Molecular and cellular studies in evolutionary physiology of natural vertebrate populations: influences of individual variation and genetic components on sampling and measurements

Evaluation of species thermal sensitivity with individual-based physiological performance

Ignoring intraspecific variations can prevent us from accurately assessing species' thermal sensitivity to global warming. Individual-based physiological performance provides a feasible solution to depict species' thermal sensitivity using a bottom-up approach. We measured the cardiac performance of intertidal bivalves (1159 individuals from multiple populations of six bivalves), determined the upper thermal limit of each individual, calculated the proportions of individuals suffering sublethal/lethal heat stress, and mapped sensitive regions to high temperatures. Results showed that high inter-individual variations of physiological performance existed in levels of populations and species, and species' thermal sensitivity was positively related to the intraspecific variations of heat tolerance. This bottom-up approach scaled up from individual, population to species emphasizes the importance of individual-based physiology performance in assessing thermal sensitivity across different hierarchical levels and enables better evaluating and forecasting of species responses to global warming.

Physiological insight into the evolution of complex phenotypes: aerobic performance and the O2 transport pathway of vertebrates

Evolutionary physiology strives to understand how the function and integration of physiological systems influence the way in which organisms evolve. Studies of the O2 transport pathway - the integrated physiological system that transports O2 from the environment to mitochondria - are well suited to this endeavour. We consider the mechanistic underpinnings across the O2 pathway for the evolution of aerobic capacity, focusing on studies of artificial selection and naturally selected divergence among wild populations of mammals and fish. We show that evolved changes in aerobic capacity do not require concerted changes across the O2 pathway and can arise quickly from changes in one or a subset of pathway steps. Population divergence in aerobic capacity can be associated with the evolution of plasticity in response to environmental variation or activity. In some cases, initial evolutionary divergence of aerobic capacity arose exclusively from increased capacities for O2 diffusion and/or utilization in active O2-consuming tissues (muscle), which may often constitute first steps in adaptation. However, continued selection leading to greater divergence in aerobic capacity is often associated with increased capacities for circulatory and pulmonary O2 transport. Increases in tissue O2 diffusing capacity may augment the adaptive benefit of increasing circulatory O2 transport owing to their interactive influence on tissue O2 extraction. Theoretical modelling of the O2 pathway suggests that O2 pathway steps with a disproportionately large influence over aerobic capacity have been more likely to evolve, but more work is needed to appreciate the extent to which such physiological principles can predict evolutionary outcomes.

Inter-individual physiological variation in responses to environmental variation and environmental change: Integrating across traits and time

Greater understanding of physiological responses to climate change demands deeper comprehension of the causes and consequences of physiological variation. Increasingly, population trait means are being deconstructed into variable signals at the level of individuals. We advocate for greater consideration of such inter-individual physiological variation and how it both depends on and interacts with environmental variability. First, we review several studies on the intertidal mussel Mytilus californianus to illustrate how the magnitude of inter-individual variation may depend on the environmental context analyzed (i.e., is the mean condition benign or stressful?) and/or on the specific physiological metric investigated. Stressful conditions may reveal or mask variation in disparate ways at different levels of analysis (e.g., transcriptome vs. proteome), but we often lack crucial information regarding the relationships among these different physiological metrics and their consequences for fitness. We then reanalyze several published datasets to ask whether individuals employ divergent strategies over time in response to acute heat stress; such time-dependence would further complicate interpretation of physiological variation. However, definitive conclusions are precluded by limited sample sizes and short timescales in extant datasets. A key remaining challenge is to extend these analytical frameworks to longer periods over which individuals in a population experience repeated, but spatially variable, episodic stress events. We conclude that variation at multiple levels of analysis should be investigated over longer periods and, where possible, within individuals (or genotypes) experiencing repeated environmental challenges. Although difficult in practice, such studies will facilitate improved understanding of potential population-level physiological responses to climate change.

