4 °C increments across 16-28 °C) after standard laboratory rearing for two generations. We tested if key morphological, evolutionary or ecological factors (e.g. species identity, sex, body mass, wing loading, geographic range size) predicted traits of TFPCs (including optimum temperature, maximum performance, thermal breadth of performance) or flight ability (success/failure to fly). Although several parameters of TFPCs varied among species these were typically not statistically significant probably owing to the relatively small pool of species assessed and the limited trait variation detected. The best explanatory model of these flight responses across species included significant positive effects of test temperature and wing area. However, the rank of geographic distribution breadth and phylogeny failed to explain significant variation in most of the traits, except for thermal performance breadth, of thermal flight performance curves among these 11 species. Future studies that employ a wider range of Drosophilidae species, especially if coupled with fine-scale estimates of species' environmental niches, would be useful.

Effect of larval density and substrate quality on the wing geometry of Stomoxys calcitrans L. (Diptera: Muscidae)

Background

In insects, oviposition decisions may lead to egg deposition in substrates with different larval density and nutritional levels. Individuals developing in such substrates may present plasticity in their phenotype. Here, we investigated the effect of two factors related to oviposition decisions, namely larval density and substrate quality, on the wing size and wing shape of the stable fly, Stomoxys calcitrans L. (Diptera: Muscidae).

Methods

We reared S. calcitrans larvae at different densities (5, 15 and 25) and on different substrates (camel, cow, donkey and sheep dung). For each fly that emerged, we recorded body weight, and detached, slide-mounted and photographed the right wing. Next, we collected 15 landmarks on each photographed wing, and applied geometric morphometric analysis to assess variation in wing size and wing shape of S. calcitrans across the different larval densities and substrate types.

Results

We observed that wing size and wing shape of S. calcitrans were affected by larval density and the nature of the developmental substrate. Flies reared in a group of 5 had larger wing centroid size, wing length, wing width, wing area and wing loading compared with those reared in a group of 25. Also, flies developed in donkey and sheep dung had larger wing centroid size, wing length, wing width, wing area and wing loading in comparison with those grown in camel and cow dung. Canonical variate analysis followed by discriminant analysis revealed significant wing shape variation in S. calcitrans across the different densities and substrates. Wing size had a significant but weak positive effect on wing shape.

Conclusions

This study demonstrates the high sensitivity of S. calcitrans wings to variation in larval density and developmental substrate, and that use of landmark-based geometric morphometric analysis could improve our understanding of how flies of veterinary importance respond to environmental variability.

Dispersal propensity, but not flight performance, explains variation in dispersal ability

Enhanced dispersal ability may lead to accelerated range expansion and increased rates of population establishment, thereby affecting population genetic structure and evolutionary potential. Morphological, behavioural and physiological traits that characterize dispersive individuals from residents are poorly understood for many invertebrate systems, especially in non-polymorphic pterygote species. Here we examined phenotypic differences between dispersal-prone and philopatric individuals from repeated mark-release-recapture (MRR) experiments using an invasive agricultural pest, Ceratitis capitata Comprehensive morphometric assessment and subsequent minimal adequate modelling using an information theoretic approach identified thorax mass : body mass ratio as a key predictor of disperser flies under semi-natural conditions. Performance differences in flight ability were then examined under controlled laboratory conditions to assess whether greater thorax mass : body mass ratio was associated with enhanced flight ability. The larger thorax : body mass ratio was associated with measurable differences in mean flight duration, most predominantly in males, and also by their willingness to disperse, scored as the number and duration of voluntary flights. No other measures of whole-animal flight performance (e.g. mean and peak vertical force, total or maximum flight duration) differed. Variation in voluntary behaviour may result in significant alterations of movement behaviour and realized dispersal in nature. This phenomenon may help explain intraspecific variation in the dispersal ability of insects.

