The Physiological Ecology of the Zebra Mussel, Dreissena polymorpha, in North America and Europe

SYNOPSIS. The zebra mussel, Dreissena polymorpha (Pallas), wasintroduced into North America in 1986. Initial North American(N.A.) studies suggested that physiological responses variedbetween N.A. and European populations. However, literature reviewindicates agreement on most aspects of physiological adaptationincluding: respiratory responses; hypoxia/anoxia tolerance;salinity limits; emersion tolerance; freezing resistance; environmentalpH limits; calcium limits; starvation responses; and bioenergeticpartitioning. The main differences among N.A. and European musselsappear to be elevated upper thermal limits and temperaturesfor optimum growth among N.A. populations. N.A. zebra musselsprobably originated from the northern shore of the Black Seain the warmest portion of the mussel's European range. However,most European physiological data come from northern Europe wherepopulations may be adapted to colder temperatures. Alternatively,N.A. research suggests that mussels may have a capacity forseasonal temperature acclimatization such that responses recordedin warmer N.A. waters may be different from those recorded innorthern Europe even after short-term laboratory acclimation.Studies of genetic variation and physiological response amongEuropean and N.A. D. polymorpha populations are required toelucidate the basis for physiological differentiation. Recentlyevolved D. polymorpha has poor resistance adaptations comparedto unionacean and sphaeriid bivalves with longer freshwaterfossil histories. Poor resistance adaptations make it less suitedfor stable habitats, instead, its high fecundities, early maturity,and rapid growth are adaptations to unstable habitats whereextensive resistance adaptations are of little value.     

Effects of shear stress on microalgae – A review

Cultivation of microalgae requires consideration of shear stress, which is generated by operations such as mixing, circulation, aeration and pumping that are designed to facilitate mass and heat transfer as well as light distribution in cultures. Excessive shear stress can cause increased cell mortality, decreased growth rate and cell viability, or even cell lysis. This review examines the sources of shear stress in different cultivation systems, shear stress tolerance of different microalgal species and the physiological factors and environmental conditions that may affect shear sensitivity, and potential approaches to mitigate the detrimental effects of shear stress. In general, green algae have the greatest tolerance to shear stress, followed by cyanobacteria, haptophytes, red algae, and diatoms, with dinoflagellates comprising the most shear-sensitive species. The shear-sensitivity of microalgae is determined primarily by cell wall strength, cell morphology and the presence of flagella. Turbulence, eddy size, and viscosity are the most prominent parameters affecting shear stress to microalgal cells during cultivation.     

Thermal tolerance,evaporative water loss,air-water oxygen consumption and zonation of intertidal prosobranchs: a new synthesis

Duration of emergence increases with tidal height on rocky shores therefore, emergence adaptations in intertidal species such as littorine and other prosobranch gastropods have been considered correlated with zonation patterns; temperature tolerance, desiccation resistance and aerial respiration rate all commonly assumed to increase progressively with increasing zonation level. Such direct correlations are rarely observed in nature. Maximal aerial gas exchange occurs in mid-shore, not high shore species. Temperature tolerance and desiccation resistance do not increase directly with shore height. Thus, hypotheses regarding physiological correlates of zonation require revaluation. A new hypothesis is presented that the high tide mark presents a single major physiological barrier on rocky shores. Above it, snails experience prolonged emergence and extensive desiccation; below it, predictable submergence and rehydration with each tidal cycle. Thus, desiccation stress is minimal below the high tide mark and maximal above it. Therefore, species restricted below high tide (the eulittoral zone) should display markedly different adaptive strategies to emergence than those above it (the eulittoral fringe). A review of the literature indicated that adaptations in eulittoral species are dominated by those allowing maintenance of activity and foraging in air including: evaporative cooling; low thermal tolerance; elevated aerial O₂ uptake rates; and high capacity for radiant heat absorption. Such adaptations exacerbate evaporative water loss. In contrast, species restricted to the eulittoral fringe display adaptive strategies that minimize desiccation and prolong survival of emergence including: foot withdrawal, preventing heat conduction from the substratum; aestivation in air; elevated thermal tolerance reducing necessity for evaporative cooling; position maintenance by cementation to the substratum and increased capacity for heat dissipation. In order to test of this hypothesis the upper thermal limits, tissue and substratum temperatures on emergence in direct sunlight and evaporative water loss and tissue temperatures on emergence in 40 °C were evaluated for specimens of six species of eulittoral and eulittoral fringe gastropods from a granite shore on Princess Royal Harbour near Albany, Western Australia. The results were consistant with adaptation to the proposed desiccation barrier at high tide. The eulittoral species, Austrocochlea constricta, Austrocochlea concamerata, Nerita atramentosa and Lepsiella vinosa, displayed adaptations dominated by maintenance of activity and foraging during emergence while the eulittoral fringe littorine species, Bembicium vittatum and Nodilittorina unifasciata displayed adaptations dominated by minization of activity and evaporative water loss during emergence. The evolution of adaptations allowing tolerance of prolonged desiccation have allowed littorine species to dominate high intertidal rocky shore gastropod faunas throughout the world's oceans.     

