Phosphoglucose isomerase (Pgi) performance and fitness effects among Arthropods and its potential role as an adaptive marker in conservation genetics

Effects of phthalate and bisphenol plasticizers on the activity of glycolytic enzymes of the moth Spodoptera littoralis

Environmental plastic pollution has significantly increased in the recent decades, and severely impacts economies, human and biodiversity health. Plastics are made of several chemical additives, including bisphenol and phthalate plasticizers such as bisphenol A (BPA) and Di(2-ethylhexyl)phthalate (DEHP). In some animal species, both BPA and DEHP are known as endocrine disruptor compounds, and can alter physiological and metabolic homeostasis, reproduction, development and/or behavior. To date, the impacts of BPA and DEHP have mainly focused on vertebrates, and to a lesser extent, on aquatic invertebrates. Yet, the few studies which examined the effects of DEHP on terrestrial insects also revealed the impacts this pollutant can have on development, hormone titrations, and metabolic profiles. In particular, it has been hypothesized in the Egyptian cotton leafworm Spodoptera littoralis that the observed metabolic alterations could result from the energetic costs necessary for DEHP detoxification or to the dysregulation of hormonally-controlled enzymatic activities. To get additional insights into the physiological effects of bisphenol and phthalate plasticizers on the moth S. littoralis, larvae were fed with food contaminated by BPA, DEHP, or the mixture of both compounds. Then, activities of four glycolytic enzymes, hexokinase, phosphoglucose isomerase, phosphofructokinase, and pyruvate kinase were measured. BPA and/or DEHP had no effects on the activities of phosphofructokinase and pyruvate kinase. Conversely, BPA-contaminated larvae were characterized by a 1.9-fold increase in phosphoglucose isomerase activity, and BPA + DEHP-fed larvae had highly variable hexokinase activity. Overall, since no disruption of glycolytic enzyme was observed in DEHP-contaminated larvae, our work tended to demonstrate that exposure to bisphenol and DEHP increased the amount of oxidative stress experienced.

Climate change effects on animal ecology: butterflies and moths as a case study

Butterflies and moths (Lepidoptera) are one of the most studied, diverse, and widespread animal groups, making them an ideal model for climate change research. They are a particularly informative model for studying the effects of climate change on species ecology because they are ectotherms that thermoregulate with a suite of physiological, behavioural, and phenotypic traits. While some species have been negatively impacted by climatic disturbances, others have prospered, largely in accordance with their diversity in life-history traits. Here we take advantage of a large repertoire of studies on butterflies and moths to provide a review of the many ways in which climate change is impacting insects, animals, and ecosystems. By studying these climate-based impacts on ecological processes of Lepidoptera, we propose appropriate strategies for species conservation and habitat management broadly across animals.

Mitonuclear mismatch alters performance and reproductive success in naturally introgressed populations of a montane leaf beetle

Coordination between nuclear and mitochondrial genomes is critical to metabolic processes underlying animals' ability to adapt to local environments, yet consequences of mitonuclear interactions have rarely been investigated in populations where individuals with divergent mitochondrial and nuclear genomes naturally interbreed. Genetic variation in the leaf beetle Chrysomela aeneicollis was assessed along a latitudinal thermal gradient in California's Sierra Nevada. Variation at mitochondrial cytochrome oxidase II (COII) and the nuclear gene phosphoglucose isomerase (PGI) shows concordance and was significantly greater along a 65 km transect than 10 other loci. STRUCTURE analyses using neutral loci identified a southern and northern subpopulation, which interbreed in the central drainage Bishop Creek. COII and PGI were used as indicators of mitochondrial and nuclear genetic variation in field and laboratory experiments conducted on beetles from this admixed population. Fecundity, larval development rate, running speed and male mating frequency were higher for beetles with geographically "matched" than "mismatched" mitonuclear genotypes. Effects of mitonuclear mismatch were largest for individuals with northern nuclear genotypes possessing southern mitochondria and were most pronounced after heat treatment or at high elevation. These findings suggest that mitonuclear incompatibility diminishes performance and reproductive success in nature, effects that could intensify at environmental extremes.

Ecological and genetic basis of metapopulation persistence of the Glanville fritillary butterfly in fragmented landscapes

Ecologists are challenged to construct models of the biological consequences of habitat loss and fragmentation. Here, we use a metapopulation model to predict the distribution of the Glanville fritillary butterfly during 22 years across a large heterogeneous landscape with 4,415 small dry meadows. The majority (74%) of the 125 networks into which the meadows were clustered are below the extinction threshold for long-term persistence. Among the 33 networks above the threshold, spatial configuration and habitat quality rather than the pooled habitat area predict metapopulation size and persistence, but additionally allelic variation in a SNP in the gene Phosphoglucose isomerase (Pgi) explains 30% of variation in metapopulation size. The Pgi genotypes are associated with dispersal rate and hence with colonizations and extinctions. Associations between Pgi genotypes, population turnover and metapopulation size reflect eco-evolutionary dynamics, which may be a common feature in species inhabiting patch networks with unstable local dynamics.