The quantitative genetic basis of clinal divergence in phenotypic plasticity

Phenotypic plasticity is thought to be an important mechanism for adapting to environmental heterogeneity. Nonetheless, the genetic basis of plasticity is still not well understood. In Drosophila melanogaster and D. simulans, body size and thermal stress resistance show clinal patterns along the east coast of Australia, and exhibit plastic responses to different developmental temperatures. The genetic basis of thermal plasticity, and whether the genetic effects underlying clinal variation in traits and their plasticity are similar, remains unknown. Here, we use line-cross analyses between a tropical and temperate population of Drosophila melanogaster and D. simulans developed at three constant temperatures (18°C, 25°C, and 29°C) to investigate the quantitative genetic basis of clinal divergence in mean thermal response (elevation) and plasticity (slope and curvature) for thermal stress and body size traits. Generally, the genetic effects underlying divergence in mean response and plasticity differed, suggesting that different genetic models may be required to understand the evolution of trait means and plasticity. Furthermore, our results suggest that nonadditive genetic effects, in particular epistasis, may commonly underlie plastic responses, indicating that current models that ignore epistasis may be insufficient to understand and predict evolutionary responses to environmental change.

The influence of larval competition on Brazilian Wolbachia-infected Aedes aegypti mosquitoes

BACKGROUND

With field releases starting in Brazil, particular interest must be given to understanding how the endosymbiotic bacterium Wolbachia pipientis affects Aedes aegypti mosquitoes with a Brazilian genetic background. Currently, there is limited information on how the bacterium affects phenotypic traits such as larval development rate, metabolic reserves and morphometric parameters in Ae. aegypti. Here, we analyze for the first time, the effect of Wolbachia on these key phenotypes and consider how this might impact the potential of the bacterium as a disease control agent in Brazil.

METHODS

We examined the influence of the wMel strain of Wolbachia in laboratory Ae. aegypti with a Brazilian genetic background, reared under different larval densities. Pupae formation was counted daily to assess differences in development rates. Levels of metabolic reserves and morphometric parameters were assessed in adults resulting from each larval condition.

RESULTS

wMel infection led to more rapid larval development at higher densities for both males and females, with no effect under less crowded conditions in females. Infection also led to reduced body size at both high and low density, but not at intermediate density, although the scale of this difference was maintained regardless of larval density, in comparison to uninfected individuals. Wing shape also varied significantly between infected and uninfected mosquitoes due to larval density. Glycogen levels in uninfected mosquitoes decreased under higher larval density, but were consistently high with Wolbachia infection, regardless of larval density.

CONCLUSIONS

We demonstrate that the wMel Wolbachia strain can positively influence some important host fitness traits, and that this interaction is directly linked to the conditions in which the host is reared. Combined with previously published data, these results suggest that this Wolbachia strain could be successfully used as part of the Eliminate Dengue Program in Brazil.

Can body traits, other than wings, reflect the flight ability of Triatominae bugs?

INTRODUCTION

Insects of the subfamily Triatominae are vectors of Trypanosoma cruzi , the Chagas disease parasite, and their flying behavior has epidemiological importance. The flying capacity is strikingly different across and within Triatominae species, as well as between sexes or individuals. Many Triatoma infestans individuals have wings but no flying muscles. In other Triatominae species, no clear relationships were found between wing length and flying behavior. If wing presence or size is not reflective of the flying behavior, which other parts of the body could be considered as reliable markers of this important function?

METHODS

The genus Mepraia has exceptional characteristics with invariably wingless females and wingless or winged males. We calculated the porous surface exposed to odorant molecules to estimate the olfactory capacity of Mepraia spinolai . The head shape and thorax size were estimated using the geometric morphometric approach and traditional morphometric techniques, respectively.

RESULTS

Alary polymorphism in M. spinolai was significantly associated with consistent modification of the thorax size, head shape, and notable change in the estimated olfactory capacity. The macropterous individuals had a larger olfactory surface and thorax size and significantly different head shape compared to those of the micropterous individuals.

