Consistent pattern of local adaptation during an experimental heat wave in a pipefish-trematode host-parasite system

A Mosquito Parasite Is Locally Adapted to Its Host but Not Temperature

AbstractClimate change will alter interactions between parasites and their hosts. Warming may affect patterns of local adaptation, shifting the environment to favor the parasite or host and thus changing the prevalence of disease. We assessed local adaptation to hosts and temperature in the facultative ciliate parasite Lambornella clarki, which infects the western tree hole mosquito Aedes sierrensis. We conducted laboratory infection experiments with mosquito larvae and parasites collected from across a climate gradient, pairing sympatric or allopatric populations across three temperatures that were either matched or mismatched to the source environment. Lambornella clarki parasites were locally adapted to their hosts, with 2.6 times higher infection rates on sympatric populations compared with allopatric populations, but they were not locally adapted to temperature. Infection peaked at the intermediate temperature of 12.5°C, notably lower than the optimum temperature for free-living L. clarki growth, suggesting that the host's immune response can play a significant role in mediating the outcome of infection. Our results highlight the importance of host selective pressure on parasites, despite the impact of temperature on infection success.

Are fish immunocompetent enough to face climate change?

Ongoing climate change has already been associated with increased disease outbreaks in wild and farmed fish. Here, we evaluate the current knowledge of climate change-related ecoimmunology in teleosts with a focus on temperature, hypoxia, salinity and acidification before exploring interactive effects of multiple stressors. Our literature review reveals that acute and chronic changes in temperature and dissolved oxygen can compromise fish immunity which can lead to increased disease susceptibility. Moreover, temperature and hypoxia have already been shown to enhance the infectivity of certain pathogens/parasites and to accelerate disease progression. Too few studies exist that have focussed on acidification, but direct immune effects seem to be limited while salinity studies have led to contrasting results. Likewise, multi-stressor experiments essential for unravelling the interactions of simultaneously changing environmental factors are still scarce. This ultimately impedes our ability to estimate to what extent climate change will hamper fish immunity. Our review about epigenetic regulation mechanisms highlights the acclimation potential of the fish immune response to changing environments. However, due to the limited number of epigenetic studies, overarching conclusions cannot be drawn. Finally, we provide an outlook on how to better estimate the effects of realistic climate change scenarios in future immune studies in fish.

Neuronal gene expression in two generations of the marine parasitic worm, Cryptocotyle lingua

Trematodes, or flukes, undergo intricate anatomical and behavioral transformations during their life cycle, yet the functional changes in their nervous system remain poorly understood. We investigated the molecular basis of nervous system function in Cryptocotyle lingua, a species of relevance for fisheries. Transcriptomic analysis revealed a streamlined molecular toolkit with the absence of key signaling pathways and ion channels. Notably, we observed the loss of nitric oxide synthase across the Platyhelminthes. Furthermore, we identified upregulated neuronal genes in dispersal larvae, including those involved in aminergic pathways, synaptic vesicle trafficking, TRPA channels, and surprisingly nitric oxide receptors. Using neuronal markers and in situ hybridization, we hypothesized their functional relevance to larval adaptations and host-finding strategies. Additionally, employing a behavior quantification toolkit, we assessed cercaria motility, facilitating further investigations into the behavior and physiology of parasitic flatworms. This study enhances our understanding of trematode neurobiology and provides insights for targeted antiparasitic strategies.

A mosquito parasite is locally adapted to its host but not temperature

Climate change will alter interactions between parasites and their hosts. Warming may affect patterns of local adaptation, shifting the environment to favor the parasite or host and thus changing the prevalence of disease. We assessed local adaptation in the facultative ciliate parasite Lambornella clarki, which infects the western tree hole mosquito Aedes sierrensis. We conducted laboratory infection experiments with mosquito larvae and parasites collected from across a climate gradient, pairing sympatric or allopatric populations across three temperatures that were either matched or mismatched to the source environment. L. clarki parasites were locally adapted to their hosts, with 2.6x higher infection rates on sympatric compared to allopatric populations, but were not locally adapted to temperature. Infection peaked at the intermediate temperature of 13°C. Our results highlight the importance of host selective pressure on parasites, despite the impact of temperature on infection success.