The 3-D Structural Basis for the Pgi Genotypic Differences in the Performance of the Butterfly Melitaea cinxia at Different Temperatures

Although genotype-by-environment interaction has long been used to unveil the genetic variation that affects Darwinian fitness, the mechanisms underlying the interaction usually remain unknown. Genetic variation at the dimeric glycolytic enzyme phosphoglucoisomerase (Pgi) has been observed to interact with temperature to explain the variation in the individual performance of the butterfly Melitaea cinxia. At relatively high temperature, individuals with Pgi-non-f genotypes generally surpass those with Pgi-f genotypes, while the opposite applies at relatively low temperature. In this study, we did protein structure predictions and BlastP homology searches with the aim to understand the structural basis for this temperature-dependent difference in the performance of M. cinxia. Our results show that, at amino acid (AA) site 372, one of the two sites that distinguish Pgi-f (the translated polypeptide of the Pgi-f allele) from Pgi-non-f (the translated polypeptide of the Pgi-non-f allele), the Pgi-non-f-related residue strengthens an electrostatic attraction between a pair of residues (Glu373-Lys472) that are from different monomers, compared to the Pgi-f-related residue. Further, BlastP searches of animal protein sequences reveal a dramatic excess of electrostatically attractive combinations of the residues at the Pgi AA sites equivalent to sites 373 and 472 in M. cinxia. This suggests that factors enhancing the inter-monomer interaction between these two sites, and therefore helping the tight association of two Pgi monomers, are favourable. Our homology-modelling results also show that, at the second AA site that distinguishes Pgi-f from Pgi-non-f in M. cinxia, the Pgi-non-f-related residue is more entropy-favourable (leading to higher structural stability) than the Pgi-f-related residue. To sum up, this study suggests a higher structural stability of the protein products of the Pgi-non-f genotypes than those of the Pgi-f genotypes, which may explain why individuals carrying Pgi-non-f genotypes outperform those carrying Pgi-f genotypes at stressful high temerature.

A horizontally transferred nuclear gene is associated with microhabitat variation in a natural plant population

Horizontal gene transfer involves the non-sexual interspecific transmission of genetic material. Even if they are initially functional, horizontally transferred genes are expected to deteriorate into non-expressed pseudogenes, unless they become adaptively relevant in the recipient organism. However, little is known about the distributions of natural transgenes within wild species or the adaptive significance of natural transgenes within wild populations. Here, we examine the distribution of a natural plant-to-plant nuclear transgene in relation to environmental variation within a wild population. Festuca ovina is polymorphic for an extra (second) expressed copy of the nuclear gene (PgiC) encoding cytosolic phosphoglucose isomerase, with the extra PgiC locus having been acquired horizontally from the distantly related grass genus Poa. We investigated variation at PgiC in samples of F. ovina from a fine-scale, repeating patchwork of grassland microhabitats, replicated within spatially separated sites. Even after accounting for spatial effects, the distributions of F. ovina individuals carrying the additional PgiC locus, and one of the enzyme products encoded by the locus, are significantly associated with fine-scale habitat variation. Our results suggest that the PgiC transgene contributes, together with the unlinked 'native' PgiC locus, to local adaptation to a fine-scale mosaic of edaphic and biotic grassland microhabitats.

Evidence for Positive Selection within the PgiC1 Locus in the Grass Festuca ovina

