LDH-B genotype-specific hatching times of Fundulus heteroclitus embryos

Ecology, the pace-of-life, epistatic selection and the maintenance of genetic variation in life-history genes

Evolutionary genetics has long struggled with understanding how functional genes under selection remain polymorphic in natural populations. Taking as a starting point that natural selection is ultimately a manifestation of ecological processes, we spotlight an underemphasized and potentially ubiquitous ecological effect that may have fundamental effects on the maintenance of genetic variation. Negative frequency dependency is a well-established emergent property of density dependence in ecology, because the relative profitability of different modes of exploiting or utilizing limiting resources tends to be inversely proportional to their frequency in a population. We suggest that this may often generate negative frequency-dependent selection (NFDS) on major effect loci that affect rate-dependent physiological processes, such as metabolic rate, that are phenotypically manifested as polymorphism in pace-of-life syndromes. When such a locus under NFDS shows stable intermediate frequency polymorphism, this should generate epistatic selection potentially involving large numbers of loci with more minor effects on life-history (LH) traits. When alternative alleles at such loci show sign epistasis with a major effect locus, this associative NFDS will promote the maintenance of polygenic variation in LH genes. We provide examples of the kind of major effect loci that could be involved and suggest empirical avenues that may better inform us on the importance and reach of this process.

Natural Selection at the Edge of Life: Allelic Polymorphism and Recruitment in High Latitude Arctic Char (Salvelinus alpinus) Generated and Maintained by Environmental Extremes

Information from extreme habitats of polymorphic populations is expected to answer questions related to evolutionary changes occurring at their niche border. Landlocked and resident/anadromous populations of the Arctic char (Salvelinus alpinus (L.) sp. complex), sampled during three successive years in northern Svalbard, were assessed for life-history characteristics, allele-frequency variation at the polymorphic EST-2* locus, and tested for genotype–environment interactions. While year-class strength correlated positively with mean air temperature during the two summers preceding spawning, the EST-2*100 allele frequency correlated positively with the air temperature in June after birth. By affecting two asynchronous and independent population variables, which are both no doubt correlated with fitness, annual temperature variation is suggested to generate and maintain polymorphism in Arctic char in the High Arctic by modifying year-class strength and selecting for variant alleles influencing cold resistance. Intra- and inter-population comparisons imply an additional selection between the two variant EST-2* alleles to operate ontogenetically, with a fixation on the 90 allele in landlocked and resident individuals and the 100 allele in anadromous individuals. The selective mechanism behind the latter processes is unknown. Because of low substrate specificity, however, esterases may form a reserve of adaptive ability towards environmental stress during contrasting conditions.

Embryonic development of the annual killifish Austrofundulus limnaeus: An emerging model for ecological and evolutionary developmental biology research and instruction

BACKGROUND

Austrofundulus limnaeus is an annual killifish from the Maracaibo basin of Venezuela. Annual killifishes are unique among vertebrates in their ability to enter into a state of dormancy at up to three distinct developmental stages termed diapause I, II, and III. These embryos are tolerant of a wide variety of environmental stresses and develop relatively slowly compared with nonannual fishes.

RESULTS

These traits make them an excellent model for research on interactions between the genome and the environment during development, and an excellent choice for developmental biology laboratories. Furthermore, A. limnaeus is relatively easy to maintain in a laboratory setting and has a high fecundity, making it an excellent candidate as an emerging model for studies of development, and for defining the limits of developmental buffering in vertebrates.

CONCLUSIONS

This study reports for the first time on the detailed development of A. limnaeus and provides a photographic and illustrated atlas of embryos on the two developmental trajectories possible in this species. Developmental Dynamics 246:779-801, 2017. © 2017 The Authors Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Steep, coincident, and concordant clines in mitochondrial and nuclear-encoded genes in a hybrid zone between subspecies of Atlantic killifish, Fundulus heteroclitus