Oxygen deprivation stress in a changing environment

Past research into flooding tolerance and oxygen shortages inplants has been motivated largely by cultivation problems ofarable crops. Unfortunately, such species are unsuitable forinvestigating the physiological and biochemical basis of anoxia-toleranceas selection has reduced any tolerance of anaerobiosis and anaerobicsoil conditions that their wild ancestors might have possessed.Restoration of anoxia-tolerance to species that have lost thisproperty is served better by physiological and molecular studiesof the mechanisms that are employed in wild species that stillpossess long-term anoxia-tolerance. Case studies developingthese arguments are presented in relation to a selection ofcrop and wild species. The flooding sensitivity and metabolismof maize is compared in relation to rice in its capacity foranaerobic germination. The sensitivity of potato to floodingis related to its disturbed energy metabolism and inabilityto maintain functioning membranes under anoxia and postinoxia.By contrast, long-term anoxia-tolerance in the American cranberry(Vaccinium macrocarpon) and the arctic grass species Deschampsiaberingensis can be related to the provision and utilizationof carbohydrate reserves. Among temperate species, the sweetflag (Acorus calamus) shows a remarkable tolerance of anoxiain both shoots and roots and is also able to mobilize carbohydrateand maintain ATP levels during anoxia as well as preservingmembrane lipids against anoxic and post-anoxic injury. Phragmitesaustralis and Spartina alterniflora, although anoxia-tolerant,are both sulphide-sensitive species which can pre-dispose themto the phenomenon of die-back in stagnant, nutrient-rich water.Glyceria maxima adapts to flooding through phenological adaptationswith a seasonal metabolic tolerance of anoxia confined to winterand spring which, combined with a facility for root aerationand early spring growth, allows rapid colonization of siteswith only shallow flooding. The diversity of responses to floodingin wild plants suggests that, depending on the life strategyand habitat of the species, many different mechanisms may beinvolved in adapting plants to survive periods of inundationand no one mechanism on its own is adequate for ensuring survival. Key words: Anoxia, hypoxia, flooding, Zea mays, Solanum tuberosum, Oryza sativa, Acorus calamus, Phragmites australis, Glyceria maxima, cranberry     

Evolution of rDNA in Nicotiana allopolyploids: a potential link between rDNA homogenization and epigenetics