Alternate patterns of temperature variation bring about very different disease outcomes at different mean temperatures

The dynamics of host-parasite interactions are highly temperature-dependent and may be modified by increasing frequency and intensity of climate-driven heat events. Here, we show that altered patterns of temperature variance lead to an almost order-of-magnitude shift in thermal performance of host and pathogen life-history traits over and above the effects of mean temperature and, moreover, that different temperature regimes affect these traits differently. We found that diurnal fluctuations of ±3°C lowered infection rates and reduced spore burden compared to constant temperatures in our focal host Daphnia magna exposed to the microsporidium parasite Ordospora colligata. In contrast, a 3-day heatwave (+6°C) did not affect infection rates, but increased spore burden (relative to constant temperatures with the same mean) at 16°C, while reducing burden at higher temperatures. We conclude that changing patterns of climate variation, superimposed on shifts in mean temperatures due to global warming, may have profound and unanticipated effects on disease dynamics.

Global warming is increasing average temperatures and causing extreme temperature fluctuations and heatwaves. These changes may affect when, where, and how often infectious disease outbreaks occur. This could have profound impacts on agriculture, human health, and wildlife.

Studying how extreme temperatures or temperature fluctuations alter infections in laboratory animals may help scientists to better understand the impact of climate change on disease. A small aquatic invertebrate, such as a water flea, is one good candidate for such studies. These tiny creatures can be grown in small glass jars in temperature-controlled aquariums.

Kunze, Luijckx et al. show that temperature fluctuations and heat waves have complex effects on parasitic infections in water fleas. In the experiments, water fleas housed with a parasite that infects them were exposed to constant temperatures, fluctuating temperatures, or three-day heatwaves, while being kept at a broad range of mean water temperatures. Then, Kunze, Luijckx et al. measured how these conditions affected the water fleas’ longevity, reproduction, and parasite infections.

This revealed that temperature variations had a unique effect on the life span, and reproduction and infection rates of the water fleas, depending on the average water temperature the animals were kept at. Heatwaves drastically increased the number of parasites in the water fleas at an average water temperature of 16 °C but had no effect at all or decreased the number of parasites at 19 °C and 22 °C, respectively. Similarly, at high average water temperatures (>24 °C), temperature fluctuations reduced the number of water fleas infected with parasites and the number of parasites in each infected flea. Moreover, the maximum temperature at which parasites were able to cause infections was 5 °C lower under fluctuating temperatures than under constant temperatures.

Kunze and Luijckx et al. show that consistent high temperatures, temperature changes, extreme weather events, and mean water temperature affect disease outcomes in water fleas. More studies are needed to assess how temperature variations change the course of diseases in other organisms and to understand the underlying mechanisms. Learning more about disease-temperature interactions will help scientists predict climate change-driven disease outbreaks.

Morphological and molecular identification of Cryptocotyle lingua metacercariae isolated from Atlantic cod (Gadus morhua) from Danish seas and whiting (Merlangius merlangus) from the English Channel

Trematode larvae (metacercariae) causing black spot disease occur frequently in gills, fins, skin and the superficial muscle layers of marine fish. Species within the genus Cryptocotyle Lühe, 1899 are frequently associated with this disease. Descriptions of the metacercarial stage are relatively limited and none has hitherto been reported from fish from the English Channel. The present study reports the morphological and molecular identifications of encysted black spot-inducing parasites from whiting (Merlangius merlangus) and Atlantic cod (Gadus morhua) caught respectively from the north coast of France (English Channel) and from Danish sea waters. Metacercariae were characterised morphologically based on microscopic observations and molecularly using Sanger sequencing of fragments of the mitochondrial cox1 gene and rDNA ITS region. Morphological data were compared with available data in the literature. Phylogenetic trees including reference sequences were built to confirm morphological and molecular identifications. This survey constitutes the first description of C. lingua metacercariae in the English Channel ecosystems.