CONCLUSIONS

We concluded that these structural changes could be associated with the flying potential of Triatominae. Thus, morphological attributes not found on wings could help determine the likely flying potential of the bugs.

Clinal patterns of desiccation and starvation resistance in ancestral and invading populations of Drosophila subobscura

As invading species expand, they eventually encounter physical and biotic stressors that limit their spread. We examine latitudinal and climatic variation in physiological tolerance in one native and two invading populations of Drosophila subobscura. These flies are native to the Palearctic region, but invaded both South and North America around 1980 and spread rapidly across 15° of latitude on each continent. Invading flies rapidly evolved latitudinal clines in chromosome inversion frequencies and in wing size that parallel those of native populations in the Old World. Here we investigate whether flies on all three continents have evolved parallel clines in desiccation and starvation tolerance, such that flies in low-latitude regions (hot, dry) might have increased stress resistance. Starvation tolerance does not vary with latitude or climate on any continent. In contrast, desiccation tolerance varies clinally with latitude on all three continents, although not in parallel. In North American and Europe, desiccation tolerance is inversely related to latitude, as expected. But in South America, desiccation tolerance increases with latitude and is greatest in relatively cool and wet areas. Differences among continents in latitudinal patterns of interspecific-competition potentially influence clinal selection for physiological resistance, but no simple pattern is evident on these continents.

Trait-specific consequences of inbreeding on adaptive phenotypic plasticity

Environmental changes may stress organisms and stimulate an adaptive phenotypic response. Effects of inbreeding often interact with the environment and can decrease fitness of inbred individuals exposed to stress more so than that of outbred individuals. Such an interaction may stem from a reduced ability of inbred individuals to respond plastically to environmental stress; however, this hypothesis has rarely been tested. In this study, we mimicked the genetic constitution of natural inbred populations by rearing replicate Drosophila melanogaster populations for 25 generations at a reduced population size (10 individuals). The replicate inbred populations, as well as control populations reared at a population size of 500, were exposed to a benign developmental temperature and two developmental temperatures at the lower and upper margins of their viable range. Flies developed at the three temperatures were assessed for traits known to vary across temperatures, namely abdominal pigmentation, wing size, and wing shape. We found no significant difference in phenotypic plasticity in pigmentation or in wing size between inbred and control populations, but a significantly higher plasticity in wing shape across temperatures in inbred compared to control populations. Given that the norms of reaction for the noninbred control populations are adaptive, we conclude that a reduced ability to induce an adaptive phenotypic response to temperature changes is not a general consequence of inbreeding and thus not a general explanation of inbreeding–environment interaction effects on fitness components.

A highly pleiotropic amino acid polymorphism in the Drosophila insulin receptor contributes to life-history adaptation

Finding the specific nucleotides that underlie adaptive variation is a major goal in evolutionary biology, but polygenic traits pose a challenge because the complex genotype-phenotype relationship can obscure the effects of individual alleles. However, natural selection working in large wild populations can shift allele frequencies and indicate functional regions of the genome. Previously, we showed that the two most common alleles of a complex amino acid insertion-deletion polymorphism in the Drosophila insulin receptor show independent, parallel clines in frequency across the North American and Australian continents. Here, we report that the cline is stable over at least a five-year period and that the polymorphism also demonstrates temporal shifts in allele frequency concurrent with seasonal change. We tested the alleles for effects on levels of insulin signaling, fecundity, development time, body size, stress tolerance, and life span. We find that the alleles are associated with predictable differences in these traits, consistent with patterns of Drosophila life-history variation across geography that likely reflect adaptation to the heterogeneous climatic environment. These results implicate insulin signaling as a major mediator of life-history adaptation in Drosophila, and suggest that life-history trade-offs can be explained by extensive pleiotropy at a single locus.