The dimeric metabolic enzyme phosphoglucose isomerase (PGI, EC 5.3.1.9) plays an essential role in energy production. In the grass Festuca ovina, field surveys of enzyme variation suggest that genetic variation at cytosolic PGI (PGIC) may be adaptively important. In the present study, we investigated the molecular basis of the potential adaptive significance of PGIC in F. ovina by analyzing cDNA sequence variation within the PgiC1 gene. Two, complementary, types of selection test both identified PGIC1 codon (amino acid) sites 200 and 173 as candidate targets of positive selection. Both candidate sites involve charge-changing amino acid polymorphisms. On the homology-modeled F. ovina PGIC1 3-D protein structure, the two candidate sites are located on the edge of either the inter-monomer boundary or the inter-domain cleft; examination of the homology-modeled PGIC1 structure suggests that the amino acid changes at the two candidate sites are likely to influence the inter-monomer interaction or the domain-domain packing. Biochemical studies in humans have shown that mutations at several amino acid sites that are located close to the candidate sites in F. ovina, at the inter-monomer boundary or the inter-domain cleft, can significantly change the stability and/or kinetic properties of the PGI enzyme. Molecular evolutionary studies in a wide range of other organisms suggest that PGI amino acid sites with similar locations to those of the candidate sites in F. ovina may be the targets of positive/balancing selection. Candidate sites 200 and 173 are the only sites that appear to discriminate between the two most common PGIC enzyme electromorphs in F. ovina: earlier studies suggest that these electromorphs are implicated in local adaptation to different grassland microhabitats. Our results suggest that PGIC1 sites 200 and 173 are under positive selection in F. ovina.

Pgi: the ongoing saga of a candidate gene

Numerous studies have found amino acid variation at the phosphoglucose isomerase (PGI) gene associated with organismal performance and fitness. Here we focus upon recent advances in the study of this gene, highlighting novel species being studied, new tools being used, and emerging insights into the evolutionary dynamics acting on this gene. Our synthesis highlights questions that are coming into focus, as well as the need for attention in specific areas, such as manipulative experiments to establish mechanistic insights and a causative role of allelic variation.

Differences in the aerobic capacity of flight muscles between butterfly populations and species with dissimilar flight abilities

Habitat loss and climate change are rapidly converting natural habitats and thereby increasing the significance of dispersal capacity for vulnerable species. Flight is necessary for dispersal in many insects, and differences in dispersal capacity may reflect dissimilarities in flight muscle aerobic capacity. In a large metapopulation of the Glanville fritillary butterfly in the Åland Islands in Finland, adults disperse frequently between small local populations. Individuals found in newly established populations have higher flight metabolic rates and field-measured dispersal distances than butterflies in old populations. To assess possible differences in flight muscle aerobic capacity among Glanville fritillary populations, enzyme activities and tissue concentrations of the mitochondrial protein Cytochrome-c Oxidase (CytOx) were measured and compared with four other species of Nymphalid butterflies. Flight muscle structure and mitochondrial density were also examined in the Glanville fritillary and a long-distance migrant, the red admiral. Glanville fritillaries from new populations had significantly higher aerobic capacities than individuals from old populations. Comparing the different species, strong-flying butterfly species had higher flight muscle CytOx content and enzymatic activity than short-distance fliers, and mitochondria were larger and more numerous in the flight muscle of the red admiral than the Glanville fritillary. These results suggest that superior dispersal capacity of butterflies in new populations of the Glanville fritillary is due in part to greater aerobic capacity, though this species has a low aerobic capacity in general when compared with known strong fliers. Low aerobic capacity may limit dispersal ability of the Glanville fritillary.

Positive selection in glycolysis among Australasian stick insects

BACKGROUND

The glycolytic pathway is central to cellular energy production. Selection on individual enzymes within glycolysis, particularly phosphoglucose isomerase (Pgi), has been associated with metabolic performance in numerous organisms. Nonetheless, how whole energy-producing pathways evolve to allow organisms to thrive in different environments and adopt new lifestyles remains little explored. The Lanceocercata radiation of Australasian stick insects includes transitions from tropical to temperate climates, lowland to alpine habitats, and winged to wingless forms. This permits a broad investigation to determine which steps within glycolysis and what sites within enzymes are the targets of positive selection. To address these questions we obtained transcript sequences from seven core glycolysis enzymes, including two Pgi paralogues, from 29 Lanceocercata species.

RESULTS

Using maximum likelihood methods a signature of positive selection was inferred in two core glycolysis enzymes. Pgi and Glyceraldehyde 3-phosphate dehydrogenase (Gaphd) genes both encode enzymes linking glycolysis to the pentose phosphate pathway. Positive selection among Pgi paralogues and orthologues predominately targets amino acids with residues exposed to the protein's surface, where changes in physical properties may alter enzyme performance.

CONCLUSION

Our results suggest that, for Lancerocercata stick insects, adaptation to new stressful lifestyles requires a balance between maintaining cellular energy production, efficiently exploiting different energy storage pools and compensating for stress-induced oxidative damage.