Early developmental stages of native populations of Ciona intestinalis under increased temperature are affected by local habitat history

Temperature modulates marine ectotherm physiology, influencing survival, abundance and species distribution. While native species could be susceptible to ocean warming, thermal tolerance might favour the spread of non-native species. Determining the success of invasive species in response to climate change is confounded by the cumulative, synergistic or antagonistic effects of environmental drivers, which vary at a geographical and temporal scale. Thus, an organism's acclimation or adaptive potential could play an important evolutionary role by enabling or conditioning species tolerance to stressful environmental conditions. We investigated developmental performance of early life stages of the ascidian Ciona intestinalis (derived from populations of anthropogenically impacted and control sites) to an extreme weather event (i.e. marine heatwave). Fertilization rate, embryo and larval development, settlement, metamorphosis success and juvenile heart rate were assessed as experimental endpoints. With the exception of fertilization and heart rates, temperature influenced all analysed endpoints. C. intestinalis derived from control sites were the most negatively affected by increased temperature conditions. By contrast, C. intestinalis from anthropogenically impacted sites showed a positive response to thermal stress, with a higher proportion of larvae development, settlement and metamorphosis success being observed under increased temperature conditions. No differences were observed for heart rates between sampled populations and experimental temperature conditions. Moreover, interaction between temperature and populations was statistically significant for embryo and larvae development, and metamorphosis. We hypothesize that selection resulting from anthropogenic forcing could shape stress resilience of species in their native range and subsequently confer advantageous traits underlying their invasive potential.

Climate change facilitates a parasite's host exploitation via temperature-mediated immunometabolic processes

Global climate change can influence organismic interactions like those between hosts and parasites. Rising temperatures may exacerbate the exploitation of hosts by parasites, especially in ectothermic systems. The metabolic activity of ectotherms is strongly linked to temperature and generally increases when temperatures rise. We hypothesized that temperature change in combination with parasite infection interferes with the host's immunometabolism. We used a parasite, the avian cestode Schistocephalus solidus, which taps most of its resources from the metabolism of an ectothermic intermediate host, the three-spined stickleback. We experimentally exposed sticklebacks to this parasite, and studied liver transcriptomes 50 days after infection at 13°C and 24°C, to assess their immunometabolic responses. Furthermore, we monitored fitness parameters of the parasite and examined immunity and body condition of the sticklebacks at 13°C, 18°C and 24°C after 36, 50 and 64 days of infection. At low temperatures (13°C), S. solidus growth was constrained, presumably also by the more active stickleback's immune system, thus delaying its infectivity for the final host to 64 days. Warmer temperature (18°C and 24°C) enhanced S. solidus growth, and it became infective to the final host already after 36 days. Overall, S. solidus produced many more viable offspring after development at elevated temperatures. In contrast, stickleback hosts had lower body conditions, and their immune system was less active at warm temperature. The stickleback's liver transcriptome revealed that mainly metabolic processes were differentially regulated between temperatures, whereas immune genes were not strongly affected. Temperature effects on gene expression were strongly enhanced in infected sticklebacks, and even in exposed-but-not-infected hosts. These data suggest that the parasite exposure in concert with rising temperature, as to be expected with global climate change, shifted the host's immunometabolism, thus providing nutrients for the enormous growth of the parasite and, at the same time suppressing immune defence.