Background: The evolution and biology of rDNA have interested biologistsfor many years, in part, because of two intriguing processes:(1) nucleolar dominance and (2) sequence homogenization. Wereview patterns of evolution in rDNA in the angiosperm genusNicotiana to determine consequences of allopolyploidy on theseprocesses. Scope: Allopolyploid species of Nicotiana are ideal for studying rDNAevolution because phylogenetic reconstruction of DNA sequenceshas revealed patterns of species divergence and their parents.From these studies we also know that polyploids formed overwidely different timeframes (thousands to millions of years),enabling comparative and temporal studies of rDNA structure,activity and chromosomal distribution. In addition studies onsynthetic polyploids enable the consequences of de novo polyploidyon rDNA activity to be determined. Conclusions: We propose that rDNA epigenetic expression patterns establishedeven in F₁ hybrids have a material influence on the likely patternsof divergence of rDNA. It is the active rDNA units that arevulnerable to homogenization, which probably acts to reducemutational load across the active array. Those rDNA units thatare epigenetically silenced may be less vulnerable to sequencehomogenization. Selection cannot act on these silenced genes,and they are likely to accumulate mutations and eventually beeliminated from the genome. It is likely that whole silencedarrays will be deleted in polyploids of 1 million years of ageand older.     

Adaptations of the Ribbed Mussel, Modiolus demissus (Dillvvyn), to the Intertidal Habitat

SYNOPSIS. This is a review of the behavioral, biochemical-physiological,and morphological adaptations of Modiolus demissus (Dillwyn)to the high intertidal habitat. The ribbed mussel, Modiolus,has fully exploited the behavioral adaptations within the limitsimposed by the bivalve body plan. Air-gaping is undoubtedlya significant behavioral adaptation which permits aerial respirationand penetration of the high intertidal zone. Modiolus fullyutilizes physiological-biochemical adaptations also. The musselis both eurythermal and euryhaline: the thermal range is atleast 56 °C and the salinity range is at least 70%, bothimpressive ranges for an organism which conforms to these environmentalparameters. The two primary adaptations made by Modiolus arean extensive tolerance to dehydration and a very high enzymethermostability. These two adaptations are resjxmsible for Modiolus'tolerance toward desiccation, variation in salinity, thermalstress, and possibly anaerobic conditions. There is no obviousmorphological adaptation. Thus, the physiological adaptations,coupled with air-gaping, are responsible for the vertical penetrationof Modiolus into the intertidal zone and its horizontal migrationover an extensive geographic range.     

Adaptations against heme toxicity in blood-feeding arthropods

A blood-sucking habit appeared independently several times in the course of arthropod evolution. However, from more than a million species of insects and arachnids presently living on earth, only about 14,000 species developed the capacity to feed on vertebrate blood. This figure suggests the existence of severe physiological constraints for the evolution of hematophagy, implying the selective advantage of special adaptations related to the use of blood as a food source. Digestion of vertebrate hemoglobin in the midgut of blood-feeding arthropods results in the production of large amounts of heme, a potentially cytotoxic molecule. Here we will review mechanisms by which heme can exert biological damage, together with a wide spectrum of adaptations developed by blood-feeding insects and ticks to counteract its deleterious effects. In spite of the existence of a great molecular diversity of protective mechanisms, different hematophagous organisms developed convergent solutions that may be physiologically equivalent.     

Consequences of Parasitism to Marine Invertebrates: Host Evolution?

Parasitism among aquatic invertebrates is common, if not ubiquitous,and can be pathological to hosts. However, host evolution inresponse to parasitism has received little attention, particularlyfor marine invertebrates. Drawing on the rich literature demonstratingprey adaptations to predators, I develop analogous predictionsfor the ways in which host life histories may be molded by theirparasites. Such adaptations are expected when the effects ofparasites are severe and when the probability of infection ishigh. Predicted life history changes include the evolution ofsemelparity, reduced age at first reproduction and reduced sizeat first reproduction. Using Recent and fossil populations oftwo bivalves species in the genusTransennella, I show that theincidence of trematode parasites may explain a trend of reducedsize through time and contribute to the maintenance of sexualdimorphism for size.     

