Predictable and Divergent Change in the Multivariate P Matrix during Parallel Adaptation.

AbstractAdaptation to replicated environmental conditions can be remarkably predictable, suggesting that parallel evolution may be a common feature of adaptive radiation. An open question, however, is how phenotypic variation itself evolves during repeated adaptation. Here, we use a dataset of morphological measurements from 35 populations of threespine stickleback, consisting of 16 parapatric lake-stream pairs and three marine populations, to understand how phenotypic variation has evolved during transitions from marine to freshwater environments and during subsequent diversification across the lake-stream boundary. We find statistical support for divergent phenotypic covariance (P) across populations, with most diversification of P occurring among freshwater populations. Despite a close correspondence between within-population phenotypic variation and among-population divergence, we find that variation in P is unrelated to total variation in population means across the set of populations. For lake-stream pairs, we find that theoretical predictions for microevolutionary change can explain more than 30% of divergence in P matrices across the habitat boundary. Together, our results indicate that divergence in variance structure occurs primarily in dimensions of trait space with low phenotypic integration, correlated with disparate lake and stream environments. Our findings illustrate how conserved and divergent features of multivariate variation can underlie adaptive radiation.

Is the local environment more important than within-host interactions in determining coinfection?

Host populations often vary in the magnitude of coinfection they experience across environmental gradients. Furthermore, coinfection often occurs sequentially, with a second parasite infecting the host after the first has established a primary infection. Because the local environment and interactions between coinfecting parasites can both drive patterns of coinfection, it is important to disentangle the relative contributions of environmental factors and within-host interactions to patterns of coinfection. Here, we develop a conceptual framework and present an empirical case study to disentangle these facets of coinfection. Across multiple lakes, we surveyed populations of five damselfly (host) species and quantified primary parasitism by aquatic, ectoparasitic water mites and secondary parasitism by terrestrial, endoparasitic gregarines. We first asked if coinfection is predicted by abiotic and biotic factors within the local environment, finding that the probability of coinfection decreased for all host species as pH increased. We then asked if primary infection by aquatic water mites mediated the relationship between pH and secondary infection by terrestrial gregarines. Contrary to our expectations, we found no evidence for a water mite-mediated relationship between pH and gregarines. Instead, the intensity of gregarine infection correlated solely with the local environment, with the magnitude and direction of these relationships varying among environmental predictors. Our findings emphasize the role of the local environment in shaping infection dynamics that set the stage for coinfection. Although we did not detect within-host interactions, the approach herein can be applied to other systems to elucidate the nature of interactions between hosts and coinfecting parasites within complex ecological communities.

Opening a can of worms: a test of the co-infection facilitation hypothesis.

Parasitic infections are a global occurrence and impact the health of many species. Coinfections, where two or more species of parasite are present in a host, are a common phenomenon across species. Coinfecting parasites can interact directly or indirectly via their manipulation of (and susceptibility to) the immune system of their shared host. Helminths, such as the cestode Schistocephalus solidus, are well known to suppress immunity of their host (threespine stickleback, Gasterosteus aculeatus), potentially facilitating other parasite species. Yet, hosts can evolve a more robust immune response (as seen in some stickleback populations), potentially turning facilitation into inhibition. Using wild-caught stickleback from 20 populations with non-zero S. solidus prevalence, we tested an a priori hypothesis that S. solidus infection facilitates infection by other parasites. Consistent with this hypothesis, individuals with S. solidus infections have 18.6% higher richness of other parasites compared to S. solidus-uninfected individuals from the same lakes. This facilitation-like trend is stronger in lakes where S. solidus is particularly successful but is reversed in lakes with sparse and smaller cestodes (indicative of stronger host immunity). These results suggest that a geographic mosaic of host-parasite co-evolution might lead to a mosaic of between-parasite facilitation/inhibition effects.

Elemental content of a host-parasite relationship in the threespine stickleback.