Steep genetic clines resulting from recent secondary contact between previously isolated taxa can either gradually erode over time or be stabilized by factors such as ecological selection or selection against hybrids. We used patterns of variation in 30 nuclear and two mitochondrial SNPs to examine the factors that could be involved in stabilizing clines across a hybrid zone between two subspecies of the Atlantic killifish, Fundulus heteroclitus. Increased heterozygote deficit and cytonuclear disequilibrium in populations near the center of the mtDNA cline suggest that some form of reproductive isolation such as assortative mating or selection against hybrids may be acting in this hybrid zone. However, only a small number of loci exhibited these signatures, suggesting locus-specific, rather than genomewide, factors. Fourteen of the 32 loci surveyed had cline widths inconsistent with neutral expectations, with two SNPs in the mitochondrial genome exhibiting the steepest clines. Seven of the 12 putatively non-neutral nuclear clines were for SNPs in genes related to oxidative metabolism. Among these putatively non-neutral nuclear clines, SNPs in two nuclear-encoded mitochondrial genes (SLC25A3 and HDDC2), as well as SNPs in the myoglobin, 40S ribosomal protein S17, and actin-binding LIM protein genes, had clines that were coincident and concordant with the mitochondrial clines. When hybrid index was calculated using this subset of loci, the frequency distribution of hybrid indices for a population located at the mtDNA cline center was non-unimodal, suggesting selection against advanced-generation hybrids, possibly due to effects on processes involved in oxidative metabolism.

Aerobic and anaerobic capacities differ in embryos of the annual killifish Austrofundulus limnaeus that develop on alternate developmental trajectories

Embryos of the annual killifish Austrofundulus limnaeus have a remarkable tolerance to anoxia during their development, especially during diapause II (DII), but little is known about potential mechanisms by which this tolerance is achieved. This study examined the aerobic and anaerobic capacities of these embryos as they develop along alternate developmental trajectories and in response to altered incubation temperature. Aerobic and anaerobic capacities were estimated by measuring the total activity of the enzymes citrate synthase (CS) and lactate dehydrogenase (LDH), respectively. Embryos of A. limnaeus exhibit high anaerobic capacity throughout development as evidenced by high LDH/CS ratios, especially during early development through DII. Anaerobic production of lactate is supported by the heart isoform of LDH, even in stages of development that exhibit extreme tolerance of anoxia. CS capacity is extremely low during DII and may indicate an active suppression of mitochondrial metabolism during this stage of dormancy. Post-DII and "escape" embryos which bypass DII increase their aerobic and anaerobic capacities in tandem as they develop. The activity of both LDH and CS continue to increase for many days after morphological development ceases during DIII. Based on this observation, it is likely that regulation of metabolic dormancy is different in DII and III. Escape embryos seem to develop along a different metabolic trajectory than do embryos that enter diapause. Importantly, these embryos complete development with different enzymatic capacities that could influence physiological and ecological performance during early larval life.

Embryonic gene expression among pollutant resistant and sensitive Fundulus heteroclitus populations

Changes in gene expression, coupled with biochemical, physiological, and behavioral alterations, play a critical role in adaptation to environmental stress. Our goal was to explore ways natural populations may have adapted to local, polluted environments. We took advantage of natural populations of Fundulus heteroclitus, one of the few studied fish species in North America that has established resistant populations in highly contaminated urban estuaries. We analyzed morphology, physiology, and gene expression of developing F. heteroclitus embryos during late organogenesis (stage 31); these embryos were from both resistant and sensitive populations and were raised in a common, unpolluted environment. While cardiac heart rates show significant differences between embryos of parents from clean and heavily contaminated Superfund sites, time-to-stage, embryo morphology, and gene expression profile analyses do not differ significantly between untreated embryos from resistant and sensitive populations. Further evaluation that includes tissue-specific approaches in gene expression analysis and larger sample sizes may be necessary to highlight important phenotypes associated with mechanisms of sensitivity and resistance among natural F. heteroclitus embryo populations. Alternatively, population differences may be masked by developmental canalization, and biologically important differences between sensitive and resistant embryos may only manifest with exposure (e.g., be dependent on gene by environment interactions).

