Local adaptation of stress related traits inDrosophila buzzatiiandDrosophila simulansin spite of high gene flow

Intraspecific variation in thermal acclimation and tolerance between populations of the winter ant, Prenolepis imparis

Thermal phenotypic plasticity, otherwise known as acclimation, plays an essential role in how organisms respond to short-term temperature changes. Plasticity buffers the impact of harmful temperature changes; therefore, understanding variation in plasticity in natural populations is crucial for understanding how species will respond to the changing climate. However, very few studies have examined patterns of phenotypic plasticity among populations, especially among ant populations. Considering that this intraspecies variation can provide insight into adaptive variation in populations, the goal of this study was to quantify the short-term acclimation ability and thermal tolerance of several populations of the winter ant, Prenolepis imparis. We tested for correlations between thermal plasticity and thermal tolerance, elevation, and body size. We characterized the thermal environment both above and below ground for several populations distributed across different elevations within California, USA. In addition, we measured the short-term acclimation ability and thermal tolerance of those populations. To measure thermal tolerance, we used chill-coma recovery time (CCRT) and knockdown time as indicators of cold and heat tolerance, respectively. Short-term phenotypic plasticity was assessed by calculating acclimation capacity using CCRT and knockdown time after exposure to both high and low temperatures. We found that several populations displayed different chill-coma recovery times and a few displayed different heat knockdown times, and that the acclimation capacities of cold and heat tolerance differed among most populations. The high-elevation populations displayed increased tolerance to the cold (faster CCRT) and greater plasticity. For high-temperature tolerance, we found heat tolerance was not associated with altitude; instead, greater tolerance to the heat was correlated with increased plasticity at higher temperatures. These current findings provide insight into thermal adaptation and factors that contribute to phenotypic diversity by revealing physiological variance among populations.

Pesticide contamination drives adaptive genetic variation in the endemic mayfly Andesiops torrens within a semi-arid agricultural watershed of Chile

Agrichemical contamination can provoke evolutionary responses in freshwater populations. It is a particularly relevant issue in semi-arid regions due to the sensitivity of endemic species to pollutants and to interactions with temperature stress. This paper investigates the presence of pesticides in rivers within a semi-arid agricultural watershed of Chile, testing for their effects on population genetic characteristics of the endemic mayfly Andesiops torrens (Insecta, Ephemeroptera). Pesticides were detected in sediment samples in ten out of the 30 sites analyzed throughout the upper part of the Limarí watershed. To study the evolutionary impact of such contamination on A. torrens, we used a genome-wide approach and analyzed 2056 single nucleotide polymorphisms (SNPs) loci in 551 individuals from all sites. Genetic differentiation was weak between populations, suggesting high gene flow across the study area. While we did not find evidence of pesticide effects on genetic diversity nor on population differentiation, the allele frequency of three outlier SNP loci correlated significantly with pesticide occurrence. Interrogation of genomic resources indicates that two of these SNPs are located within functional genes that encode for the low-density lipoprotein receptor-related protein 2 and Dumpy, both potentially involved in insect cuticle resistance processes. Such genomic signatures of local adaptation are indicative of past adverse effects of pesticide exposure on the locally adapted populations. Our results reveal that A. torrens is sensitive to pesticide exposure, but that a high gene flow may confer resilience to contamination. This research supports the contention that A. torrens is an ideal model organism to study evolutionary responses induced by pesticides on non-target, endemic species.