Assessing quality of life-shortening Wolbachia-infected Aedes aegypti mosquitoes in the field based on capture rates and morphometric assessments

Background

Recent releases have been carried out with Aedes aegypti mosquitoes infected with the wMelPop mosquito cell-line adapted (wMelPop-CLA) strain of Wolbachia. This infection introduced from Drosophila provides strong blockage of dengue and other arboviruses but also has large fitness costs in laboratory tests. The releases were used to evaluate the fitness of released infected mosquitoes, and (following termination of releases) to test for any effects of wMelPop-CLA on wing size and shape when mosquitoes were reared under field conditions.

Methods

We monitored gravid females via double sticky traps to assess the reproductive success of wMelPop-CLA-infected females and also sampled the overall mosquito population post-release using Biogent Sentinel traps. Morphometric analyses were used to evaluate infection effects on wing shape as well as size.

Results

Oviposition success as assessed through double sticky traps was unrelated to size of released mosquitoes. However, released mosquitoes with lower wing loading were more successful. Furthermore, wMelPop-CLA-infected mosquitoes had 38.3% of the oviposition success of uninfected mosquitoes based on the predicted infection frequency after release. Environmental conditions affected wing shape and particularly size across time in uninfected mosquitoes, but not in naturally-reared wMelPop-CLA-infected mosquitoes. Although the overall size and shape do not differ between naturally-reared wMelPop-CLA-infected and uninfected mosquitoes, the infected mosquitoes tended to have smaller wings than uninfected mosquitoes during the cooler November in comparison to December.

Conclusion

These results confirm the lower fitness of wMelPop-CLA infection under field conditions, helping to explain challenges associated with a successful invasion by this strain. In the long run, invasion may depend on releasing strains carrying insecticide resistance or egg desiccation resistance, combined with an active pre-release population suppression program.

Body size and wing shape measurements as quality indicators of Aedes aegypti mosquitoes destined for field release

There is increasing interest in rearing modified mosquitoes for mass release to control vector-borne diseases, particularly Wolbachia-infected Aedes aegypti for suppression of dengue. Successful introductions require release of high quality mosquitoes into natural populations. Potential indicators of quality are body size and shape. We tested to determine if size, wing/thorax ratio, and wing shape are associated with field fitness of Wolbachia-infected Ae. aegypti. Compared with field-collected mosquitoes, released mosquitoes were larger in size, with lower size variance and different wing shape but similar in wing-thorax ratio and its associated variance. These differences were largely attributed to nutrition and to a minor extent to wMel Wolbachia infection. Survival potential of released female mosquitoes was similar to those from the field. Females at oviposition sites tended to be larger than those randomly collected from BG-Sentinel traps. Rearing conditions should thus aim for large size without affecting wing/thorax ratios.

Polymorphism in the neurofibromin gene, Nf1, is associated with antagonistic selection on wing size and development time in Drosophila melanogaster

In many invertebrates, body size shows genetically based clines, with size increasing in colder climates. Large body size is typically associated with prolonged development times. We consider variation in the CNS-specific gene neurofibromin 1 (Nf1) and its association with body size and development time. We identified two major Nf1 haplotypes in natural populations, Nf1-insertion-A and Nf1-deletion-G. These haplotypes are characterized by a 45-base insertion/deletion (INDEL) in Nf1 intron 2 and an A/G synonymous substitution (locus L17277). Linkage disequilibrium (LD) between the INDEL and adjacent sites is high but appears to be restricted within the Nf1 gene interval. In Australia, the frequency of the Nf1-insertion-A haplotype increases with latitude where wing size is larger, independent of the chromosomal inversion In(3R)Payne. Unexpectedly, the Nf1-insertion-A haplotype is negatively associated with wing size. We found that the Nf1-insertion-A haplotype is enriched in females with shorter development time. This suggests that the Nf1 haplotype cline may be driven by selection for development time rather than size; females from southern (higher latitude) D. melanogaster populations maintain a rapid development time despite being relatively larger, and the higher incidence of Nf1-insertion-A in Southern Australia may contribute to this pattern, whereas the effects of the Nf1 haplotypes on size may be countered by other loci with antagonistic effects on size and development time. Our results point to the potential complexity involved in identifying selection on genetic variants exhibiting pleiotropic effects when studies are based on spatial patterns or association studies.