Individual variation in aquatic toxicology: Not only unwanted noise

The mean value of any parameter and its changes are usually discussed, when ecotoxicological studies are carried out. However, also the variation of any parameter and its changes can be important components of the responses to environmental contamination. Although the homogeneity of variances is commonly tested, testing is done for the use of correct statistical methods, not because of exploring the possibility that variability and its changes could be important components of environmental responses. We evaluated recent aquatic toxicological literature and found that in the majority of articles indicating that homogeneity of variances was tested and giving the result of testing, the assumption of homogeneity was not fulfilled. Further, it was observed that in some studies experimental treatment clearly affected the variability. In this commentary we discuss the reasons for variability: measurement errors, experimental design, genetic heterogeneity and phenotypic plasticity, and conclude that even after accounting for experimental design and genetic makeup significant variability remains. This plasticity may change in environmental responses as suggested by a hypothetical example, and as confirmed by experimental data. As a consequence, the changes of variability can be significant, even when the means do not differ. Because of this, variability and its changes should always be analysed and reported. This will be easy, since the datasets are exactly the same for comparing the variances and means, and as normally variances are tested for homogeneity. It is likely that much new information about the responses of organisms to environmental contamination will be obtained. However, the present journal practises tend to discourage one from concentrating on anything but the mean. In contrast, we think it is imperative that variability is always included as an endpoint in data analysis in the future.

Effects of environmental stressors on lipid metabolism in aquatic invertebrates

Lipid metabolism is crucial for the survival and propagation of the species, since lipids are an essential cellular component across animal taxa for maintaining homeostasis in the presence of environmental stressors. This review aims to summarize information on the lipid metabolism under environmental stressors in aquatic invertebrates. Fatty acid synthesis from glucose via de novo lipogenesis (DNL) pathway is mostly well-conserved across animal taxa. The structure of free fatty acid (FFA) from both dietary and DNL pathway could be transformed by elongase and desaturase. In addition, FFA can be stored in lipid droplet as triacylglycerol, upon attachment to glycerol. However, due to the limited information on both gene and lipid composition, in-depth studies on the structural modification of FFA and their storage conformation are required. Despite previously validated evidences on the disturbance of the normal life cycle and lipid homeostasis by the environmental stressors (e.g., obesogens, salinity, temperature, pCO₂, and nutrients) in the aquatic invertebrates, the mechanism behind these effects are still poorly understood. To overcome this limitation, omics approaches such as transcriptomic and proteomic analyses have been used, but there are still gaps in our knowledge on aquatic invertebrates as well as the lipidome. This paper provides a deeper understanding of lipid metabolism in aquatic invertebrates.

DNA Barcoding Marine Biodiversity: Steps from Mere Cataloguing to Giving Reasons for Biological Differences

DNA barcoding has become a useful tool in many contexts and has opened up a completely new avenue for taxonomy. DNA barcoding has its widest application in biodiversity and ecological research to detect and describe diversity whenever morphological discrimination is difficult or impossible (e.g., in the case of species lacking diagnostic characters, early life stages, or cryptic species). In this chapter, we outline the utility of including physiological parameters as part of species description in publicly available databases that catalog taxonomic information resulting from barcoding projects. Cryptic species or different life stages of a species often differ in their physiological traits. Thus, if physiological aspects were included in species definitions, the presently cryptic species could be distinguished. We furthermore give suggestions for physiological information that should be included in a species description and describe potential applications of DNA barcoding for research with physiological components.

The association of very low-density lipoprotein receptor (VLDLR) haplotypes with egg production indicates VLDLR is a candidate gene for modulating egg production

The very low-density lipoprotein receptor (VLDLR) transports egg yolk precursors into oocytes. However, our knowledge of the distribution patterns of VLDLR variants among breeds and their relationship to egg production is still incomplete. In this study, eight single nucleotide polymorphisms (SNPs) that account for 87% of all VLDLR variants were genotyped in Nick Chick (NC, n=91), Lohmann Brown (LohB, n=50) and Lueyang (LY, n=381) chickens, the latter being an Chinese indigenous breed. Egg production by NC and LY chickens was recorded from 17 to 50 weeks. Only four similar haplotypes were found in NC and LohB, of which two accounted for 100% of all NC haplotypes and 92.5% of LohB haplotypes. In contrast, there was considerable haplotypic diversity in LY. Comparison of egg production in LY showed that hens with NC-like haplotypes had a significantly higher production (p < 0.05) than those without the haplotypes. However, VLDLR expression was not significantly different between the haplotypes. These findings indicate a divergence in the distribution of VLDLR haplotypes between selected and non-selected breeds and suggest that the near fixation of VLDLR variants in NC and LohB is compatible with signature of selection. These data also support VLDLR as a candidate gene for modulating egg production.

