Experimental parasite infection reveals costs and benefits of paternal effects

The intersection of host in vivo metabolism and immune responses to infection with kinetoplastid and apicomplexan parasites

SUMMARYProtozoan parasite infection dramatically alters host metabolism, driven by immunological demand and parasite manipulation strategies. Immunometabolic checkpoints are often exploited by kinetoplastid and protozoan parasites to establish chronic infection, which can significantly impair host metabolic homeostasis. The recent growth of tools to analyze metabolism is expanding our understanding of these questions. Here, we review and contrast host metabolic alterations that occur in vivo during infection with Leishmania, trypanosomes, Toxoplasma, Plasmodium, and Cryptosporidium. Although genetically divergent, there are commonalities among these pathogens in terms of metabolic needs, induction of the type I immune responses required for clearance, and the potential for sustained host metabolic dysbiosis. Comparing these pathogens provides an opportunity to explore how transmission strategy, nutritional demand, and host cell and tissue tropism drive similarities and unique aspects in host response and infection outcome and to design new strategies to treat disease.

Low-dose of formalin-inactivated Vibrio alginolyticus protects Crassostrea gigas from secondary infection and confers broad-spectrum Vibrio resistance on offspring

An increasing number of evidences have shown that invertebrate taxa can be primed to produce immune memory to resist the secondary infection of pathogens, which was considered as a viable option to protect invertebrates from pathogens. In this work, we compared the protective effect of several different immune priming methods on the Vibrio alginolyticus secondary infection of the Crassostrea gigas. The results showed that C. gigas primed with live V. alginolyticus had higher ROS level, which led to hemocytes necrosis and higher mortality rate in the later stage. Low-dose of formalin-inactivated V. alginolyticus (including 5 × 104 CFU/mL and 5 × 105 CFU/mL) elicited appropriate immune response in C. gigas, protecting C. gigas from V. alginolyticus infection. Immersion with 5 × 104 CFU/mL formalin-inactivated V. alginolyticus was performed to prime C. gigas immunity in the trans-generational immune priming. Trans-generational immune priming significantly increased the resistance of larvae to various Vibrio species. Overall, these results suggested that low-dose of formalin-inactivated V. alginolyticus can protect C. gigas from secondary infection and confer broad-spectrum Vibrio resistance on offspring. This work provided valuable information toward a new direction for the protection of C. gigas from Vibrio infection.

Cross-continental experimental infections reveal distinct defence mechanisms in populations of the three-spined stickleback Gasterosteus aculeatus

Epidemiological traits of host-parasite associations depend on the effects of the host, the parasite and their interaction. Parasites evolve mechanisms to infect and exploit their hosts, whereas hosts evolve mechanisms to prevent infection and limit detrimental effects. The reasons why and how these traits differ across populations still remain unclear. Using experimental cross-infection of three-spined stickleback Gasterosteus aculeatus and their species-specific cestode parasites Schistocephalus solidus from Alaskan and European populations, we disentangled host, parasite and interaction effects on epidemiological traits at different geographical scales. We hypothesized that host and parasite main effects would dominate both within and across continents, although interaction effects would show geographical variation of natural selection within and across continents. We found that mechanisms preventing infection (qualitative resistance) occurred only in a combination of hosts and parasites from different continents, while mechanisms limiting parasite burden (quantitative resistance) and reducing detrimental effects of infection (tolerance) were host-population specific. We conclude that evolution favours distinct defence mechanisms on different geographical scales and that it is important to distinguish concepts of qualitative resistance, quantitative resistance and tolerance in studies of macroparasite infections.

Mapping the past, present and future research landscape of paternal effects

BACKGROUND

Although in all sexually reproducing organisms an individual has a mother and a father, non-genetic inheritance has been predominantly studied in mothers. Paternal effects have been far less frequently studied, until recently. In the last 5 years, research on environmentally induced paternal effects has grown rapidly in the number of publications and diversity of topics. Here, we provide an overview of this field using synthesis of evidence (systematic map) and influence (bibliometric analyses).