Context Dependence of Local Adaptation to Abiotic and Biotic Environments: A Quantitative and Qualitative Synthesis

Understanding how spatially variable selection shapes adaptation is an area of long-standing interest in evolutionary ecology. Recent meta-analyses have quantified the extent of local adaptation, but the relative importance of abiotic and biotic factors in driving population divergence remains poorly understood. To address this gap, we combined a quantitative meta-analysis and a qualitative metasynthesis to (1) quantify the magnitude of local adaptation to abiotic and biotic factors and (2) characterize major themes that influence the motivation and design of experiments that seek to test for local adaptation. Using local-foreign contrasts as a metric of local adaptation (or maladaptation), we found that local adaptation was greater in the presence than in the absence of a biotic interactor, especially for plants. We also found that biotic environments had stronger effects on fitness than abiotic environments when ignoring whether those environments were local versus foreign. Finally, biotic effects were stronger at low latitudes, and abiotic effects were stronger at high latitudes. Our qualitative analysis revealed that the lens through which local adaptation has been examined differs for abiotic and biotic factors. It also revealed biases in the design and implementation of experiments that make quantitative results challenging to interpret and provided directions for future research.

Comparative transcriptional profiling analysis of the effect of heat waves during embryo incubation on the hatchlings of the Chinese soft-shelled turtle (Pelodiscus sinensis)

Temperature is one of most the important environmental factors that affect the ontogenesis of organisms. In this study, we incubated Chinese soft‐shelled turtle eggs at 28°C (control temperature, C treatment), a temperature with a 16°C cold shock and a 36°C heat shock twice per week (S treatment) or a ramp‐programmed temperature of 29 ± 9°C (with 12 hr (+) and 12 hr (−) every day) (F treatment). The incubation period, hatching success, hatchling weight, and locomotor performance were significantly different between the controls and the different heat treatment groups. The pathogen challenge results illustrated that hatchlings from the S treatment group were more resistant to bacterial infection, whereas hatchlings from the F treatment group were more vulnerable. We used RNA‐seq quantification analysis to identify differentially expressed genes (DEGs) of hatchlings in the S treatment group. Based on the functional annotation results for the DEGs, 9 genes were chosen to verify the RNA‐seq results. The background expression of DEGs was also analyzed for the three treatments, as was the regulation of the pathogen challenge. The results showed that 8 DEGs were related to the immune response after pathogen challenge and that temperature was an important factor in differential regulation of the immunity pathways.

Trans-generational plasticity in response to immune challenge is constrained by heat stress

Trans‐generational plasticity (TGP) is the adjustment of phenotypes to changing habitat conditions that persist longer than the individual lifetime. Fitness benefits (adaptive TGP) are expected upon matching parent–offspring environments. In a global change scenario, several performance‐related environmental factors are changing simultaneously. This lowers the predictability of offspring environmental conditions, potentially hampering the benefits of TGP. For the first time, we here explore how the combination of an abiotic and a biotic environmental factor in the parental generation plays out as trans‐generational effect in the offspring. We fully reciprocally exposed the parental generation of the pipefish Syngnathus typhle to an immune challenge and elevated temperatures simulating a naturally occurring heatwave. Upon mating and male pregnancy, offspring were kept in ambient or elevated temperature regimes combined with a heat‐killed bacterial epitope treatment. Differential gene expression (immune genes and DNA‐ and histone‐modification genes) suggests that the combined change of an abiotic and a biotic factor in the parental generation had interactive effects on offspring performance, the temperature effect dominated over the immune challenge impact. The benefits of certain parental environmental conditions on offspring performance did not sum up when abiotic and biotic factors were changed simultaneously supporting that available resources that can be allocated to phenotypic trans‐generational effects are limited. Temperature is the master regulator of trans‐generational phenotypic plasticity, which potentially implies a conflict in the allocation of resources towards several environmental factors. This asks for a reassessment of TGP as a short‐term option to buffer environmental variation in the light of climate change.

Can environmental change affect host/parasite-mediated speciation?