Parasite infections are ubiquitous and their effects on hosts could play a role in ecosystem processes. Ecological stoichiometry provides a framework to study linkages between consumers and their resource, such as parasites and their host, and ecosystem process; however, the stoichiometric traits of host-parasite associations are rarely quantified. Specifically, it is unclear whether parasites' elemental ratios closely resemble those of their host or if infection is related to host stoichiometry, especially in vertebrate hosts. To answer such questions, we measured the elemental content (%C, %N, and %P) and molar ratios (C:N, C:P, and N:P) of parasitized and unparasitized Gasterosteus aculeatus (three-spined stickleback) and their cestode parasite, Schistocephalus solidus. Host and parasite elemental content were distinct from each other, and parasites were generally higher in %C and lower in %N and %P. Parasite infections were related to host C:N, with infected hosts being lower in C:N. Parasite elemental content was independent of their host, but parasite body mass and parasite density were important drivers of parasite stoichiometry. Overall, these potential effects of parasite infections on host stoichiometry along with parasites' distinct elemental compositions suggest parasites may further contribute to differences in how individual hosts store and recycle nutrients.

Contemporary changes in phenotypic variation, and the potential consequences for eco-evolutionary dynamics.

Most studies assessing rates of phenotypic change focus on population mean trait values, whereas a largely overlooked additional component is changes in population trait variation. Theoretically, eco-evolutionary dynamics mediated by such changes in trait variation could be as important as those mediated by changes in trait means. To date, however, no study has comprehensively summarised how phenotypic variation is changing in contemporary populations. Here, we explore four questions using a large database: How do changes in trait variances compare to changes in trait means? Do different human disturbances have different effects on trait variance? Do different trait types have different effects on changes in trait variance? Do studies that established a genetic basis for trait change show different patterns from those that did not? We find that changes in variation are typically small; yet we also see some very large changes associated with particular disturbances or trait types. We close by interpreting and discussing the implications of our findings in the context of eco-evolutionary studies.

Species divergence under competition and shared predation.

Species competing for resources also commonly share predators. While competition often drives divergence between species, the effects of shared predation are less understood. Theoretically, competing prey species could either diverge or evolve in the same direction under shared predation depending on the strength and symmetry of their interactions. We took an empirical approach to this question, comparing antipredator and trophic phenotypes between sympatric and allopatric populations of threespine stickleback and prickly sculpin fish that all live in the presence of a trout predator. We found divergence in antipredator traits between the species: in sympatry, antipredator adaptations were relatively increased in stickleback but decreased in sculpin. Shifts in feeding morphology, diet and habitat use were also divergent but driven primarily by stickleback evolution. Our results suggest that asymmetric ecological character displacement indirectly made stickleback more and sculpin less vulnerable to shared predation, driving divergence of antipredator traits between sympatric species.

Understanding the evolution of immune genes in jawed vertebrates.

Driven by co-evolution with pathogens, host immunity continuously adapts to optimize defence against pathogens within a given environment. Recent advances in genetics, genomics and transcriptomics have enabled a more detailed investigation into how immunogenetic variation shapes the diversity of immune responses seen across domestic and wild animal species. However, a deeper understanding of the diverse molecular mechanisms that shape immunity within and among species is still needed to gain insight into-and generate evolutionary hypotheses on-the ultimate drivers of immunological differences. Here, we discuss current advances in our understanding of molecular evolution underpinning jawed vertebrate immunity. First, we introduce the immunome concept, a framework for characterizing genes involved in immune defence from a comparative perspective, then we outline how immune genes of interest can be identified. Second, we focus on how different selection modes are observed acting across groups of immune genes and propose hypotheses to explain these differences. We then provide an overview of the approaches used so far to study the evolutionary heterogeneity of immune genes on macro and microevolutionary scales. Finally, we discuss some of the current evidence as to how specific pathogens affect the evolution of different groups of immune genes. This review results from the collective discussion on the current key challenges in evolutionary immunology conducted at the ESEB 2021 Online Satellite Symposium: Molecular evolution of the vertebrate immune system, from the lab to natural populations.