RESULTS

We find that motivations for studies into paternal effects are diverse. For example, from the ecological and evolutionary perspective, paternal effects are of interest as facilitators of response to environmental change and mediators of extended heredity. Medical researchers track how paternal pre-fertilization exposures to factors, such as diet or trauma, influence offspring health. Toxicologists look at the effects of toxins. We compare how these three research guilds design experiments in relation to objects of their studies: fathers, mothers and offspring. We highlight examples of research gaps, which, in turn, lead to future avenues of research.

CONCLUSIONS

The literature on paternal effects is large and disparate. Our study helps in fostering connections between areas of knowledge that develop in parallel, but which could benefit from the lateral transfer of concepts and methods.

Long-term survey of sea turtles (Caretta caretta) reveals correlations between parasite infection, feeding ecology, reproductive success and population dynamics

Long-term monitoring of host-parasite interactions is important for understanding the consequences of infection on host fitness and population dynamics. In an eight-year survey of the loggerhead sea turtle (Caretta caretta) population nesting in Cabo Verde, we determined the spatiotemporal variation of Ozobranchus margoi, a sanguivorous leech best known as a vector for sea turtle fibropapilloma virus. We quantified O. margoi association with turtles’ δ¹⁵N and δ¹³C stable isotopes to identify where infection occurs. We then measured the influence of infection on reproduction and offspring fitness. We found that parasite prevalence has increased from 10% of the population in 2010, to 33% in 2017. Stable isotope analysis of host skin samples suggests transmission occurs within the host’s feeding grounds. Interestingly, we found a significant interaction between individual size and infection on the reproductive success of turtles. Specifically, small, infected females produced fewer offspring of poorer condition, while in contrast, large, infected turtles produced greater clutch sizes and larger offspring. We interpret this interaction as evidence, upon infection, for a size-dependent shift in reproductive strategy from bet hedging to terminal investment, altering population dynamics. This link between infection and reproduction underscores the importance of using long-term monitoring to quantify the impact of disease dynamics over time.

Between-Generation Phenotypic and Epigenetic Stability in a Clonal Snail

Epigenetic variation might play an important role in generating adaptive phenotypes by underpinning within-generation developmental plasticity, persistent parental effects of the environment (e.g., transgenerational plasticity), or heritable epigenetically based polymorphism. These adaptive mechanisms should be most critical in organisms where genetic sources of variation are limited. Using a clonally reproducing freshwater snail (Potamopyrgus antipodarum), we examined the stability of an adaptive phenotype (shell shape) and of DNA methylation between generations. First, we raised three generations of snails adapted to river currents in the lab without current. We showed that habitat-specific adaptive shell shape was relatively stable across three generations but shifted slightly over generations two and three toward a no-current lake phenotype. We also showed that DNA methylation specific to high-current environments was stable across one generation. This study provides the first evidence of stability of DNA methylation patterns across one generation in an asexual animal. Together, our observations are consistent with the hypothesis that adaptive shell shape variation is at least in part determined by transgenerational plasticity, and that DNA methylation provides a potential mechanism for stability of shell shape across one generation.

Experimental Parasite Infection Causes Genome-Wide Changes in DNA Methylation

Parasites are arguably among the strongest drivers of natural selection, constraining hosts to evolve resistance and tolerance mechanisms. Although, the genetic basis of adaptation to parasite infection has been widely studied, little is known about how epigenetic changes contribute to parasite resistance and eventually, adaptation. Here, we investigated the role of host DNA methylation modifications to respond to parasite infections. In a controlled infection experiment, we used the three-spined stickleback fish, a model species for host-parasite studies, and their nematode parasite Camallanus lacustris. We showed that the levels of DNA methylation are higher in infected fish. Results furthermore suggest correlations between DNA methylation and shifts in key fitness and immune traits between infected and control fish, including respiratory burst and functional trans-generational traits such as the concentration of motile sperm. We revealed that genes associated with metabolic, developmental, and regulatory processes (cell death and apoptosis) were differentially methylated between infected and control fish. Interestingly, genes such as the neuropeptide FF receptor 2 and the integrin alpha 1 as well as molecular pathways including the Th1 and Th2 cell differentiation were hypermethylated in infected fish, suggesting parasite-mediated repression mechanisms of immune responses. Altogether, we demonstrate that parasite infection contributes to genome-wide DNA methylation modifications. Our study brings novel insights into the evolution of vertebrate immunity and suggests that epigenetic mechanisms are complementary to genetic responses against parasite-mediated selection.