Consistent effects of a major QTL for thermal resistance in field-released Drosophila melanogaster

Molecular genetic markers can be used to identify quantitative trait loci (QTL) for thermal resistance and this has allowed characterization of a major QTL for knockdown resistance to high temperature in Drosophila melanogaster. The QTL showed trade-off associations with cold resistance under laboratory conditions. However, assays of thermal tolerance conducted in the laboratory may not necessarily reflect performance at varying temperatures in the field. Here we tested if lines with different genotypes in this QTL show different thermal performance under high and low temperatures in the field using a release recapture assay. We found that lines carrying the QTL genotype for high thermal tolerance were significantly better at locating resources in the field releases under hot temperatures while the QTL line carrying the contrasting genotype were superior at cold temperatures. Further, we studied copulatory success between the different QTL genotypes at different temperatures. We found higher copulatory success in males of the high tolerance QTL genotype under hot temperature conditions, while there was no difference in females at cold temperatures. The results allow relating components of field fitness at different environmental temperatures with genotypic variation in a QTL for thermal tolerance.

The influence of host fruit and temperature on the body size of adult Ceratitis capitata (Diptera: Tephritidae) under laboratory and field conditions

The adult body size of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), varies in natural conditions. Body size is an important fitness indicator in the Mediterranean fruit fly; larger individuals are more competitive at mating and have a greater dispersion capacity and fertility. Both temperature during larval development and host fruit quality have been cited as possible causes for this variation. We studied the influence of host fruit and temperature during larval development on adult body size (wing area) in the laboratory, and determined body size variation in field populations of the Mediterannean fruit fly in eastern Spain. Field flies measured had two origins: 1) flies periodically collected throughout the year in field traps from 32 citrus groves, during the period 2003-2007; and 2) flies evolved from different fruit species collected between June and December in 2003 and 2004. In the lab, wing area of male and female adults varied significantly with temperature during larval development, being larger at the lowest temperature. Adult size also was significantly different depending on the host fruit in which larvae developed. The size of the flies captured at the field, either from traps or from fruits, varied seasonally showing a gradual pattern of change along the year. The largest individuals were obtained during winter and early spring and the smallest during late summer. In field conditions, the size of the adult Mediterannean fruit fly seems apparently more related with air temperature than with host fruit. The implications of this adult size pattern on the biology of C. capitata and on the application of the sterile insect technique are discussed.

Molecular basis of adaptive shift in body size in Drosophila melanogaster: functional and sequence analyses of the Dca gene

Latitudinal body size clines in animals conforming to Bergmann's rule occur on many continents but isolating their underlying genetic basis remains a challenge. In Drosophila melanogaster, the gene Dca accounts for approximately 5-10% of the natural wing size variation (McKechnie SW, Blacket MJ, Song SV, Rako L, Carroll X, Johnson TK, Jensen LT, Lee SF, Wee CW, Hoffmann AA. 2010. A clinally varying promoter polymorphism associated with adaptive variation in wing size in Drosophila. Mol Ecol. 19:775-784). We present here functional evidence that Dca is a negative regulator of wing size. A significant negative latitudinal cline of Dca gene expression was detected in synchronized third instar larvae. In addition, we clarified the evolutionary history of the three most common Dca promoter alleles (Dca237-1, Dca237-2, and Dca247) and showed that the insertion allele (Dca247), whose frequency increases with latitude, is associated with larger wing centroid size and higher average cell number in male flies. Finally, we showed that the overall linkage disequilibrium (LD) was low in the Dca promoter and that the insertion/deletion polymorphism that defines the Dca alleles was in strong LD with two other upstream sites. Our results provide strong support that Dca is a candidate for climatic adaptation in D. melanogaster.