Genetic diversity and gene flow decline with elevation in montane mayflies

Montane environments around the globe are biodiversity 'hotspots' and important reservoirs of genetic diversity. Montane species are also typically more vulnerable to environmental change than their low-elevation counterparts due to restricted ranges and dispersal limitations. Here we focus on two abundant congeneric mayflies (Baetis bicaudatus and B. tricaudatus) from montane streams over an elevation gradient spanning 1400 m. Using single-nucleotide polymorphism genotypes, we measured population diversity and vulnerability in these two species by: (i) describing genetic diversity and population structure across elevation gradients to identify mechanisms underlying diversification; (ii) performing spatially explicit landscape analyses to identify environmental drivers of differentiation; and (iii) identifying outlier loci hypothesized to underlie adaptive divergence. Differences in the extent of population structure in these species were evident depending upon their position along the elevation gradient. Heterozygosity, effective population sizes and gene flow all declined with increasing elevation, resulting in substantial population structure in the higher elevation species (B. bicaudatus). At lower elevations, populations of both species are more genetically similar, indicating ongoing gene flow. Isolation by distance was detected at lower elevations only, whereas landscape barriers better predicted genetic distance at higher elevations. At higher elevations, dispersal was restricted due to landscape effects, resulting in greater population isolation. Our results demonstrate differentiation over small spatial scales along an elevation gradient, and highlight the importance of preserving genetic diversity in more isolated high-elevation populations.

Physiological effects of heat stress on Hawaiian picture-wing Drosophila: genome-wide expression patterns and stress-related traits

Climate change is compounding the threats to the future of biodiversity, already impacted by habitat loss, invasive species and diseases. In the Hawaiian Islands, many of the endemic species have narrow habitat ranges that make them especially vulnerable to climate change. The Hawaiian Drosophila, a remarkably diverse group of species with 11 listed as federally endangered, are thought to be sensitive to temperature changes. To examine the species differences in sensitivity of Hawaiian picture-wing Drosophila to temperature changes, wild populations of Drosophila sproati, a relatively common species, and Drosophila silvestris, a rare species, were collected from two locations on Hawaii Island and bred in common laboratory conditions. Adult flies were exposed to hot and cold temperatures and compared with adult flies at control temperatures. Drosophila silvestris adults were less tolerant to heat stress than D. sproati for both survival and sperm mobility. In contrast, D. silvestris adults were more tolerant to cold stress than D. sproati for adult survival. The expression of 4950 Gene Ontology annotated gene transcripts was also analysed in high-temperature-treated and control males to identify candidate genes related to heat tolerance. There were more than twice as many transcripts differentially expressed after high temperature treatment for D. silvestris (246 transcripts) as for D. sproati (106 transcripts), with 13 Gene Ontology terms enriched between temperatures for D. silvestris and merely three in D. sproati. The combined results are consistent with D. sproati occurring more widely today as well as occurring at lower elevations than D. silvestris and with a genetically based temperature response, which is more severe in D. silvestris at high temperatures than that in D. sproati. These experiments demonstrate the potential for different capacities of species to adapt to future climate change conditions as well as providing an explanation for historical changes in the distribution of species.

Evidence for adaptive divergence of thermal responses among Bemisia tabaci populations from tropical Colombia following a recent invasion

There is an increasing evidence that populations of ectotherms can diverge genetically in response to different climatic conditions, both within their native range and (in the case of invasive species) in their new range. Here, we test for such divergence in invasive whitefly Bemisia tabaci populations in tropical Colombia, by considering heritable variation within and between populations in survival and fecundity under temperature stress, and by comparing population differences with patterns established from putatively neutral microsatellite markers. We detected significant differences among populations linked to mean temperature (for survival) and temperature variation (for fecundity) in local environments. A QST  - FST analysis indicated that phenotypic divergence was often larger than neutral expectations (QST  > FST ). Particularly, for survival after a sublethal heat shock, this divergence remained linked to the local mean temperature after controlling for neutral divergence. These findings point to rapid adaptation in invasive whitefly likely to contribute to its success as a pest species. Ongoing evolutionary divergence also provides challenges in predicting the likely impact of Bemisia in invaded regions.

Widespread phenotypic and genetic divergence along altitudinal gradients in animals

Altitudinal gradients offer valuable study systems to investigate how adaptive genetic diversity is distributed within and between natural populations and which factors promote or prevent adaptive differentiation. The environmental clines along altitudinal gradients tend to be steep relative to the dispersal distance of many organisms, providing an opportunity to study the joint effects of divergent natural selection and gene flow. Temperature is one variable showing consistent altitudinal changes, and altitudinal gradients can therefore provide spatial surrogates for some of the changes anticipated under climate change. Here, we investigate the extent and patterns of adaptive divergence in animal populations along altitudinal gradients by surveying the literature for (i) studies on phenotypic variation assessed under common garden or reciprocal transplant designs and (ii) studies looking for signatures of divergent selection at the molecular level. Phenotypic data show that significant between-population differences are common and taxonomically widespread, involving traits such as mass, wing size, tolerance to thermal extremes and melanization. Several lines of evidence suggest that some of the observed differences are adaptively relevant, but rigorous tests of local adaptation or the link between specific phenotypes and fitness are sorely lacking. Evidence for a role of altitudinal adaptation also exists for a number of candidate genes, most prominently haemoglobin, and for anonymous molecular markers. Novel genomic approaches may provide valuable tools for studying adaptive diversity, also in species that are not amenable to experimentation.