Stress responses in fish: From molecular to evolutionary processes

In the context of global changes, fish are increasingly exposed to multiple stressors that have cascading effects from molecules to the whole individual, thereby affecting wild fish populations through selective processes. In this review, we synthetize recent advances in molecular biology and evolutionary biology to outline some potentially important effects of stressors on fish across biological levels. Given the burgeoning literature, we highlight four promising avenues of research. First, (1) the exposure to multiple stressors can lead to unexpected synergistic or antagonistic effects, which should be better taken into account to improve our predictions of the effects of actual and future human activities on aquatic organisms. Second, (2) we argue that such interactive effects might be due to switches in energy metabolism leading to threshold effects. Under multiple stress exposure, fish could switch from a "compensation" strategy, i.e. a reallocation of energy to defenses and repair to a "conservation" strategy, i.e. blocking of stress responses leading to strong deleterious effects and high mortality. Third, (3) this could have cascading effects on fish survival and population persistence but multiscale studies are still rare. We propose emerging tools merging different levels of biological organization to better predict population resilience under multiple stressors. Fourth (4), there are strong variations in sensitivity among populations, which might arise from transgenerational effects of stressors through plastic, genetic, and epigenetic mechanisms. This can lead to local adaptation or maladaptation, with strong impacts on the evolutionary trajectories of wild fish populations. With this review, we hope to encourage future research to bridge the gap between molecular ecology, ecotoxicology and evolutionary biology to better understand the evolution of responses of fishes to current and future multiple stressors in the context of global changes.

Nongenetic paternal effects via seminal fluid

Mounting evidence suggests that nongenetic paternal effects on offspring may be widespread among animal taxa, but the mechanisms underlying this form of nongenetic inheritance are not yet fully understood. Here, we show that seminal fluids underlie paternal effects on early offspring survival in an insect, the cricket Teleogryllus oceanicus, and quantify the contribution of this paternal effect to the inheritance of this important fitness trait. We used castrated males within a full-sib half-sib experimental design to show that seminal fluid donors were responsible for variation in the survival of developing embryos to hatching, and in their subsequent survival to adulthood. Increased expression of two seminal fluid protein genes, previously found to be positively associated with sperm quality, was found to be negatively associated with embryo survival. These nongenetic paternal effects hold important implications for the evolution of adaptive maternal responses to sperm competition, and more broadly for the interpretation of sire effects from classic quantitative genetic breeding designs.

Transgenerational plasticity and selection shape the adaptive potential of sticklebacks to salinity change

In marine climate change research, salinity shifts have been widely overlooked. While widespread desalination effects are expected in higher latitudes, salinity is predicted to increase closer to the equator. We took advantage of the steep salinity gradient of the Baltic Sea as a space-for-time design to address effects of salinity change on populations. Additionally, genetic diversity, a prerequisite for adaptive responses, is reduced in Baltic compared to Atlantic populations. On the one hand, adaptive transgenerational plasticity (TGP) might buffer the effects of environmental change, which may be of particular importance under reduced genetic variation. On the other hand, physiological trade-offs due to environmental stress may hamper parental provisioning to offspring thereby intensifying the impact of climate change across generations (nonadaptive TGP). Here, we studied both hypothesis of adaptive and nonadaptive TGP in the three-spined stickleback (Gasterosteus aculeatus) fish model along the strong salinity gradient of the Baltic Sea in a space-for-time experiment. Each population tolerated desalination well, which was not altered by parental exposure to low salinity. Despite a common marine ancestor, populations locally adapted to low salinity lost their ability to cope with fully marine conditions, resulting in lower survival and reduced relative fitness. Negative transgenerational effects were evident in early life stages, but disappeared after selection via mortality occurred during the first 12-30 days posthatch. Modeling various strengths of selection, we showed that nonadaptive transgenerational plasticity accelerated evolution by increasing directional selection within the offspring generation. Qualitatively, when genetic diversity is large, we predict that such effects will facilitate rapid adaptation and population persistence, while below a certain threshold populations suffer a higher risk of local extinction. Overall, our results suggest that transgenerational plasticity and selection are not independent of each other and thereby highlight a current gap in TGP studies.