Parasitism can be a driver of species divergence and thereby significantly alter species formation processes. While we still need to better understand how parasite-mediated speciation functions, it is even less clear how this process is affected by environmental change. Both rapid and gradual changes of the environment can modify host immune responses, parasite virulence and the specificity of their interactions. They will thereby change host-parasite evolutionary trajectories and the potential for speciation in both hosts and parasites. Here, we summarise mechanisms of host-parasite interactions affecting speciation and subsequently consider their susceptibility to environmental changes. We mainly focus on the effects of temperature change and nutrient input to ecosystems as they are major environmental stressors. There is evidence for both disruptive and accelerating effects of those pressures on speciation that seem to be context-dependent. A prerequisite for parasite-driven host speciation is that parasites significantly alter the host's Darwinian fitness. This can rapidly lead to divergent selection and genetic adaptation; however, it is likely preceded by more short-term plastic and transgenerational effects. Here, we also consider how these first responses and their susceptibility to environmental changes could lead to alterations of the species formation process and may provide alternative pathways to speciation.

Isolation over 35 years in a heated biotest basin causes selection on MHC class IIß genes in the European perch (Perca fluviatilis L.)

Genes that play key roles in host immunity such as the major histocompatibility complex (MHC) in vertebrates are expected to be major targets of selection. It is well known that environmental conditions can have an effect on host–parasite interactions and may thus influence the selection on MHC. We analyzed MHC class IIß variability over 35 years in a population of perch (Perca fluviatilis) from the Baltic Sea that was split into two populations separated from each other. One population was subjected to heating from cooling water of a nuclear power plant and was isolated from the surrounding environment in an artificial lake, while the other population was not subjected to any change in water temperature (control). The isolated population experienced a change of the allelic composition and a decrease in allelic richness of MHC genes compared to the control population. The two most common MHC alleles showed cyclic patterns indicating ongoing parasite–host coevolution in both populations, but the alleles that showed a cyclic behavior differed between the two populations. No such patterns were observed at alleles from nine microsatellite loci, and no genetic differentiation was found between populations. We found no indications for a genetic bottleneck in the isolated population during the 35 years. Additionally, differences in parasitism of the current perch populations suggest that a change of the parasite communities has occurred over the isolation period, although the evidence in form of in-depth knowledge of the change of the parasite community over time is lacking. Our results are consistent with the hypothesis of a selective sweep imposed by a change in the parasite community.

Impact of thermal stress on evolutionary trajectories of pathogen resistance in three-spined stickleback (Gasterosteus aculeatus)

Background

Pathogens are a major regulatory force for host populations, especially under stressful conditions. Elevated temperatures may enhance the development of pathogens, increase the number of transmission stages, and can negatively influence host susceptibility depending on host thermal tolerance. As a net result, this can lead to a higher prevalence of epidemics during summer months. These conditions also apply to marine ecosystems, where possible ecological impacts and the population-specific potential for evolutionary responses to changing environments and increasing disease prevalence are, however, less known. Therefore, we investigated the influence of thermal stress on the evolutionary trajectories of disease resistance in three marine populations of three-spined sticklebacks Gasterosteus aculeatus by combining the effects of elevated temperature and infection with a bacterial strain of Vibrio sp. using a common garden experiment.

Results

We found that thermal stress had an impact on fish weight and especially on survival after infection after only short periods of thermal acclimation. Environmental stress reduced genetic differentiation (Q_ST) between populations by releasing cryptic within-population variation. While life history traits displayed positive genetic correlations across environments with relatively weak genotype by environment interactions (GxE), environmental stress led to negative genetic correlations across environments in pathogen resistance. This reversal of genetic effects governing resistance is probably attributable to changing environment-dependent virulence mechanisms of the pathogen interacting differently with host genotypes, i.e. G_PathogenxG_HostxE or (G_PathogenxE)x(G_HostxE) interactions, rather than to pure host genetic effects, i.e. G_HostxE interactions.