Adaptive latitudinal cline of photoperiodic diapause induction in the parasitoid Nasonia vitripennis in Europe

Living in seasonally changing environments requires adaptation to seasonal cycles. Many insects use the change in day length as a reliable cue for upcoming winter and respond to shortened photoperiod through diapause. In this study, we report the clinal variation in photoperiodic diapause induction in populations of the parasitoid wasp Nasonia vitripennis collected along a latitudinal gradient in Europe. In this species, diapause occurs in the larval stage and is maternally induced. Adult Nasonia females were exposed to different photoperiodic cycles and lifetime production of diapausing offspring was scored. Females switched to the production of diapausing offspring after exposure to a threshold number of photoperiodic cycles. A latitudinal cline was found in the proportion of diapausing offspring, the switch point for diapause induction measured as the maternal age at which the female starts to produce diapausing larvae, and the critical photoperiod for diapause induction. Populations at northern latitudes show an earlier switch point, higher proportions of diapausing individuals and longer critical photoperiods. Since the photoperiodic response was measured under the same laboratory conditions, the observed differences between populations most likely reflect genetic differences in sensitivity to photoperiodic cues, resulting from local adaptation to environmental cycles. The observed variability in diapause response combined with the availability of genomic tools for N. vitripennis represent a good opportunity to further investigate the genetic basis of this adaptive trait.

Investigating the molecular basis of local adaptation to thermal stress: population differences in gene expression across the transcriptome of the copepod Tigriopus californicus

BACKGROUND

Geographic variation in the thermal environment impacts a broad range of biochemical and physiological processes and can be a major selective force leading to local population adaptation. In the intertidal copepod Tigriopus californicus, populations along the coast of California show differences in thermal tolerance that are consistent with adaptation, i.e., southern populations withstand thermal stresses that are lethal to northern populations. To understand the genetic basis of these physiological differences, we use an RNA-seq approach to compare genome-wide patterns of gene expression in two populations known to differ in thermal tolerance.

RESULTS

Observed differences in gene expression between the southern (San Diego) and the northern (Santa Cruz) populations included both the number of affected loci as well as the identity of these loci. However, the most pronounced differences concerned the amplitude of up-regulation of genes producing heat shock proteins (Hsps) and genes involved in ubiquitination and proteolysis. Among the hsp genes, orthologous pairs show markedly different thermal responses as the amplitude of hsp response was greatly elevated in the San Diego population, most notably in members of the hsp70 gene family. There was no evidence of accelerated evolution at the sequence level for hsp genes. Among other sets of genes, cuticle genes were up-regulated in SD but down-regulated in SC, and mitochondrial genes were down-regulated in both populations.

CONCLUSIONS

Marked changes in gene expression were observed in response to acute sub-lethal thermal stress in the copepod T. californicus. Although some qualitative differences were observed between populations, the most pronounced differences involved the magnitude of induction of numerous hsp and ubiquitin genes. These differences in gene expression suggest that evolutionary divergence in the regulatory pathway(s) involved in acute temperature stress may offer at least a partial explanation of population differences in thermal tolerance observed in Tigriopus.

Adaptation to a seasonally varying environment: a strong latitudinal cline in reproductive diapause combined with high gene flow in Drosophila montana

Adaptation to seasonal changes in the northern hemisphere includes an ability to predict the forthcoming cold season from gradual changes in environmental cues early enough to prepare for the harsh winter conditions. The magnitude and speed of changes in these cues vary between the latitudes, which induces strong selection pressures for local adaptation.