Thermal variation, thermal extremes and the physiological performance of individuals

In this review we consider how small-scale temporal and spatial variation in body temperature, and biochemical/physiological variation among individuals, affect the prediction of organisms' performance in nature. For ‘normal’ body temperatures – benign temperatures near the species' mean – thermal biology traditionally uses performance curves to describe how physiological capabilities vary with temperature. However, these curves, which are typically measured under static laboratory conditions, can yield incomplete or inaccurate predictions of how organisms respond to natural patterns of temperature variation. For example, scale transition theory predicts that, in a variable environment, peak average performance is lower and occurs at a lower mean temperature than the peak of statically measured performance. We also demonstrate that temporal variation in performance is minimized near this new ‘optimal’ temperature. These factors add complexity to predictions of the consequences of climate change. We then move beyond the performance curve approach to consider the effects of rare, extreme temperatures. A statistical procedure (the environmental bootstrap) allows for long-term simulations that capture the temporal pattern of extremes (a Poisson interval distribution), which is characterized by clusters of events interspersed with long intervals of benign conditions. The bootstrap can be combined with biophysical models to incorporate temporal, spatial and physiological variation into evolutionary models of thermal tolerance. We conclude with several challenges that must be overcome to more fully develop our understanding of thermal performance in the context of a changing climate by explicitly considering different forms of small-scale variation. These challenges highlight the need to empirically and rigorously test existing theories.

Transcription and redox enzyme activities: comparison of equilibrium and disequilibrium levels in the three-spined stickleback

Evolutionary and acclimatory responses require functional variability, but in contrast with mRNA and protein abundance data, most physiological measurements cannot be obtained in a high-throughput manner. Consequently, one must either rely on high-throughput transcriptomic or proteomic data with only predicted functional information, or accept the limitation that most physiological measurements can give fewer data than those provided by transcriptomics or proteomics. We evaluated how transcriptional and redox enzyme activity data agreed with regard to population differentiation (i.e. a system in steady state in which any time lag between transcription, translation and post-translational effects would be irrelevant) and in response to an acute 6°C increase in temperature (i.e. a disequilibrium state wherein translation could not have caught up with transcription) in the three-spined stickleback (Gasterosteus aculeatus). Transcriptional and enzyme activity data corresponded well with regard to population differentiation, but less so with regard to acute temperature increase. The data thus suggest that transcriptional and functional measurements can lead to similar conclusions when a biological system is in a steady state. The responses to acute changes must, as has been demonstrated earlier, be based on changes in cellular conditions or properties of existing proteins without significant de novo synthesis of new gene products.

Climate change and ocean acidification-interactions with aquatic toxicology

The possibilities for interactions between toxicants and ocean acidification are reviewed from two angles. First, it is considered how toxicant responses may affect ocean acidification by influencing the carbon dioxide balance. Second, it is introduced, how the possible changes in environmental conditions (temperature, pH and oxygenation), expected to be associated with climate change and ocean acidification, may interact with the toxicant responses of organisms, especially fish. One significant weakness in available data is that toxicological research has seldom been connected with ecological and physiological/biochemical research evaluating the responses of organisms to temperature, pH or oxygenation changes occurring in the natural environment. As a result, although there are significant potential interactions between toxicants and natural environmental responses pertaining to climate change and ocean acidification, it is very poorly known if such interactions actually occur, and can be behind the observed disturbances in the function and distribution of organisms in our seas.