Recent advances in vertebrate and invertebrate transgenerational immunity in the light of ecology and evolution

Parental experience with parasites and pathogens can lead to increased offspring resistance to infection, through a process known as transgenerational immune priming (TGIP). Broadly defined, TGIP occurs across a wide range of taxa, and can be viewed as a type of phenotypic plasticity, with hosts responding to the pressures of relevant local infection risk by altering their offspring's immune defenses. There are ever increasing examples of both invertebrate and vertebrate TGIP, which go beyond classical examples of maternal antibody transfer. Here we critically summarize the current evidence for TGIP in both invertebrates and vertebrates. Mechanisms underlying TGIP remain elusive in many systems, but while it is unlikely that they are conserved across the range of organisms with TGIP, recent insight into epigenetic modulation may challenge this view. We place TGIP into a framework of evolutionary ecology, discussing costs and relevant environmental variation. We highlight how the ecology of species or populations should affect if, where, when, and how TGIP is realized. We propose that the field can progress by incorporating evolutionary ecology focused designs to the study of the so far well chronicled, but mostly descriptive TGIP, and how rapidly developing -omic methods can be employed to further understand TGIP across taxa.

Hook, Line and Infection: A Guide to Culturing Parasites, Establishing Infections and Assessing Immune Responses in the Three-Spined Stickleback

The three-spined stickleback (Gasterosteus aculeatus) is a model organism with an extremely well-characterized ecology, evolutionary history, behavioural repertoire and parasitology that is coupled with published genomic data. These small temperate zone fish therefore provide an ideal experimental system to study common diseases of coldwater fish, including those of aquacultural importance. However, detailed information on the culture of stickleback parasites, the establishment and maintenance of infections and the quantification of host responses is scattered between primary and grey literature resources, some of which is not readily accessible. Our aim is to lay out a framework of techniques based on our experience to inform new and established laboratories about culture techniques and recent advances in the field. Here, essential knowledge on the biology, capture and laboratory maintenance of sticklebacks, and their commonly studied parasites is drawn together, highlighting recent advances in our understanding of the associated immune responses. In compiling this guide on the maintenance of sticklebacks and a range of common, taxonomically diverse parasites in the laboratory, we aim to engage a broader interdisciplinary community to consider this highly tractable model when addressing pressing questions in evolution, infection and aquaculture.

Recent evolution of extreme cestode growth suppression by a vertebrate host

Parasites can be a major cause of natural selection on hosts, which consequently evolve a variety of strategies to avoid, eliminate, or tolerate infection. When ecologically similar host populations present disparate infection loads, this natural variation can reveal immunological strategies underlying adaptation to infection and population divergence. For instance, the tapeworm Schistocephalus solidus persistently infects 0-80% of threespine stickleback (Gasterosteus aculeatus) in lakes on Vancouver Island. To test whether these heterogeneous infection rates result from evolved differences in immunity, we experimentally exposed laboratory-reared fish from ecologically similar high-infection and no-infection populations to controlled doses of Schistocephalus We observed heritable between-population differences in several immune traits: Fish from the naturally uninfected population initiated a stronger granulocyte response to Schistocephalus infection, and their granulocytes constitutively generate threefold more reactive oxygen species in cell culture. Despite these immunological differences, Schistocephalus was equally successful at establishing initial infections in both host populations. However, the no-infection fish dramatically suppressed tapeworm growth relative to high-infection fish, and parasite size was intermediate in F1 hybrid hosts. Our results show that stickleback recently evolved heritable variation in their capacity to suppress helminth growth by two orders of magnitude. Data from many natural populations indicate that growth suppression is widespread but not universal and, when present, is associated with reduced infection prevalence. Host suppression of helminth somatic growth may be an important immune strategy that aids in parasite clearance or in mitigating the fitness costs of persistent infection.