Developmental plasticity, modularity, and heterochrony during the phylotypic stage of the zebra fish, Danio rerio

We studied early embryonic development of zebra fish and tested if changes in the external raising conditions could elicit phenotypic changes during the phylotypic stage which, classically, is considered as a conserved embryonic stage. In particular, we tested for internal constraints, plasticity, and heterochrony during the early embryonic development. Our tested hypotheses predict (i) no change associated with developmental stability/internal constraints, (ii) change of the rate of development associated with developmental flexibility, and (iii) heterochronic disruption of developmental pattern associated with a modular organization of the embryo. We measured 14 traits of embryos raised in different conditions (temperature, salinity, oxygen concentration). The results of our study show that zebra fish embryos respond flexibly to changes in external parameters even during the conserved "phylotypic stage." It also showed that internal constraints canalize early development when exposed to moderate external challenges. Hypoxic conditions, however, elicited a heterochronic delay of the onset of the development of the Anlagen of the eye and the otic vesicle from the remaining embryo. Therefore, we concluded that the eye and the otic vesicle are modules that may develop, to a certain degree, independently of the rest of the embryo. Because these modules become recognizable only under specific raising conditions, we suggest that the modularization acts as buffering mechanism against extreme developmental deviations. Our results provide support to the idea that modularity is present during the phylotypic stage, but it is not effective under normal conditions.

Genomic approaches with natural fish populations

Natural populations v. inbred stocks provide a much richer resource for identifying the effects of nucleotide substitutions because natural populations have greater polymorphism. Additionally, natural populations offer an advantage over most common research organisms because they are subject to natural selection, and analyses of these adaptations can be used to identify biologically important changes. Among fishes, these analyses are enhanced by having a wide diversity of species (>28 000 species, more than any other group of vertebrates) living in a huge range of environments (from below freezing to > 46 degrees C, in fresh water to salinities >40 ppt.). Moreover, fishes exhibit many different life-history and reproductive strategies and have many different phenotypes and social structures. Although fishes provide numerous advantages over other vertebrate models, there is still a dearth of available genomic tools for fishes. Fishes make up approximately half of all known vertebrate species, yet <0.2% of fish species have significant genomic resources. Nonetheless, genomic approaches with fishes have provided some of the first measures of individual variation in gene expression and insights into environmental and ecological adaptations. Thus, genomic approaches with natural fish populations have the potential to revolutionize fundamental studies of diverse fish species that offer myriad ecological and evolutionary questions.

Linking genotypes to phenotypes and fitness: how mechanistic biology can inform molecular ecology

The accessibility of new genomic resources, high-throughput molecular technologies and analytical approaches such as genome scans have made finding genes contributing to fitness variation in natural populations an increasingly feasible task. Once candidate genes are identified, we argue that it is necessary to take a mechanistic approach and work up through the levels of biological organization to fully understand the impacts of genetic variation at these candidate genes. We demonstrate how this approach provides testable hypotheses about the causal links among levels of biological organization, and assists in designing relevant experiments to test the effects of genetic variation on phenotype, whole-organism performance capabilities and fitness. We review some of the research programs that have incorporated mechanistic approaches when examining naturally occurring genetic and phenotypic variation and use these examples to highlight the value of developing a comprehensive understanding of the relationship between genotype and fitness. We give suggestions to guide future research aimed at uncovering and understanding the genetic basis of adaptation and argue that further integration of mechanistic approaches will help molecular ecologists better understand the evolution of natural populations.

Adaptive plasticity of killifish (Fundulus heteroclitus) embryos: dehydration-stimulated development and differential aquaporin-3 expression