Performance, personality, and energetics: correlation, causation, and mechanism

The study of phenotypic evolution should be an integrative endeavor that combines different approaches and crosses disciplinary and phylogenetic boundaries to consider complex traits and organisms that historically have been studied in isolation from each other. Analyses of individual variation within populations can act to bridge studies focused at the levels of morphology, physiology, biochemistry, organismal performance, behavior, and life history. For example, the study of individual variation recently facilitated the integration of behavior into the concept of a pace-of-life syndrome and effectively linked the field of energetics with research on animal personality. Here, we illustrate how studies on the pace-of-life syndrome and the energetics of personality can be integrated within a physiology-performance-behavior-fitness paradigm that includes consideration of ecological context. We first introduce key concepts and definitions and then review the rapidly expanding literature on the links between energy metabolism and personality traits commonly studied in nonhuman animals (activity, exploration, boldness, aggressiveness, sociability). We highlight some empirical literature involving mammals and squamates that demonstrates how emerging fields can develop in rather disparate ways because of historical accidents and/or particularities of different kinds of organisms. We then briefly discuss potentially interesting avenues for future conceptual and empirical research in relation to motivation, intraindividual variation, and mechanisms underlying trait correlations. The integration of performance traits within the pace-of-life-syndrome concept has the potential to fill a logical gap between the context dependency of selection and how energetics and personality are expected to interrelate. Studies of how performance abilities and/or aspects of Darwinian fitness relate to both metabolic rate and personality traits are particularly lacking.

Neural sensitivity to sex steroids predicts individual differences in aggression: implications for behavioural evolution

Testosterone (T) regulates many traits related to fitness, including aggression. However, individual variation in aggressiveness does not always relate to circulating T, suggesting that behavioural variation may be more closely related to neural sensitivity to steroids, though this issue remains unresolved. To assess the relative importance of circulating T and neural steroid sensitivity in predicting behaviour, we measured aggressiveness during staged intrusions in free-living male and female dark-eyed juncos (Junco hyemalis). We compared aggressiveness to plasma T levels and to the abundance of androgen receptor (AR), aromatase (AROM) and oestrogen receptor alpha (ORα) mRNA in behaviourally relevant brain areas (avian medial amygdala, hypothalamus and song control regions). We also asked whether patterns of covariation among behaviour and endocrine parameters differed in males and females, anticipating that circulating T may be a better predictor of behaviour in males than in females. We found that circulating T related to aggressiveness only in males, but that gene expression for ORα, AR and AROM covaried with individual differences in aggressiveness in both sexes. These findings are among the first to show that individual variation in neural gene expression for three major sex steroid-processing molecules predicts individual variation in aggressiveness in both sexes in nature. The results have broad implications for our understanding of the mechanisms by which aggressive behaviour may evolve.

The relationship between phenotypic and environmental variation: do physiological responses reduce interindividual differences?

What is the effect of a variable environment on phenotypic variation? Does the physiological response to a new environment increase or decrease the differences among individuals? We provide a speculative hypothesis suggesting that the induction of a physiological response to environmental change minimizes phenotypic differences among individuals in outbred genetically variable populations. Although this suggestion runs counter to the general idea that environmental variation induces phenotypic variation, we provide evidence that this is not always the case. One explanation for this counterintuitive hypothesis is that in a variable environment, the physiological mechanism that maintains homeostasis changes the concentrations of active transcription factors (TFs). This change in TFs reduces the effectiveness of nucleotide polymorphisms in TF binding sites and thus reduces the variation among individuals in mRNA expression and in the phenotypes affected by these mRNA transcripts. Thus, there are fewer differences among individuals in a variable environment compared with the variation observed in a constant environment. Our conjecture is that the physiological mechanisms that maintain homeostasis in response to environmental variation canalize phenotypic variation. If our hypothesis is correct, then the physiological canalization of gene expression in a variable environment hides genetic variation and thereby reduces the evolutionary costs of polymorphism. This hypothesis provides a new perspective on the mechanisms by which high levels of genetic variation can persist in real-world populations.