We studied adaptation to seasonal changes in Drosophila montana, a northern maltfly, by defining the photoperiodic conditions leading to adult reproductive diapause along a latitudinal cline in Finland and by measuring genetic differentiation and the amount of gene flow between the sampling sites with microsatellites. Our data revealed a clear correlation between the latitude and the critical day length (CDL), in which half of the females of different cline populations enter photoperiodic reproductive diapause. There was no sign of limited gene flow between the cline populations, even though these populations showed isolation by distance. Our results show that local adaptation may occur even in the presence of high gene flow, when selection for locally adaptive life-history traits is strong. A wide range of variation in the CDLs of the fly strains within and between the cline populations may be partly due to gene flow and partly due to the opposing selection pressures for fly reproduction and overwinter survival. This variation in the timing of diapause will enhance populations’ survival over the years that differ in the severity of the winter and in the length of the warm period and may also help them respond to long-term changes in environmental conditions.

Complex patterns of local adaptation in heat tolerance in Drosophila simulans from eastern Australia

Latitudinal clines are considered a powerful means of investigating evolutionary responses to climatic selection in nature. However, most clinal studies of climatic adaptation in Drosophila have involved species that contain cosmopolitan inversion polymorphisms that show clinal patterns themselves, making it difficult to determine whether the traits or inversions are under selection. Further, although climatic selection is unlikely to act on only one life stage in metamorphic organisms, a few studies have examined clinal patterns across life stages. Finally, clinal patterns of heat tolerance may also depend on the assay used. To unravel these potentially confounding effects on clinal patterns of thermal tolerance, we examined adult and larval heat tolerance traits in populations of Drosophila simulans from eastern Australia using static and dynamic (ramping 0.06 °C min(-1)) assays. We also used microsatellites markers to clarify whether demographic factors or selection are responsible for population differentiation along clines. Significant cubic clinal patterns were observed for adult static basal, hardened and dynamic heat knockdown time and static basal heat survival in larvae. In contrast, static, hardened larval heat survival increased linearly with latitude whereas no clinal association was found for larval ramping survival. Significant associations between adult and larval traits and climatic variables, and low population differentiation at microsatellite loci, suggest a role for climatic selection, rather than demographic processes, in generating these clinal patterns. Our results suggest that adaptation to thermal stress may be species and life-stage specific, complicating our efforts to understand the evolutionary responses to selection for increasing thermotolerance.

A comprehensive assessment of geographic variation in heat tolerance and hardening capacity in populations of Drosophila melanogaster from eastern Australia

We examined latitudinal variation in adult and larval heat tolerance in Drosophila melanogaster from eastern Australia. Adults were assessed using static and ramping assays. Basal and hardened static heat knockdown time showed significant linear clines; heat tolerance increased towards the tropics, particularly for hardened flies, suggesting that tropical populations have a greater hardening response. A similar pattern was evident for ramping heat knockdown time at 0.06°C min(-1) increase. There was no cline for ramping heat knockdown temperature (CT(max) ) at 0.1°C min(-1) increase. Acute (static) heat knockdown temperature increased towards temperate latitudes, probably reflecting a greater capacity of temperate flies to withstand sudden temperature increases during summer in temperate Australia. Larval viability showed a quadratic association with latitude under heat stress. Thus, patterns of heat resistance depend on assay methods. Genetic correlations in thermotolerance across life stages and evolutionary potential for critical thermal limits should be the focus of future studies.

Genetic variability in tolerance to copper contamination in a herbivorous marine invertebrate

Anthropogenic stresses such as metal contamination can have profound ecological impacts in a wide range of habitats. Reduced survival of organisms in contaminated habitats has the potential to result in the evolution of genotypes tolerant to deleterious contaminants. Local adaptation to contamination requires directional selection, genetic variation in traits relating to tolerance, spatial variability in exposure to the contaminant and limited gene flow between populations. This study assesses variation in tolerance in the herbivorous marine invertebrate Peramphithoe parmerong whose algal diets in Sydney Harbour readily accumulate the metal copper. A quantitative genetics approach (a full-sib, split family design) was used to quantify variation among families in survival on the contaminated diet. A significant genotype-by-environment interaction in offspring survival between the copper contaminated and uncontaminated diet treatments revealed variation in tolerance to copper by P. parmerong. Amphipods that survived 30 days of exposure to copper contaminated diets were slightly smaller and ate less algae than those reared on uncontaminated food. This reflects an additional sub-lethal effect associated with the consumption of contaminated algae. However, there was no evidence of acclimation to contaminated diets, nor a cost of reduced feeding for those genotypes with increased tolerance. This study provides strong evidence for the potential of a marine invertebrate to evolve tolerance to contaminants found in their diet.