Embryos of the marine killifish Fundulus heteroclitus are adapted to survive aerially. However, it is unknown if they are able to control development under dehydration conditions. Here, we show that air-exposed blastula embryos under saturated relative humidity were able to stimulate development, and hence the time of hatching was advanced with respect to embryos continuously immersed in seawater. Embryos exposed to air at later developmental stages did not hatch until water was added, while development was not arrested. Air-exposed embryos avoided dehydration probably because of their thickened egg envelope, although it suffered significant evaporative water loss. The potential role of aquaporins as part of the embryo response to dehydration was investigated by cloning the aquaporin-0 (FhAqp0), -1a (FhAqp1a), and -3 (FhAqp3) cDNAs. Functional expression in Xenopus laevis oocytes showed that FhaAqp1a was a water-selective channel, whereas FhAqp3 was permeable to water, glycerol, and urea. Expression of fhaqp0 and fhaqp1a was prominent during organogenesis, and their mRNA levels were similar between water- and air-incubated embryos. However, fhaqp3 transcripts were highly and transiently accumulated during gastrulation, and the protein product was localized in the basolateral membrane of the enveloping epithelial cell layer and in the membrane of ingressing and migrating blastomers. Interestingly, both fhaqp3 transcripts and FhAqp3 polypeptides were downregulated in air-exposed embryos. These data demonstrate that killifish embryos respond adaptively to environmental desiccation by accelerating development and that embryos are able to transduce dehydration conditions into molecular responses. The reduced synthesis of FhAqp3 may be one of these mechanisms to regulate water and/or solute transport in the embryo.

Fundulus as the premier teleost model in environmental biology: opportunities for new insights using genomics

A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.

Different genes underlie adaptive melanism in different populations of rock pocket mice

Identifying the genes responsible for adaptation has been an elusive goal in evolutionary biology. Rock pocket mice (Chaetodipus intermedius) provide a useful system for studying the genetics of adaptation: most C. intermedius are light-coloured and live on light-coloured rocks, but in several different geographical regions, C. intermedius are melanic and live on dark-coloured basalt lava, presumably as an adaptation for crypsis. Previous work demonstrated that mutations at the melanocortin-1 receptor gene (Mc1r) are responsible for the dark/light difference in mice from one population in Arizona. Here, we investigate whether melanism has evolved independently in populations of dark C. intermedius from New Mexico, and whether the same or different genes underlie the dark phenotype in mice from these populations compared with the dark mice from Arizona. Seventy-six mice were collected from pairs of dark and light localities representing four different lava flows and adjacent light-coloured rocks; lava flows were separated by 70-750 km. Spectrophotometric analysis of mouse pelage and of rock samples revealed a strong positive association between coat colour and substrate colour. No significant differences were observed in the colour of rocks among the four lava flows, suggesting that mice in these separate populations have experienced similar selection for crypsis. Despite this similarity in environment, melanic mice from the three New Mexico populations were slightly, but significantly, darker than melanic mice from Arizona. The entire Mc1r gene was sequenced in all mice. The previously identified mutations responsible for the light/dark difference in mice from Arizona were absent in all melanic mice from three different populations in New Mexico. Five new Mc1r polymorphisms were observed among mice from New Mexico, but none showed any association with coat colour. These results indicate that adaptive melanism has arisen at least twice in C. intermedius and that these similar phenotypic changes have a different genetic basis.

Genetic structure of Fundulus heteroclitus from PAH-contaminated and neighboring sites in the Elizabeth and York Rivers

Population genetic characteristics of mummichog, Fundulus heteroclitus, from the heavily industrialized Elizabeth River and nearby York River (Virginia USA) were assessed relative to sediment PAH concentrations. Allozyme genotype frequencies for all loci were consistent with random mating expectations at each locality and age class. Fish from all sites had comparable levels of enzyme polymorphism and heterozygosity regardless of the associated sediment PAH concentrations. Allozyme frequencies for 12 of 15 loci were homogeneous for mummichog from all localities except that allozyme frequencies were significantly different for the Idh-2 locus of (adult and juvenile) mummichog at the heavily-contaminated Atlantic Wood site relative to all other sites. Additionally, allele frequency differences were noted for Ldh-C and Gpi-1 among juvenile mummichog. Values for F(st) were 0.0254 and 0.0141 in the juvenile and adult samples, respectively, indicating greater among-locality genetic differentiation for juvenile mummichog than for adults. Juvenile mummichog are more likely to remain in their natal area while adult samples reflect movement of fish during two or more winter seasons. Correlation analysis suggested that genetic differentiation was not correlated with geographic distance at the spatial scale studied here; however, there was a significant correlation between genetic distance and differences among sites in organic carbon-normalized PAH concentrations. Mummichog collected at the heavily PAH-contaminated AW locality were genetically distinct from those at neighboring sites.