Gene expression patterns and stress response in marine copepods

Aquatic organisms are constantly exposed to both physical (e.g. temperature and salinity variations) and chemical (e.g. endocrine disruptor chemicals, heavy metals, hydrocarbons, diatom toxins, and other toxicants) stressors which they react to by activating a series of defense mechanisms. This paper reviews the literature on the defense systems, including detoxification enzymes and proteins (e.g. glutathione S-transferases, heat shock proteins, superoxide dismutase and catalase), studied in copepods at the molecular level. The data indicate high inter- and intra-species variability in copepod response, depending on the type of stressor tested, the concentration and exposure time, and the enzyme isoform studied. Ongoing -omics approaches will allow the identification of new genes which will give a more comprehensive overview of how copepods respond to specific stressors in laboratory and/or field conditions and the effects of these responses on higher trophic levels.

Egg yolk environment differentially influences physiological and morphological development of broiler and layer chicken embryos

Maternal effects are important in epigenetic determination of offspring phenotypes during all life stages. In the chicken (Gallus gallus domesticus), transgenerational transfer of egg yolk factors may set the stage for morphological and physiological phenotypic differences observed among breeds. To investigate the effect of breed-specific yolk composition on embryonic broiler and layer chicken phenotypes, we employed an ex ovo, xenobiotic technique that allowed the transfer of broiler and layer chicken embryos from their natural yolks to novel yolk environments. Embryonic day two broiler embryos developing on broiler yolk culture medium (YCM) had significantly higher heart rates than layer embryos developing on layer YCM (176±7 beats min(-1) and 147±7 beats min(-1), respectively). Broiler embryos developing on layer YCM exhibited heart rates typical of layer embryos developing normally on layer YCM. However, layer embryo heart rates were not affected by development on broiler YCM. Unlike O(2) consumption, development rate and body mass of embryos were significantly affected by exposure to different yolk types, with both broiler and layer embryos displaying traits that reflected yolk source rather than embryo genotype. Analysis of hormone concentrations of broiler and layer egg yolks revealed that testosterone concentrations were higher in broiler yolk (4.63±2.02 pg mg(-1) vs 3.32±1.92 pg mg(-1)), whereas triiodothyronine concentrations were higher in layer yolk (1.05±0.18 pg mg(-1) vs 0.46±0.22 pg mg(-1)). Thus, a complex synergistic effect of breed-specific genotype and yolk environment exists early in chicken development, with yolk thyroid hormone and yolk testosterone as potential mediators of the physiological and morphological effects.

Gene expression profiling of copper-induced responses in the intertidal copepod Tigriopus japonicus using a 6K oligochip microarray

The intertidal copepod Tigriopus japonicus has shown promising results in classical acute and chronic toxicity studies. Recently, a large number of genes have been identified from this species and their mRNA expression has been studied independently against exposure to marine environmental pollutants. T. japonicus is a promising organism for the study of mechanistic aspects of marine environmental pollutants using genomics. In this study, a 6K oligochip for T. japonicus that included mostly unique sets of genes from approximately 26K ESTs, was developed. A total of 5463 spots (2313 mRNAs upregulated and 3150 downregulated) were identified to be significantly expressed on microarray by hierarchical clustering of genes after exposure to copper for different time durations (10 microg/L for 6, 12 and 24h). However, mRNAs of only 138 and 375 genes were observed to be consistently upregulated and downregulated, respectively, at all time points. Most of the changes of mRNA expression were observed at the short exposure of 6h. It was observed that mRNA expression of several genes involved in growth, metabolism, reproduction and hormonal regulation was modulated in Cu-exposed T. japonicus. mRNA expression of genes involved in detoxification and antioxidant functions was also modulated. This indicates that Cu-induced gene transcription is complicated in T. japonicus similar to other crustaceans. Cu specifically upregulated mRNAs of genes of some isoforms of cytochrome P450 (CYP). On the other hand, a majority of downregulated mRNAs were of genes encoding for proteins important for growth and development. The expression profile of mRNAs of selected genes was verified by the quantitative real time RT-PCR. The mRNA expression profiles provide insight into the mechanism of action of copper in T. japonicus. These results demonstrate the suitability of a T. japonicus oligochip microarray for risk assessment of trace metals in the marine environment. As yet, major breakthroughs in invertebrate toxicogenomics have mainly been in Daphnia and Drosophila. Daphnia's use is limited to freshwater ecotoxicogenomics. Here we propose an oligochip microarray-based approach for risk assessment of trace metals in a potential model marine test species.