Resistance to environmental stress in Drosophila ananassae: latitudinal variation and adaptation among populations

Geographical variation in traits related to fitness is often the result of adaptive evolution. Stress resistance traits in Drosophila often show clinal variation, suggesting that selection affects resistance traits either directly or indirectly. Multiple stress resistance traits were investigated in 45 natural populations of Drosophila ananassae collected from all over India. There was significant positive correlation between starvation resistance and lipid content. Significant negative correlations between desiccation and lipid content and between desiccation and heat resistance were also found. Flies from lower latitudes had higher starvation resistance, heat resistance and lipid content but the pattern was reversed for desiccation resistance. These results suggest that flies from different localities varied in their susceptibility to starvation because of difference in their propensity to store body lipid. Multiple regression analysis provided evidence of climatic selection driven by latitudinal variation in the seasonal amplitude of temperature and humidity changes within the Indian. Finally, our results suggest a high degree of variation in stress resistance at the population level in D. ananassae.

Protein and carbohydrate composition of larval food affects tolerance to thermal stress and desiccation in adult Drosophila melanogaster

Larval nutrition may affect a range of different life history traits as well as responses to environmental stress in adult insects. Here we test whether raising larvae of fruit flies, Drosophila melanogaster, on two different nutritional regimes affects resistance to cold, heat and desiccation as well as egg production and egg-to-adult viability. We raised larvae on a carbohydrate-enriched and a protein-enriched growth medium. We found that flies developed on the high protein medium had increased heat and desiccation tolerance compared to flies developed on the carbohydrate-enriched medium. In contrast, flies developed on the carbohydrate-enriched growth medium recovered faster from chill coma stress compared to flies developed on a protein-enriched medium. We also found gender differences in stress tolerance, with female flies being more tolerant to chill coma, heat knockdown and desiccation stress compared to males. Egg production was highest in females that had developed on the protein-enriched medium. However, there was a sex-specific effect of nutrition on egg-to-adult viability, with higher viability for males developing on the sucrose-enriched medium, while female survival was highest when developing on the protein-enriched medium. Our study indicates that larval nutrition has a strong impact on the ability to cope with stress, and that the optimal nutrient composition varies with the type of stress.

Physiological climatic limits in Drosophila: patterns and implications

Physiological limits determine susceptibility to environmental changes, and can be assessed at the individual, population or species/lineage levels. Here I discuss these levels in Drosophila, and consider implications for determining species susceptibility to climate change. Limits at the individual level in Drosophila depend on experimental technique and on the context in which traits are evaluated. At the population level, evidence from selection experiments particularly involving Drosophila melanogaster indicate high levels of heritable variation and evolvability for coping with thermal stresses and aridity. An exception is resistance to high temperatures, which reaches a plateau in selection experiments and has a low heritability/evolvability when temperatures are ramped up to a stressful level. In tropical Drosophila species, populations are limited in their ability to evolve increased desiccation and cold resistance. Population limits can arise from trait and gene interactions but results from different laboratory studies are inconsistent and likely to underestimate the strength of interactions under field conditions. Species and lineage comparisons suggest phylogenetic conservatism for resistance to thermal extremes and other stresses. Plastic responses set individual limits but appear to evolve slowly in Drosophila. There is more species-level variation in lower thermal limits and desiccation resistance compared with upper limits, which might reflect different selection pressures and/or low evolvability. When extremes are considered, tropical Drosophila species do not appear more threatened than temperate species by higher temperatures associated with global warming, contrary to recent conjectures. However, species from the humid tropics may be threatened if they cannot adapt genetically to drier conditions.