Temporal change in the genetic structure between and within cohorts of a marine fish, Diplodus sargus, induced by a large variance in individual reproductive success

Temporal changes at 16 allozyme loci in the Diplodus sargus population of Banyuls-sur-Mer (Mediterranean Sea, France) were monitored. Temporal genetic variation within a single population was examined over two temporal scales: (i) among three year-classes sampled at the same age, and (ii) within a single year-class sampled three times over a two-year period. We observed a significant change in the genotypic structure within the same cohort during the first two years following settlement and before recruitment into the adult population. In addition, comparison of year-classes showed that cohorts differed significantly one year after settlement, whereas they became similar later on before recruitment into the adult population. The observed changes in the genetic structure within and between year-classes may be the result of complex selective processes or genetic drift. Linkage disequilibrium and genetic relatedness data suggest that these changes are due to large variation in reproductive success, followed by homogenization through adult movement. Overall, these results demonstrated a rapid genetic change within a population.

Environmental adaptations as windows on molecular evolution

Changes in gene regulation may play an important role in adaptive evolution, particularly during adaptation to a changing environment. However, little is known about the molecular mechanisms underlying adaptively significant variation in gene regulation. To address this question, we are using environmental adaptations in populations of a fish, Fundulus heteroclitus as a window into the molecular evolution of gene regulation. F. heteroclitus are found along the East Coast of North America, with populations distributed along a steep thermal gradient. At the extremes of the species range, populations have undergone local adaptation to their habitat temperatures. A variety of genes differ in their regulation between these populations. We have determined the mechanism responsible for changes in lactate dehydrogenase-B (Ldh-B) gene regulation. A limited number of mutations in the regulatory sequence of this gene result in changes in its expression. Both the phenotypic (increased LDH activity) and genotypic (changes in Ldh-B regulatory sequences) differences between populations have been shown to be affected by natural selection, rather than genetic drift. Therefore, even a small number of mutations within important regulatory sequences can provide evolutionarily significant variation and have an impact on environmental adaptation.

Functional and physiological consequences of genetic variation at phosphoglucose isomerase: heat shock protein expression is related to enzyme genotype in a montane beetle

Allele frequency variation at the phosphoglucose isomerase (PGI) locus in Californian populations of the beetle Chrysomela aeneicollis suggests that PGI may be undergoing natural selection. We quantified (i) apparent Michaelis-Menten constant (K(m)) of fructose 6-phosphate at different temperatures and (ii) thermal stability for three common PGI genotypes (1-1, 1-4, and 4-4). We also measured air temperature (T(a)) and beetle body temperature (T(b)) in three montane drainages in the Sierra Nevada, California. Finally, we measured 70-kDa heat shock protein (Hsp70) expression in field-collected and laboratory-acclimated beetles. We found that PGI allele 1 predominated in the northernmost drainage, Rock Creek (RC), which was also significantly cooler than the southernmost drainage, Big Pine Creek (BPC), where PGI allele 4 predominated. Allele frequencies and air temperatures were intermediate in the middle drainage, Bishop Creek (BC). Differences among genotypes in K(m) (1-1 > 1-4 > 4-4) and thermal stability (4-4 > 1-4 > 1-1) followed a pattern consistent with temperature adaptation. In nature, T(b) was closely related to T(a). Hsp70 expression in adult beetles decreased with elevation and differed among drainages (BPC > BC > RC). After laboratory acclimation (8 days, 20 degrees C day, 4 degrees C night) and heat shock (4 h, 28-36 degrees C), Hsp70 expression was greater for RC than BPC beetles. In RC, field-collected beetles homozygous for PGI 1-1 had higher Hsp70 levels than heterozygotes or a 4-4 homozygote. These results reveal functional and physiological differences among PGI genotypes, which suggest that montane populations of this beetle are locally adapted to temperature.