Conclusion

To cope with climatic changes and the associated increase in pathogen virulence, host species require wide thermal tolerances and pathogen-resistant genotypes. The higher resistance we found for some families at elevated temperatures showed that there is evolutionary potential for resistance to Vibrio sp. in both thermal environments. The negative genetic correlation of pathogen resistance between thermal environments, on the other hand, indicates that adaptation to current conditions can be a weak predictor for performance in changing environments. The observed feedback on selective gradients exerted on life history traits may exacerbate this effect, as it can also modify the response to selection for other vital components of fitness.

Infection success of Echinoparyphium aconiatum (Trematoda) in its snail host under high temperature: role of host resistance

Background

Extreme weather events such as summer heat waves become more frequent owing to global climate change and are predicted to alter disease dynamics. This is because high temperatures can reduce host immune function. Predicting the impact of climate change on host-parasite interactions is, however, difficult as temperature may also affect parasite infective stages and other host characteristics determining the outcome of interaction.

Methods

Two experiments were conducted to investigate these phenomena in a Lymnaea stagnalis–Echinoparyphium aconiatum (Trematoda) interaction. In the first experiment, the effects of exposure of snails to experimental heat waves [maintenance at 25°C vs. 15°C (control)] with different durations (3 days, 7 days) on the infection success of parasite cercariae was examined. In the second experiment, the infection success was examined under similar conditions, while controlling for the possible temperature effects on cercariae and at least partly also for host physiological changes that take place rapidly compared to alterations in immune function (exposure to cercariae at intermediate 20°C).

Results

In the first experiment, increased infection success at 25°C was found independently of the duration of the heat wave. In the second experiment, increased infection success was found only in snails maintained at 25°C for 7 days, a treatment in which snail immune defence is known to be impaired.

Conclusions

These results suggest that the effects of host resistance in determining overall parasite infection success can be overridden by effects of temperature on parasite transmission stages and/or alterations in other host traits than immune defence.

Heat and immunity: an experimental heat wave alters immune functions in three-spined sticklebacks (Gasterosteus aculeatus)

Global climate change is predicted to lead to increased temperatures and more extreme climatic events. This may influence host-parasite interactions, immunity and therefore the impact of infectious diseases on ecosystems. However, little is known about the effects of rising temperatures on immune defence, in particular in ectothermic animals, where the immune system is directly exposed to external temperature change. Fish are ideal models for studying the effect of temperature on immunity, because they are poikilothermic, but possess a complete vertebrate immune system with both innate and adaptive immunity. We used three-spined sticklebacks ( Gasterosteus aculeatus) originating from a stream and a pond, whereby the latter supposedly were adapted to higher temperature variation. We studied the effect of increasing and decreasing temperatures and a simulated heat wave with subsequent recovery on body condition and immune parameters. We hypothesized that the immune system might be less active at low temperatures, but will be even more suppressed at temperatures towards the upper tolerable temperature range. Contrary to our expectation, we found innate and adaptive immune activity to be highest at a temperature as low as 13 °C. Exposure to a simulated heat wave induced long-lasting immune disorders, in particular in a stickleback population that might be less adapted to temperature variation in its natural environment. The results show that the activity of the immune system of an ectothermic animal species is temperature dependent and suggest that heat waves associated with global warming may immunocompromise host species, thereby potentially facilitating the spread of infectious diseases.

Are heat waves susceptible to mitigate the expansion of a species progressing with global warming?