Worker thermal tolerance in the thermophilic ant Cataglyphis cursor (Hymenoptera, Formicidae)

In the thermophilic ant genus Cataglyphis, species differing in their physical caste system have developed alternative mechanisms to face extreme heat by physiological and/or behavioural adaptations. In this study, we tested whether thermal tolerance is related to worker size in the ant Cataglyphis cursor that presents intermediate worker size compared with previously studied species (size range 3.5–10 mm). Thermal tolerance at two temperatures was tested in the laboratory on colonies originating from two habitats (seaside versus vineyard), known to differ in average worker size. As expected large workers were more resistant to high temperature than small workers, but the effect of worker size on thermal resistance was less pronounced under the more extreme temperatures. The pattern of thermal tolerance was similar in the two habitat types. After controlling for worker size, worker thermal tolerance significantly varied amongst colonies, but this variation was not related to colony size. Our results suggest that a higher thermal tolerance can confer an advantage to larger workers especially during foraging and are discussed in the context of the evolution of worker size in ants.     

Emerging trends in the functional genomics of the abiotic stress response in crop plants

Plants are exposed to different abiotic stresses, such as water deficit, high temperature, salinity, cold, heavy metals and mechanical wounding, under field conditions. It is estimated that such stress conditions can potentially reduce the yield of crop plants by more than 50%. Investigations of the physiological, biochemical and molecular aspects of stress tolerance have been conducted to unravel the intrinsic mechanisms developed during evolution to mitigate against stress by plants. Before the advent of the genomics era, researchers primarily used a gene-by-gene approach to decipher the function of the genes involved in the abiotic stress response. However, abiotic stress tolerance is a complex trait and, although large numbers of genes have been identified to be involved in the abiotic stress response, there remain large gaps in our understanding of the trait. The availability of the genome sequences of certain important plant species has enabled the use of strategies, such as genome-wide expression profiling, to identify the genes associated with the stress response, followed by the verification of gene function by the analysis of mutants and transgenics. Certain components of both abscisic acid-dependent and -independent cascades involved in the stress response have already been identified. Information originating from the genome-wide analysis of abiotic stress tolerance will help to provide an insight into the stress-responsive network(s), and may allow the modification of this network to reduce the loss caused by stress and to increase agricultural productivity.     

Phosphorylation events catalyzed by major cell signaling proteins differ in response to thermal and osmotic stress among native (Mytilus californianus and Mytilus trossulus) and invasive (Mytilus galloprovincialis) species of mussels

Sharp environmental gradients encountered within the intertidal zone have driven the evolution of physiological adaptations that allow its inhabitants to maintain cellular function in the presence of fluctuating abiotic factors. These adaptations are mediated by gene-regulatory networks that, despite their inherent complexity, must remain evolvable and capable of responding to different selection pressures associated with specific ecological niches. Phosphorylation events catalyzed by cell-signaling enzymes represent a parsimonious mechanism to integrate new functional or regulatory properties into these gene-regulatory networks. In this study, proteins phosphorylated on consensus sequences for protein kinases A, B, and C; cyclin-dependent kinases; and mitogen-activated protein kinases, as well as the abundance of phosphorylated stress-activated protein kinase (phospho-SAPK/JNK), were quantified in order to ascertain whether phosphorylation events are divergent among native (Mytilus californianus and Mytilus trossulus) and invasive (Mytilus galloprovincialis) species of mussels that differ in their tolerance toward environmental stress. Abundances of phosphorylated substrate proteins for each of the major signaling proteins that were investigated, as well as the abundance of phospho-SAPK/JNK, differed both within and between species during thermal and osmotic stress. These data suggest that modulating protein function via phosphorylation may be an important mechanism to integrate novel properties into stress-regulatory networks. In turn, differential phosphorylation during environmental stress may contribute to species-specific tolerances toward abiotic stress, interspecies dynamics, and biogeographic patterns in Mytilus congeners.     