Lack of variation at phosphoglucose isomerase (pgi) in bumblebees: implications for conservation genetics studies

Assessing genetic variation underlying ecologically important traits is increasingly of interest and importance in population and conservation genetics. For some groups generally useful markers exist for examining the relative role of selection and drift in shaping genetic diversity e.g. the major histocompatibility complex in vertebrates and self-incompatibility loci in plants. For invertebrates there is no such generally useful locus. However, phosphoglucose isomerase (Pgi) has been proposed as a useful functional marker in the conservation genetics of invertebrates. Where thermal microclimate varies, balanced polymorphisms may be maintained due to trade-offs between thermally stable and kinetically advantageous allelic forms. We here report very low levels of Pgi variation in bumblebees rendering this locus to be of little use as an adaptive marker in a conservation genetics context in this group. Potential explanations for this lack of variation are considered.

Diversifying selection underlies the origin of allozyme polymorphism at the phosphoglucose isomerase locus in Tigriopus californicus

The marine copepod Tigriopus californicus lives in intertidal rock pools along the Pacific coast, where it exhibits strong, temporally stable population genetic structure. Previous allozyme surveys have found high frequency private alleles among neighboring subpopulations, indicating that there is limited genetic exchange between populations. Here we evaluate the factors responsible for the diversification and maintenance of alleles at the phosphoglucose isomerase (Pgi) locus by evaluating patterns of nucleotide variation underlying previously identified allozyme polymorphism. Copepods were sampled from eleven sites throughout California and Baja California, revealing deep genetic structure among populations as well as genetic variability within populations. Evidence of recombination is limited to the sample from Pescadero and there is no support for linkage disequilibrium across the Pgi locus. Neutrality tests and codon-based models of substitution suggest the action of natural selection due to elevated non-synonymous substitutions at a small number of sites in Pgi. Two sites are identified as the charge-changing residues underlying allozyme polymorphisms in T. californicus. A reanalysis of allozyme variation at several focal populations, spanning a period of 26 years and over 200 generations, shows that Pgi alleles are maintained without notable frequency changes. Our data suggest that diversifying selection accounted for the origin of Pgi allozymes, while McDonald-Kreitman tests and the temporal stability of private allozyme alleles suggests that balancing selection may be involved in the maintenance of amino acid polymorphisms within populations.

Eco-evolutionary spatial dynamics in the Glanville fritillary butterfly

Demographic population dynamics, gene flow, and local adaptation may influence each other and lead to coupling of ecological and evolutionary dynamics, especially in species inhabiting fragmented heterogeneous environments. Here, I review long-term research on eco-evolutionary spatial dynamics in the Glanville fritillary butterfly inhabiting a large network of approximately 4,000 meadows in Finland. The metapopulation persists in a balance between frequent local extinctions and recolonizations. The genetic spatial structure as defined by neutral markers is much more coarse-grained than the demographic spatial structure determined by the fragmented habitat, yet small-scale spatial structure has important consequences for the dynamics. I discuss three examples of eco-evolutionary spatial dynamics. (i) Extinction-colonization metapopulation dynamics influence allele frequency changes in the phosphoglucose isomerase (Pgi) gene, which leads to strong associations between genetic variation in Pgi and dispersal, recolonization, and local population dynamics. (ii) Inbreeding in local populations increases their risk for extinction, whereas reciprocal effects between inbreeding, population size, and emigration represent likely eco-evolutionary feedbacks. (iii) Genetically determined female oviposition preference for two host plant species exhibits a cline paralleling a gradient in host plant relative abundances, and host plant preference of dispersing females in relation to the host plant composition of habitat patches influences immigration (gene flow) and recolonization (founder events). Eco-evolutionary spatial dynamics in heterogeneous environments may not lead to directional evolutionary changes unless the environment itself changes, but eco-evolutionary dynamics may contribute to the maintenance of genetic variation attributable to fluctuating selection in space and time.

Functional genomics of life history variation in a butterfly metapopulation

In fragmented landscapes, small populations frequently go extinct and new ones are established with poorly understood consequences for genetic diversity and evolution of life history traits. Here, we apply functional genomic tools to an ecological model system, the well-studied metapopulation of the Glanville fritillary butterfly. We investigate how dispersal and colonization select upon existing genetic variation affecting life history traits by comparing common-garden reared 2-day adult females from new populations with those from established older populations. New-population females had higher expression of abdomen genes involved in egg provisioning and thorax genes involved in the maintenance of flight muscle proteins. Physiological studies confirmed that new-population butterflies have accelerated egg maturation, apparently regulated by higher juvenile hormone titer and angiotensin converting enzyme mRNA, as well as enhanced flight metabolism. Gene expression varied between allelic forms of two metabolic genes (Pgi and Sdhd), which themselves were associated with differences in flight metabolic rate, population age and population growth rate. These results identify likely molecular mechanisms underpinning life history variation that is maintained by extinction-colonization dynamics in metapopulations.