A number of organisms, especially insects, are extending their range in response of the increasing trend of warmer temperatures. However, the effects of more frequent climatic anomalies on these species are not clearly known. The pine processionary moth, Thaumetopoea pityocampa, is a forest pest that is currently extending its geographical distribution in Europe in response to climate warming. However, its population density largely decreased in its northern expansion range (near Paris, France) the year following the 2003 heat wave. In this study, we tested whether the 2003 heat wave could have killed a large part of egg masses. First, the local heat wave intensity was determined. Then, an outdoor experiment was conducted to measure the deviation between the temperatures recorded by weather stations and those observed within sun-exposed egg masses. A second experiment was conducted under laboratory conditions to simulate heat wave conditions (with night/day temperatures of 20/32°C and 20/40°C compared to the control treatment 13/20°C) and measure the potential effects of this heat wave on egg masses. No effects were noticed on egg development. Then, larvae hatched from these egg masses were reared under mild conditions until the third instar and no delayed effects on the development of larvae were found. Instead of eggs, the 2003 heat wave had probably affected directly or indirectly the young larvae that were already hatched when it occurred. Our results suggest that the effects of extreme climatic anomalies occurring over narrow time windows are difficult to determine because they strongly depend on the life stage of the species exposed to these anomalies. However, these effects could potentially reduce or enhance the average warming effects. As extreme weather conditions are predicted to become more frequent in the future, it is necessary to disentangle the effects of the warming trend from the effects of climatic anomalies when predicting the response of a species to climate change.

Effects of local adaptation and interspecific competition on species' responses to climate change

Local adaptation and species interactions have been shown to affect geographic ranges; therefore, we need models of climate impact that include both factors. To identify possible dynamics of species when including these factors, we ran simulations of two competing species using an individual-based, coupled map-lattice model using a linear climatic gradient that varies across latitude and is warmed over time. Reproductive success is governed by an individual's adaptation to local climate as well as its location relative to global constraints. In exploratory experiments varying the strength of adaptation and competition, competition reduces genetic diversity and slows range change, although the two species can coexist in the absence of climate change and shift in the absence of competitors. We also found that one species can drive the other to extinction, sometimes long after climate change ends. Weak selection on local adaptation and poor dispersal ability also caused surfing of cooler-adapted phenotypes from the expanding margin backwards, causing loss of warmer-adapted phenotypes. Finally, geographic ranges can become disjointed, losing centrally-adapted genotypes. These initial results suggest that the interplay between local adaptation and interspecific competition can significantly influence species' responses to climate change, in a way that demands future research.

Absence of major histocompatibility complex class II mediated immunity in pipefish, Syngnathus typhle: evidence from deep transcriptome sequencing

The major histocompatibility complex (MHC)-mediated adaptive immune system is the hallmark of gnathostome immune defence. Recent work suggests that cod-like fishes (Gadidae) lack important components of the MHC class II mediated immunity. Here, we report a putative independent loss of functionality of this pathway in another species, the pipefish Syngnathus typhle, that belongs to a distantly related fish family (Syngnathidae). In a deep transcriptome sequencing approach comprising several independent normalized and non-normalized expressed sequence tag (EST) libraries with approximately 7.5 × 10(8) reads, sequenced with two next generation platforms (454 and Illumina), we were unable to identify MHC class IIα/β genes as well as genes encoding associated receptors. Along with the recent findings in cod, our results suggest that immune systems of the Euteleosts may be more variable than previously assumed.

Salinity change impairs pipefish immune defence

Global change is associated with fast and severe alterations of environmental conditions. Superimposed onto existing salinity variations in a semi-enclosed brackish water body such as the Baltic Sea, a decrease in salinity is predicted due to increased precipitation and freshwater inflow. Moreover, we predict that heavy precipitation events will accentuate salinity fluctuations near shore. Here, we investigated how the immune function of the broad-nosed pipefish (Syngnathus typhle), an ecologically important teleost with sex-role reversal, is influenced by experimentally altered salinities (control: 18 PSU, lowered: 6 PSU, increased: 30 PSU) upon infection with bacteria of the genus Vibrio. Salinity changes resulted in increased activity and proliferation of immune cells. However, upon Vibrio infection, individuals at low salinity were unable to mount specific immune response components, both in terms of monocyte and lymphocyte cell proliferation and immune gene expression compared to pipefish kept at ambient salinities. We interpret this as resource allocation trade-off, implying that resources needed for osmoregulation under salinity stress are lacking for subsequent activation of the immune defence upon infection. Our data suggest that composition of small coastal fish communities may change due to elevated environmental stress levels and the incorporated consequences thereof.