Adaptive variation in lactate dehydrogenase-B gene expression: role of a stress-responsive regulatory element

Although changes in gene regulation may play an important role in adaptive evolution, there have been few attempts to investigate the molecular mechanisms responsible for adaptively significant variation in gene expression. Here we describe the mechanism underlying an adaptive difference in the expression of the lactate dehydrogenase-B gene (Ldh-B) between northern and southern populations of the fish Fundulus heteroclitus. Ldh-B regulatory sequences from northern and southern individuals, coupled to a luciferase reporter gene, were introduced into the livers of live fish. Deletion studies indicated that sequence changes between 400 and 500 bp upstream of the transcription start site resulted in a 2-fold difference in reporter gene transcription. These sequence changes can account for the previously observed 2-fold difference in Ldh-B transcription between populations. Variation in transcription factors did not play an important role. Sequences within the functionally important region resemble a mammary tumor virus glucocorticoid responsive element (MTV-GRE) in southern alleles, whereas northern alleles differ from the consensus by 1 bp. To test the hypothesis that this element is involved in the variation between populations of F. heteroclitus, we exposed transiently transgenic fish containing Ldh-B regulatory sequence/reporter gene constructs to handling stress or injected cortisol. Both treatments increased reporter gene transcription driven by southern alleles but not northern alleles, as expected if an MTV-GRE sequence were involved. This finding suggests that sequence variation in a GRE is the cause of the adaptive differences in Ldh-B gene expression between populations and demonstrates that small changes in gene regulatory sequences can have important evolutionary consequences.

Strong natural selection causes microscale allozyme variation in a marine snail

Natural selection is one of the most fundamental processes in biology. However, there is still a controversy over the importance of selection in microevolution of molecular traits. Despite the general lack of data most authors hold the view that selection on molecular characters may be important, but at lower rates than selection on most phenotypic traits. Here we present evidence that natural selection may contribute substantially to molecular variation on a scale of meters only. In populations of the marine snail Littorina saxatilis living on exposed rocky shores, steep microclines in allele frequencies between splash and surf zone groups are present in the enzyme aspartate aminotransferase (allozyme locus Aat; EC. 2.6.1.1). We followed one population over 7 years, including a period of strong natural perturbation. The surf zone part of the population dominated by the allele Aat100 was suddenly eliminated by a bloom of a toxin-producing microflagellate. Downshore migration of splash zone snails with predominantly Aat120 alleles resulted in a drastic increase in surf zone frequency of Aat120, from 0.4 to 0.8 over 2 years. Over the next four to six generations, however, the frequency of Aat120 returned to the original value. We estimated the coefficient of selection of Aat120 in the surf zone to be about 0.4. Earlier studies show similar or even sharper Aat clines in other countries. Thus, we conclude that microclinal selection is an important evolutionary force in this system.

Rapid enzyme assays investigating the variation in the glycolytic pathway in field-caught populations of Fundulus heteroclitus

Variation in enzyme expression may be important in evolutionary adaptation, yet is seldom studied. Furthermore, no studies have examined the expression of all enzymes in a defined metabolic pathway. Enzyme concentration is a measure of enzyme expression and was ascertained by assaying maximal activity. Presented here is an analysis of variation of maximal enzyme activity for all the enzymes in a single metabolic pathway, glycolysis, from three clinically distributed populations of the fish, Fundulus heteroclitus. Techniques for rapidly analyzing maximal enzyme activity for all the enzymes of an entire metabolic pathway from many individuals are described. The high degree of repeatability (mean coefficient of variation for replicates, 4.4%) and sensitivity (less than 3 mg of tissue is required to measure all 10 enzymes) of these assays demonstrate the utility of such an approach for analyzing variation among populations for a large numbers of enzymes. Results from these studies indicate that (1) the average coefficient of variation for all enzyme determinations within a population is 45.3% and (2) between populations, the activity of 5 of the 10 glycolytic enzymes are significantly different. This considerable variation occurs even in populations where there is little allelic variation. These data demonstrating substantial variation in enzyme expression support the idea that changes in gene regulation may be as important as, or even more important than, changes in biochemical kinetic parameters in evolutionary processes.