Experimental evidence that parasites drive eco-evolutionary feedbacks

Host resistance to parasites is a rapidly evolving trait that can influence how hosts modify ecosystems. Eco-evolutionary feedbacks may develop if the ecosystem effects of host resistance influence selection on subsequent host generations. In a mesocosm experiment, using a recently diverged (<100 generations) pair of lake and stream three-spined sticklebacks, we tested how experimental exposure to a common fish parasite (Gyrodactylus spp.) affects interactions between hosts and their ecosystems in two environmental conditions (low and high nutrients). In both environments, we found that stream sticklebacks were more resistant to Gyrodactylus and had different gene expression profiles than lake sticklebacks. This differential infection led to contrasting effects of sticklebacks on a broad range of ecosystem properties, including zooplankton community structure and nutrient cycling. These ecosystem modifications affected the survival, body condition, and gene expression profiles of a subsequent fish generation. In particular, lake juvenile fish suffered increased mortality in ecosystems previously modified by lake adults, whereas stream fish showed decreased body condition in stream fish-modified ecosystems. Parasites reinforced selection against lake juveniles in lake fish-modified ecosystems, but only under oligotrophic conditions. Overall, our results highlight the overlapping timescales and the interplay of host-parasite and host-ecosystem interactions. We provide experimental evidence that parasites influence host-mediated effects on ecosystems and, thereby, change the likelihood and strength of eco-evolutionary feedbacks.

Grandparental immune priming in the pipefish Syngnathus typhle

Background

Phenotypic changes in response to environmental influences can persist from one generation into the next. In many systems parental parasite experience influences offspring immune responses, known as transgenerational immune priming (TGIP). TGIP in vertebrates is mainly maternal and short-term, supporting the adaptive immune system of the offspring during its maturation. However, if fathers and offspring have a close physical connection, evolution of additional paternal immune priming can be adaptive. Biparental TGIP may result in maximized immunological protection. Here, we investigate multigenerational biparental TGIP in the sex-role reversed pipefish Syngnathus typhle by exposing grandparents to an immune challenge with heat-killed bacteria and assessing gene expression (44 target genes) of the F2-generation.

Results

Grandparental immune challenge induced gene expression of immune genes in one-week-old grandoffspring. Similarly, genes mediating epigenetic regulation including DNA-methylation and histone modifications were involved in grandparental immune priming. While grand-maternal impact was strong on genes of the complement component system, grand-paternal exposure changed expression patterns of genes mediating innate immune defense.

Conclusion

In a system with male pregnancy, grandparents influenced the immune system of their grandoffspring in a sex-specific manner, demonstrating multigenerational biparental TGIP. The involvement of epigenetic effects suggests that TGIP via the paternal line may not be limited to the pipefish system that displays male pregnancy. While the benefits and costs of grandparental TGIP depend on the temporal heterogeneity of environmental conditions, multigenerational TGIP may affect host-parasite coevolution by dampening the amplitude of Red Queen Dynamics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0885-3) contains supplementary material, which is available to authorized users.

The trans-generational impact of population density signals on host-parasite interactions

Background

The density of a host population is a key parameter underlying disease transmission, but it also has implications for the expression of disease through its effect on host physiology. In response to higher densities, individuals are predicted to either increase their immune investment in response to the elevated risk of parasitism, or conversely to decrease their immune capacity as a consequence of the stress of a crowded environment. However, an individual’s health is shaped by many different factors, including their genetic background, current environmental conditions, and maternal effects. Indeed, population density is often sensed through the presence of info-chemicals in the environment, which may influence a host’s interaction with parasites, and also those of its offspring. All of which may alter the expression of disease, and potentially uncouple the presumed link between changes in host density and disease outcomes.

Results

In this study, we used the water flea Daphnia magna and its obligate bacterial parasite Pasteuria ramosa, to investigate how signals of high host density impact on host-parasite interactions over two consecutive generations. We found that the chemical signals from crowded treatments induced phenotypic changes in both the parental and offspring generations. In the absence of a pathogen, life-history changes were genotype-specific, but consistent across generations, even when the signal of density was removed. In contrast, the influence of density on infected animals depended on the trait and generation of exposure. When directly exposed to signals of high-density, host genotypes responded differently in how they minimised the severity of disease. Yet, in the subsequent generation, the influence of density was rarely genotype-specific and instead related to ability of the host to minimise the onset of infection.