Variation in yolk precursor receptor mRNA expression is a key determinant of reproductive phenotype in the zebra finch (Taeniopygia guttata)

The vitellogenin/very low density lipoprotein receptor (VTG/VLDL-R), a 95 kDa protein that belongs to the low density lipoprotein receptor gene family,mediates the uptake of yolk precursors by developing follicles during oocyte growth. However, the extent to which variation in VTG/VLDL-R expression plays a role in determining inter-individual variation in reproductive phenotype(e.g. follicle or egg size) is not known. Here we show that the mRNA sequence of the zebra finch (Taeniopygia guttata) VTG/VLDL-R shows a high degree of sequence identity (92%) with chicken VTG/VLDL-R mRNA. Using quantitative real-time PCR we measured transcriptional expression of VTG/VLDL-R mRNA in various tissues, and for different stages of oocyte growth,in individual female zebra finches. VTG/VLDL-R mRNA was expressed at high levels in vitellogenic oocytes and in skeletal muscle, and was also detectable in liver, but these tissues expressed different splice variants: the short-form LR8–in oocytes and liver, and the LR8+ form in skeletal muscle. There was significant temporal variation in VTG/VLDL-R expression during follicle growth, with highest levels in ovary and a gradual decrease from pre-F3 to F1 vitellogenic follicles. Variation in ovary mRNA expression was correlated with inter-individual variation in clutch size and laying interval. Furthermore, variation in F3 follicle VTG/VLDL-R mRNA expression was correlated with inter-individual variation in egg mass and F1 follicle mass,suggesting that VTG/VLDL receptor mRNA expression is a key determinant of inter-individual variation in reproductive phenotype.

Population origin, development and temperature of development affect the amounts of HSP70, HSP90 and the putative hypoxia-inducible factor in the tadpoles of the common frog Rana temporaria

We raised Rana temporaria tadpoles from three different populations from southern, mid and northern Sweden (the total north-to-south distance between populations is approximately 1500 km) at two temperatures, and measured the differences in HSP70, HSP90 and putative HIF-1alpha levels (Rana temporaria HIF-1alpha was sequenced in the present study) with immunoblotting. The levels of the studied proteins increased with developmental stage. Also, the levels increased with latitude at the lower but not at the higher developmental temperature. This shows that there is a clear difference between the populations at the molecular level but that this difference can be modified by the environmental conditions experienced during development. The proteins analyzed may be involved in the regulation of developmental processes. If this is the case, the tadpoles from the northernmost population have the most advanced complement of regulatory proteins at developmental stages approaching metamorphosis.

Individual variation in endocrine systems: moving beyond the 'tyranny of the Golden Mean'

Twenty years ago, Albert Bennett published a paper in the influential book New directions in ecological physiology arguing that individual variation was an 'underutilized resource'. In this paper, I review our state of knowledge of the magnitude, mechanisms and functional significance of phenotypic variation, plasticity and flexibility in endocrine systems, and argue for a renewed focus on inter-individual variability. This will provide challenges to conventional wisdom in endocrinology itself, e.g. re-evaluation of relatively simple, but unresolved questions such as structure-function relationships among hormones, binding globulins and receptors, and the functional significance of absolute versus relative hormone titres. However, there are also abundant opportunities for endocrinologists to contribute solid mechanistic understanding to key questions in evolutionary biology, e.g. how endocrine regulation is involved in evolution of complex suites of traits, or how hormone pleiotropy regulates trade-offs among life-history traits. This will require endocrinologists to embrace the raw material of adaptation (heritable, individual variation and phenotypic plasticity) and to take advantage of conceptual approaches widely used in evolutionary biology (selection studies, reaction norms, concepts of evolutionary design) as well as a more explicit focus on the endocrine basis of life-history traits that are of primary interest to evolutionary biologists (cf. behavioural endocrinology).