Macroevolutionary and macroecological approaches to understanding the evolution of stress tolerance in plants

Environmental stress response in plants has been studied using a wide range of approaches, from lab-based investigation of biochemistry and genetics, to glasshouse studies of physiology and growth rates, to field-based trials and ecological surveys. It is also possible to investigate the evolution of environmental stress responses using macroevolutionary and macroecological analyses, analysing data from many different species, providing a new perspective on the way that environmental stress shapes the evolution and distribution of biodiversity. “Macroevoeco” approaches can produce intriguing results and new ways of looking at old problems. In this review, we focus on studies using phylogenetic analysis to illuminate macroevolutionary patterns in the evolution of environmental stress tolerance in plants. We follow a particular thread from our own research—evolution of salt tolerance—as a case study that illustrates a macroevolutionary way of thinking that opens up a range of broader questions on the evolution of environmental stress tolerances. We consider some potential future applications of macroevolutionary and macroecological analyses to understanding how diverse groups of plants evolve in response to environmental stress, which may allow better prediction of current stress tolerance and a way of predicting the capacity of species to adapt to changing environmental stresses over time.     

Insect responses to heat: physiological mechanisms,evolution and ecological implications in a warming world

Surviving changing climate conditions is particularly difficult for organisms such as insects that depend on environmental temperature to regulate their physiological functions. Insects are extremely threatened by global warming, since many do not have enough physiological tolerance even to survive continuous exposure to the current maximum temperatures experienced in their habitats. Here, we review literature on the physiological mechanisms that regulate responses to heat and provide heat tolerance in insects: (i) neuronal mechanisms to detect and respond to heat; (ii) metabolic responses to heat; (iii) thermoregulation; (iv) stress responses to tolerate heat; and (v) hormones that coordinate developmental and behavioural responses at warm temperatures. Our review shows that, apart from the stress response mediated by heat shock proteins, the physiological mechanisms of heat tolerance in insects remain poorly studied. Based on life‐history theory, we discuss the costs of heat tolerance and the potential evolutionary mechanisms driving insect adaptations to high temperatures. Some insects may deal with ongoing global warming by the joint action of phenotypic plasticity and genetic adaptation. Plastic responses are limited and may not be by themselves enough to withstand ongoing warming trends. Although the evidence is still scarce and deserves further research in different insect taxa, genetic adaptation to high temperatures may result from rapid evolution. Finally, we emphasize the importance of incorporating physiological information for modelling species distributions and ecological interactions under global warming scenarios. This review identifies several open questions to improve our understanding of how insects respond physiologically to heat and the evolutionary and ecological consequences of those responses. Further lines of research are suggested at the species, order and class levels, with experimental and analytical approaches such as artificial selection, quantitative genetics and comparative analyses.     

More than 400 million years of evolution and some plants still can't make it on their own: plant stress tolerance via fungal symbiosis

All plants in natural ecosystems are thought to be symbioticwith mycorrhizal and/or endophytic fungi. Collectively, thesefungi express different symbiotic lifestyles ranging from parasitismto mutualism. Analysis of Colletotrichum species indicates thatindividual isolates can express either parasitic or mutualisticlifestyles depending on the host genotype colonized. The endophytecolonization pattern and lifestyle expression indicate thatplants can be discerned as either disease, non-disease, or non-hosts.Fitness benefits conferred by fungi expressing mutualistic lifestylesinclude biotic and abiotic stress tolerance, growth enhancement,and increased reproductive success. Analysis of plant–endophyteassociations in high stress habitats revealed that at leastsome fungal endophytes confer habitat-specific stress toleranceto host plants. Without the habitat-adapted fungal endophytes,the plants are unable to survive in their native habitats. Moreover,the endophytes have a broad host range encompassing both monocotsand eudicots, and confer habitat-specific stress tolerance toboth plant groups. Key words: Colletotrichum, fungal endophytes, stress tolerance, symbiosis, symbiotic lifestyle Received 19 June 2007; Revised 25 November 2007 Accepted 30 November 2007     

Sources of Variation in Physiological Phenotypes and Their Evolutionary Significance