Concordant mitochondrial and nuclear DNA phylogenies for populations of the teleost fish Fundulus heteroclitus

Molecular phylogenies using mitochondrial DNA and nuclear alleles of the lactate dehydrogenase B locus were found to be concordant for populations of Fundulus heteroclitus ranging from Canada to Florida. Both mitochondrial DNA and lactate dehydrogenase alleles show a clear separation between the northern individuals (from Nova Scotia and Maine) and the southern ones (from Georgia and Florida), with a mixed population found in the geographic intermediate (New Jersey). An historical isolation, possibly as ancient as 0.5-1 million years old, may have played a role in shaping the situation observed today.

Regulation of fitness in yeast overexpressing glycolytic enzymes: responses to heat shock and nitrogen starvation

Current models based on the analysis of linear metabolic pathways at steady-state predict that large increases over wild type in the activity of one enzyme will not alter an organism's fitness. This prediction is tested at steps in a highly branched pathway under two conditions known to alter steady-state: heat shock and nitrogen starvation. Saccharomyces cerevisiae transformants overproducing 1 of 4 enzymes in glycolysis (hexokinase B, phosphoglucose isomerase, phosphofructokinase, or pyruvate kinase) were subjected to heat shock in both exponential and stationary phases of growth. In neither phase does enzyme overexpression alter heat shock sensitivity. When starved for nitrogen in acetate medium, transformants overproducing hexokinase, phosphoglucose isomerase, and phosphofructokinase sporulate at the same rate and with the same frequency as cells harbouring only the plasmid vector. Current models therefore correctly predict the relationship between activity and components of fitness for 3 of 4 enzymes. By contrast, cells overexpressing pyruvate kinase sporulate poorly. This defect is not observed among cells transformed with a plasmid containing a Tn5 disrupted copy of the PYK gene. These findings are consistent with reports that implicate the PYK locus in yeast cell cycle control and suggest that it may be challenging to model relations between fitness and activity for multifunctional proteins.

Evidence of lactate dehydrogenase-B allozyme effects in the teleost, Fundulus heteroclitus

The evolutionary significance of protein polymorphisms has long been debated. Exponents of the balanced theory advocate that selection operates to maintain polymorphisms, whereas the neoclassical school argues that most genetic variation is neutral. Some studies have suggested that protein polymorphisms are not neutral, but their significance has been questioned because one cannot eliminate the possibility that linked loci were responsible for the observed differences. Evidence is presented that an enzymatic phenotype can affect carbon flow through a metabolic pathway. Glucose flux differences between lactate dehydrogenase-B phenotypes of Fundulus heteroclitus were reversed by substituting the Ldh-B gene product of one homozygous genotype with that of another.

Molecular basis of evolutionary adaptation at the lactate dehydrogenase-B locus in the fish Fundulus heteroclitus

At the extremes of its natural distribution, populations of the common killifish Fundulus heteroclitus experience a difference of more than 15 degrees C in mean annual temperature. These populations are virtually fixed for two different codominant alleles at the heart-type lactate dehydrogenase locus (Ldh-B) which code for allozymes with different and adaptive kinetic responses to temperature. Two populations near the extremes of the species range (i.e., Maine and Georgia) were further studied for thermal adaptation at this locus. In the absence of any kinetic differences one would predict that to maintain a constant reaction velocity, 2 to 3 times as much enzyme would be required for each 10 degrees C decrease in environmental temperature. Consistent with this adaptive strategy and in addition to the adaptive kinetic characteristics, the LDH-B4 enzyme (EC 1.1.1.27) concentration and its mRNA concentration were approximately twice as great in the northern population as in the southern population. Acclimation experiments allow us to conclude that these differences are due to a combination of fixed genetic traits (evolutionary adaptation) and plastic responses to temperature (physiological acclimation). Furthermore, our calculations show that the LDH-B4 reaction velocities are essentially equivalent for these two populations, even though they live in significantly different thermal environments.