Conclusion

Our findings reveal that population level correlations between host density and infection capture only part of the complex relationship between crowding and the severity of disease. We suggest that besides its role in horizontal transmission, signals of density can influence parasite epidemiology by modifying mechanisms of resistance across multiple generations, and elevating variability via genotype-by-environment interactions. Our results help resolve why some studies are able to find a positive correlation between high density and resistance, while others uncover a negative correlation, or even no direct relationship at all.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0828-4) contains supplementary material, which is available to authorized users.

Bacteria-type-specific biparental immune priming in the pipefish Syngnathus typhle

The transfer of acquired and specific immunity against previously encountered bacteria from mothers to offspring boosts the immune response of the next generation and supports the development of a successful pathogen defense. While most studies claim that the transfer of immunity is a maternal trait, in the sex-role-reversed pipefish Syngnathus typhle, fathers nurse the embryos over a placenta-like structure, which opens the door for additional paternal immune priming. We examined the potential and persistence of bacteria-type-specific parental immune priming in the pipefish S. typhle over maturation time using a fully reciprocal design with two different bacteria species (Vibrio spp. and Tenacibaculum maritimum). Our results suggest that S. typhle is able to specifically prime the next generation against prevalent local bacteria and to a limited extent even also against newly introduced bacteria species. Long-term protection was thereby maintained only against prevailing Vibrio bacteria. Maternal and paternal transgenerational immune priming can complement each other, as they affect different pathways of the offspring immune system and come with distinct degree of specificity. The differential regulation of DNA-methylation genes upon parental bacteria exposure in premature pipefish offspring indicates that epigenetic regulation processes are involved in transferring immune-related information across generations. The identified trade-offs between immune priming and reproduction determine TGIP as a costly trait, which might constrain the evolution of long-lasting TGIP, if parental and offspring generations do not share the same parasite assembly.

Local adaptation drives thermal tolerance among parasite populations: a common garden experiment

Understanding the evolutionary responses of organisms to thermal regimes is of prime importance to better predict their ability to cope with ongoing climate change. Although this question has attracted interest in free-living organisms, whether or not infectious diseases have evolved heterogeneous responses to climate is still an open question. Here, we ran a common garden experiment using the fish ectoparasite Tracheliastes polycolpus, (i) to test whether parasites living in thermally heterogeneous rivers respond differently to an experimental thermal gradient and (ii) to determine the evolutionary processes (natural selection or genetic drift) underlying these responses. We demonstrated that the reaction norms involving the survival rate of the parasite larvae (i.e. the infective stage) across a temperature gradient significantly varied among six parasite populations. Using a Qst/Fst approach and phenotype-environment associations, we further showed that the evolution of survival rate partly depended upon temperature regimes experienced in situ, and was mostly underlined by diversifying selection, but also-to some extent-by stabilizing selection and genetic drift. This evolutionary response led to population divergences in thermal tolerance across the landscape, which has implications for predicting the effects of future 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.

The contribution of post-copulatory mechanisms to incipient ecological speciation in sticklebacks

Ecology can play a major role in species diversification. As individuals are adapting to contrasting habitats, reproductive barriers may evolve at multiple levels. While pre-mating barriers have been extensively studied, the evolution of post-mating reproductive isolation during early stages of ecological speciation remains poorly understood. In diverging three-spined stickleback ecotypes from two lakes and two rivers, we observed differences in sperm traits between lake and river males. Interestingly, these differences did not translate into ecotype-specific gamete precedence for sympatric males in competitive in vitro fertilization experiments, potentially owing to antagonistic compensatory effects. However, we observed indirect evidence for impeded development of inter-ecotype zygotes, possibly suggesting an early stage of genetic incompatibility between ecotypes. Our results show that pre-zygotic post-copulatory mechanisms play a minor role during this first stage of ecotype divergence, but suggest that genetic incompatibilities may arise at early stages of ecological speciation.