We offer the thesis that environmental physiologists and evolutionarybiologists can find fertile common ground in the study of howindividual variation in physiological phenotypes originatesand develops. The sources of such individual variation are oftencomplex; the consequences affect how natural selection willact on a suite of traits, of which some may seem, at first glance,far removed from the usual domain of environmental physiology.We illustrate our thesis in two ways. First, we offer two examplesdrawn from studies of thermal tolerance in the poeciliid fishHeterandria formosa. We show how fitness variation can be acomplex function of the gestational temperature and thermaltolerance and how these effects can produce environmentallyinduced variation among populations in thermal tolerance thatmimics a pattern of adaptive variation. Second, we review twocase studies that illuminate how environmental effects on amultivariate phenotype can channel the action of natural selection.The phenotypic plasticity of male life history in Poecilia latipinnain response to temperature embraces a spectrum of traits; theeffects of each one upon fitness will influence the abilityof selection to mold the response of any one of them to temperature.The phenotypic covariances in thermal tolerance and life-historytraits in Heterandria formosa differ slightly between populationsfrom different parts of the species range, apparently becauseof differences between them in thermal sensitivity; this differenceinsures that the multivariate nature of selection will be correspondinglydifferent in those different populations.     

Blood Oxygen Transport and Delivery in Reptiles

Cenozoic reptiles are characterized by physiological morphologicaland ecological systems with low energy requirements comparedto those of mammals. Ectothermy and low resting rates of metabolismare the primary physiological adaptations of reptiles that producelow energy demand. Adjustments of the oxygen-transport systemto different thermoregulatory characteristics among reptilesmay be reflected in blood viscosity oxygen capacity oxygen affinityand the temperature sensitivity of oxygenation. Other adaptationsreduce the energy cost of oxygen transport. Reptiles have lowhematocrits and large, widely spaced capillaries that contributeto a low fluid resistance in the vascular system but also limitthe oxygen transport capacity. The low oxygen affinity characteristicof the blood of most reptiles appears to facilitate diffusionof oxygen to the tissues, overcoming the intrinsic limitationsimposed by the morphological specializations of the cardiovascularsystem. The low blood oxygen affinity permits virtually allof the oxygen carried by the blood to be delivered to the tissuesduring periods of stress. It may also help to maintain a relativelyhigh arterial Po₂ even when a right-to-left shunt occurs inthe heart. Reptilian erythrocytes are capable of reducing methemoglobinrapidly. The high concentrations of methemoglobin and polymerizedhemoglobin that occur in vivo may indicate that these compoundshave a functional role. In their blood physiology as in otheraspects of their biology reptiles are specialized animals thatreflect selective forces quite different from those that haveshaped the evolution of mammals.     

The Secret to Successful Deep-Sea Invasion: Does Low Temperature Hold the Key?

There is a general consensus that today’s deep-sea biodiversity has largely resulted from recurrent invasions and speciations occurring through homogenous waters during periods of the Phanerozoic eon. Migrations likely continue today, primarily via isothermal water columns, such as those typical of Polar Regions, but the necessary ecological and physiological adaptations behind them are poorly understood. In an evolutionary context, understanding the adaptations, which allow for colonisation to high-pressure environments, may enable us to predict future events. In this investigation, we examine pressure tolerance during development, in the shallow-water neogastropod Buccinum undatum using thermally acclimated egg masses from temperate and sub-polar regions across the species range. Fossil records indicate neogastropods to have a deep-water origin, suggesting shallow-water species may be likely candidates for re-emergence into the deep sea. Our results show population level differences in physiological thresholds, which indicate low temperature acclimation to increase pressure tolerance. These findings imply this species is capable of deep-sea penetration through isothermal water columns prevailing at high latitudes. This study gives new insight into the fundamentals behind past and future colonisation events. Such knowledge is instrumental to understand better how changes in climate envelopes affect the distribution and radiation of species along latitudinal as well as bathymetric temperature gradients.