Microparasites have a higher evolutionary potential than their hosts due to an increased mutation rate and a shorter generation time that usually results in parasites being locally adapted to their sympatric hosts. This pattern may not apply to generalist pathogens as adaptation to sympatric host genotypes is disadvantageous due to a narrowing of the host range, in particular under strong gene flow among host populations. Under this scenario, we predict that the immune defense of hosts reveals adaptation to locally common pathogen phylotypes. This was tested in four host populations of the pipefish Syngnathus typhie and associated bacteria of the genus Vibrio. We investigated the population divergence among host and bacteria populations and verified that gene flow is higher among host populations than among parasite populations. Next, we experimentally assessed the strength of innate immune defense of pipefish hosts using in vitro assays that measured antimicrobial activity of blood plasma against sympatric and allopatric Vibrio phylotypes. Pipefish plasma displays stronger antimicrobial activity against sympatric Vibrio phylotypes compared to allopatric ones. This suggests that host defense is genetically adapted against local bacteria with a broad and unspecialized host spectrum, a situation that is typical for marine systems with weak host population structure.

Extreme climate events such as heat waves are expected to increase in frequency under global change. As one indirect effect, they can alter magnitude and direction of species interactions, for example those between hosts and parasites. We simulated a summer heat wave to investigate how a changing environment affects the interaction between the broad-nosed pipefish (Syngnathus typhle) as a host and its digenean trematode parasite (Cryptocotyle lingua). In a fully reciprocal laboratory infection experiment, pipefish from three different coastal locations were exposed to sympatric and allopatric trematode cercariae. In order to examine whether an extreme climatic event disrupts patterns of locally adapted host-parasite combinations we measured the parasite's transmission success as well as the host's adaptive and innate immune defence under control and heat wave conditions. Independent of temperature, sympatric cercariae were always more successful than allopatric ones, indicating that parasites are locally adapted to their hosts. Hosts suffered from heat stress as suggested by fewer cells of the adaptive immune system (lymphocytes) compared to the same groups that were kept at 18°C. However, the proportion of the innate immune cells (monocytes) was higher in the 18°C water. Contrary to our expectations, no interaction between host immune defence, parasite infectivity and temperature stress were found, nor did the pattern of local adaptation change due to increased water temperature. Thus, in this host-parasite interaction, the sympatric parasite keeps ahead of the coevolutionary dynamics across sites, even under increasing temperatures as expected under marine global warming.

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.

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Given the ubiquity of the parasites and their important fitness consequences on mate and offspring condition, selection for the ability to distinguish healthy from parasitized potential mates is a key process to enhance Darwinian fitness. In this study, we experimentally evaluated how the immunological experience of two potential partners influences mate choice, using the sex-role-reversed pipefish Syngnathus typhle. We exposed S. typhle to immune challenges with heat-killed Vibrio bacteria and investigated whether the activation of the immune system determined mate preferences. Our results demonstrate that the immune status of the potential partners influenced female mate preference, such that females that were exposed to an immune challenge became choosy and favored unchallenged males. Males, however, did not show any preferences for female immune status. In this context, we discuss mate choice decisions and behavioral plasticity as a complex result of immune challenge, severity of infection, as well as trans-generational effects.

The importance of microbiome research is rapidly gaining momentum for understanding its role in development, evolution, ecology, health and disease. Recent progress in community and single-cell genomic approaches has provided an unprecedented amount of information on the abundance and ecology of microbes in different host organisms and turned them into metaorganisms. A metaorganism is a host and its complete microbial community which is commonly referred to as the microbiome. Over half the cells in a human body are not human but belong to the multitude of species that compose our microbiome. However, linkages between metaorganisms from different taxa and their in situ level of intraspecific dependence (be it growth, division or metabolic activity) are much more scarce. Visualization therefore is crucial for understanding host–microbe interactions as well as overarching concepts in different host organisms. Here we introduce an innovative user-friendly method for interactive visualization of microbiome multi-omics data. The new communication format combines science and visual communication design. Interactive media are used to transform scientific findings on host–microbe interactions in an intuitive way. The method provides access to additional layers of information that cannot be visualized using a traditional platform. We demonstrate the usefulness of this visualization approach using the interactive scientific poster ‘Digital Meta’, which is designed to support not only interdisciplinary co-working but also communication with the general public.