Variation in morphological characters of two invasive leafminers, Liriomyza huidobrensis and L. sativae, across a tropical elevation gradient

Changes in morphological traits along elevation and latitudinal gradients in ectotherms are often interpreted in terms of the temperature-size rule, which states that the body size of organisms increases under low temperatures, and is therefore expected to increase with elevation and latitude. However other factors like host plant might contribute to spatial patterns in size as well, particularly for polyphagous insects. Here elevation patterns for trait size and shape in two leafminer species are examined, Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae) and L. sativae Blanchard, along a tropical elevation gradient in Java, Indonesia. Adult leafminers were trapped from different locations in the mountainous area of Dieng in the province of Central Java. To separate environmental versus genetic effects, L. huidobrensis originating from 1378 m and 2129 m ASL were reared in the laboratory for five generations. Size variation along the elevation gradient was only found in L. huidobrensis and this followed expectations based on the temperature-size rule. There were also complex changes in wing shape along the gradient. Morphological differences were influenced by genetic and environmental effects. Findings are discussed within the context of adaptation to different elevations in the two species.

Genetic mapping of adaptive wing size variation in Drosophila simulans

Many ecologically important traits exhibit latitudinal variation. Body size clines have been described repeatedly in insects across multiple continents, suggesting that similar selective forces are shaping these geographical gradients. It is unknown whether these parallel clinal patterns are controlled by the same or different genetic mechanism(s). We present here, quantitative trait loci (QTL) analysis of wing size variation in Drosophila simulans. Our results show that much of the wing size variation is controlled by a QTL on Chr 3L with relatively minor contribution from other chromosome arms. Comparative analysis of the genomic positions of the QTL indicates that the major QTL on Chr 3 are distinct in D. simulans and D. melanogaster, whereas the QTL on Chr 2R might overlap between species. Our results suggest that parallel evolution of wing size clines could be driven by non-identical genetic mechanisms but in both cases involve a major QTL as well as smaller effects of other genomic regions.

Connecting genetic variation to phenotypic clines

From early allozyme work to recent genome-wide scans, many studies have reported associations between molecular markers and latitude. These geographic patterns are tantalizing because they hint at the possibility of identifying specific mutations responsible for climatic adaptation. Unfortunately, few studies have done so because these exciting first glances often prove extremely challenging to follow up. Many difficulties can hinder connecting genetic and phenotypic variation in this context, and without such links, distinguishing the action of spatially varying selection from the other evolutionary processes capable of generating these patterns can be quite thorny. Nevertheless, two papers in this issue report excellent progress in overcoming these obstacles and provide persuasive evidence supporting the involvement of specific natural variants in clinal adaptation of Drosophila melanogaster populations (Fig. 1). In the first paper, Paaby et al. (2010) describe replicated allele frequency clines for a coding polymorphism in the Insulin-like Receptor (InR) gene on two continents, findings that strongly point to selection acting at this locus and that likely reflect life history adaptation. McKechnie et al. (2010) report compelling functional evidence that cis-regulatory variation in the Dca (drosophila cold acclimation) gene contributes to an adaptive cline in wing size. Notably, these papers employ largely alternative and complementary approaches, and together they exemplify how diverse strategies may be interwoven to draw convincing connections between genotype, phenotype, and evolutionary process.

A clinally varying promoter polymorphism associated with adaptive variation in wing size in Drosophila

Body size often shows adaptive clines in many ectotherms across altitude and latitude, but little is known about the genetic basis of these adaptive clines. Here we identify a polymorphism in the Dca (Drosophila cold acclimation) gene in Drosophila melanogaster that influences wing size, affects wing:thorax allometry and also controls a substantial proportion of the clinal wing-size variation. A polymorphism in the promoter region of Dca had two common alleles showing strong reciprocal clinal variation in frequency with latitude along the east coast of Australia. The Dca-237 allele increased towards the tropics where wing size is smaller. A within-population association study highlighted that an increase in the frequency of this allele decreased wing size but did not influence thorax size. A manipulated increase in the level of expression of Dca achieved through UAS-GAL4 was associated with a decrease in wing size but had no effect on thorax size. This was consistent with higher Dca expression levels in family lines with higher frequency of the Dca-237 allele. Genetic variation in the promoter region of the Dca gene appears to influence adaptive size variation in the eastern Australian cline of Drosophila melanogaster and accounts for more than 10% of the genetic variation in size within and between populations.