Fish as model systems

Fish represent the largest and most diverse group of vertebrates. Their evolutionary position relative to other vertebrates and their ability to adapt to a wide variety of environments make them ideal for studying both organismic and molecular evolution. A number of other characteristics make them excellent experimental models for studies in embryology, neurobiology, endocrinology, environmental biology, and other areas. In fact, they have played a critical role in the development of several of these disciplines. Research techniques that enable scientists to make isogenic lines in a single generation, create and maintain mutants, culture cells, and transfer cloned genes into embryos signal an increasing role for fish as experimental models.

Allozymes of glucose-6-phosphate isomerase differentially modulate pentose-shunt metabolism in the sea anemone Metridium senile

We tested the hypothesis that kinetic differences among allelic variants of glucose-6-phosphate isomerase (GPI; D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) from the sea anemone Metridium senile differentially modulate glucose metabolism at the glycolysis-pentose-shunt branch point. Fractional contribution of pentose shunt and absolute flux of glucose in glycolysis were measured in fasted or fed anemones acclimated to 5 degrees C or 15 degrees C. When fed, anemones of genotype Gpiss routed a greater fraction of glucose through the shunt than did Gpiff anemones; the effect was more pronounced at 5 degrees C than at 15 degrees C. This confirms predictions from kinetic and population data and is consistent with thermal selection maintaining the variation. Relative levels of shunt metabolism increased at 5 degrees C, compared with 15 degrees C, in fed anemones regardless of genotype, but the proportion of glucose metabolized by the pentose shunt was unchanged by temperature in fasted anemones. Glucose flux through the shunt was constant at approximately 5 pmol.mg-1.hr-1 in fed anemones at 5 degrees C and 15 degrees C and in fasted anemones at 15 degrees C, indicating apparently near-perfect thermal acclimation of the absolute flux of glucose through the shunt in fed, but not in fasted, anemones. Rates of glucose oxidation and flux through the shunt in freshly collected anemones were similar to those of anemones fed and acclimated at 15 degrees C in the laboratory. If these differences affect organismal-level processes, Gpi variation could contribute to Darwinian fitness in thermally varying environments.

Some aspects of isozymes of lactate dehydrogenase, malate dehydrogenase and glucosephosphate isomerase in fish

1. The present paper reports some aspects of the isozymes of LDH, MDH and GPI in fish. 2. In Petromyzontiformes LDH is encoded by a single Ldh-A gene locus. In Myxiniformes and in most vertebrates LDH is encoded by two gene loci, A and B. A third Ldh-C locus is characteristic of the bony fishes Actinopterygii. 3. In fish the MDH isozymes are generally encoded by three gene loci Mdh-M, Mdh-A and Mdh-B. 4. In most diploid bony fish the GPI is controlled by two independent gene loci Gpi-A and Gpi-B. 5. The relationships of isozymes with evolution of vertebrates, tissual specificity, ontogenetic changes, with physiological and metabolic roles are discussed.

Motor training and physical fitness: possible short- and long-term influences on the development of individual differences in behavior

Individual differences in the behavior of young and adult animals have been documented in diverse species. Possible sources of such variation are of interest to scientists representing many disciplines, including behavior, genetics, and population and evolutionary biology. Two variables that may be important in the ontogeny and maintenance of behavioral differences are (1) individual physical (aerobic and anaerobic) fitness and (2) possible genetic variations underlying individual abilities to engage in, and to benefit from, motor training early in life. The differential development of aerobic and anaerobic capacities may play a significant role in the ontogeny of individual differences in the performance of various motor skills. There also may be short- and long-term consequences of variations in physical fitness that influence individual abilities to perform energy demanding acts during aggressive encounters, interactions with prey or predators, and courtship and breeding. Genetic studies of a limited number of species indicate that specific genotypes are correlated with individual variations in motor performance, even among conspecifics. Multidisciplinary research concerning possible relationships among the ontogeny of physical fitness, genetics, and variations in behavior is needed. Recent work on the relationship between individual differences in physical fitness and variations in the behavior of adult cold-blooded vertebrates provides a good model for comparative research on warm-blooded species.