Clinal variation in post-winter male fertility retention; an adaptive overwintering strategy in Drosophila melanogaster

In insects including Drosophila melanogaster, females can overwinter at the adult stage by adopting a shallow reproductive diapause, but almost nothing is known about male reproductive diapause. In this study, we test for the maintenance of fertility in overwintering males from the eastern Australian D. melanogaster cline. Males from southern temperate populations maintained in field cages in temperate Melbourne had a higher fertility in spring compared with males from tropical locations. Temperate males successfully inseminated more females, and there were also more offspring produced per inseminated female. The resulting linear post-winter fertility clines were unrelated to male body size. In contrast, there was no clinal variation for fertility in nonoverwintering males held in the laboratory. The cline in overwintering male fertility is likely to have evolved as an adaptive response to latitudinal climatic variation over the last 100 years.

Life-history consequences of adaptation to larval nutritional stress in Drosophila

Many animal species face periods of chronic nutritional stress during which the individuals must continue to develop, grow, and/or reproduce despite low quantity or quality of food. Here, we use experimental evolution to study adaptation to such chronic nutritional stress in six replicate Drosophila melanogaster populations selected for the ability to survive and develop within a limited time on a very poor larval food. In unselected control populations, this poor food resulted in 20% lower egg-to-adult viability, 70% longer egg-to-adult development, and 50% lower adult body weight (compared to the standard food on which the flies were normally maintained). The evolutionary changes associated with adaptation to the poor food were assayed by comparing the selected and control lines in a common environment for different traits after 29-64 generations of selection. The selected populations evolved improved egg-to-adult viability and faster development on poor food. Even though the adult dry weight of selected flies when raised on the poor food was lower than that of controls, their average larval growth rate was higher. No differences in proportional pupal lipid content were observed. When raised on the standard food, the selected flies showed the same egg-to-adult viability and the same resistance to larval heat and cold shock as the controls and a slightly shorter developmental time. However, despite only 4% shorter development time, the adults of selected populations raised on the standard food were 13% smaller and showed 20% lower early-life fecundity than the controls, with no differences in life span. The selected flies also turned out less tolerant to adult malnutrition. Thus, fruit flies have the genetic potential to adapt to poor larval food, with no detectable loss of larval performance on the standard food. However, adaptation to larval nutritional stress is associated with trade-offs with adult fitness components, including adult tolerance to nutritional stress.

Costs and benefits of cold acclimation in field-released Drosophila

One way animals can counter the effects of climatic extremes is via physiological acclimation, but acclimating to one extreme might decrease performance under different conditions. Here, we use field releases of Drosophila melanogaster on two continents across a range of temperatures to test for costs and benefits of developmental or adult cold acclimation. Both types of cold acclimation had enormous benefits at low temperatures in the field; in the coldest releases only cold-acclimated flies were able to find a resource. However, this advantage came at a huge cost; flies that had not been cold-acclimated were up to 36 times more likely to find food than the cold-acclimated flies when temperatures were warm. Such costs and strong benefits were not evident in laboratory tests where we found no reduction in heat survival of the cold-acclimated flies. Field release studies, therefore, reveal costs of cold acclimation that standard laboratory assays do not detect. Thus, although physiological acclimation may dramatically improve fitness over a narrow set of thermal conditions, it may have the opposite effect once conditions extend outside this range, an increasingly likely scenario as temperature variability increases under global climate change.