Locomotor effects of a fibrosis-based immune response in stickleback fish.

The vertebrate immune system provides an impressively effective defense against parasites and pathogens. However, these benefits must be balanced against a range of costly side-effects including energy loss and risks of auto-immunity. These costs might include biomechanical impairment of movement, but little is known about the intersection between immunity and biomechanics. Here, we show that a fibrosis immune response to Schistocephalus solidus infection in freshwater threespine stickleback (Gasterosteus aculeatus) has collateral effects on their locomotion. Although fibrosis is effective at reducing infection, some populations of stickleback actively suppress this immune response, possibly because the costs of fibrosis outweigh the benefits. We quantified the locomotor effects of the fibrosis immune response in the absence of parasites to investigate whether there are incidental costs of fibrosis that could help explain why some fish forego this effective defense. To do this, we induced fibrosis in stickleback and then tested their C-start escape performance. Additionally, we measured the severity of fibrosis, body stiffness and body curvature during the escape response. We were able to estimate performance costs of fibrosis by including these variables as intermediates in a structural equation model. This model revealed that among control fish without fibrosis, there is a performance cost associated with increased body stiffness. However, fish with fibrosis did not experience this cost but rather displayed increased performance with higher fibrosis severity. This result demonstrates that the adaptive landscape of immune responses can be complex with the potential for wide-reaching and unexpected fitness consequences.

Frequency dependence limits divergent evolution by favouring rare immigrants over residents.

Two distinct forms of natural selection promote adaptive biological diversity. Divergent selection occurs when different environments favour different phenotypes, leading to increased differences between populations. Negative frequency-dependent selection occurs when rare variants within a population are favoured over common ones, increasing diversity within populations. These two diversifying forces promote genetic variation at different spatial scales, and may act in opposition, but their relative effects remain unclear because they are rarely measured concurrently. Here we show that negative frequency-dependent selection within populations can favor rare immigrants over locally adapted residents. We reciprocally transplanted lake and stream ecotypes of threespine stickleback into lake and stream habitats, while manipulating the relative abundance of residents versus immigrants. We found negative frequency-dependence: survival was highest for the locally rare ecotype, rather than natives. Also, individuals with locally rare major histocompatibility complex (MHC) class IIb genotypes were infected by fewer parasites. This negative frequency-dependent selection will tend to favour rare immigrants over common residents, amplifying the effect of migration and undermining the efficacy of divergent natural selection to drive population differences. The only signal of divergent selection was a tendency for foreign fish to have higher parasite loads than residents, after controlling for MHC genotype rarity. Frequency-dependent ecological interactions have long been thought to promote speciation. Our results suggest a more nuanced view in which negative frequency dependence alters the fate of migrants to promote or constrain evolutionary divergence between populations.

Complex community-wide consequences of consumer sexual dimorphism.

Sexual dimorphism is a ubiquitous source of within-species variation, yet the community-level consequences of sex differences remain poorly understood. Here, we analyse a bitrophic model of two competing resource species and a sexually reproducing consumer species. We show that consumer sex differences in resource acquisition can have striking consequences for consumer-resource coexistence, abundance and dynamics. Under both direct interspecific competition and apparent competition between two resource species, sexual dimorphism in consumers' attack rates can mediate coexistence of the resource species, while in other cases can lead to exclusion when stable coexistence is typically expected. Slight sex differences in total resource acquisition also can reverse competitive outcomes and lead to density cycles. These effects are expected whenever both consumer sexes require different amounts or types of resources to reproduce. Our results suggest that consumer sexual dimorphism, which is common, has wide-reaching implications for the assembly and dynamics of natural communities.

The evolution of hybrid fitness during speciation.

The evolution of postzygotic reproductive isolation is an important component of speciation. But before isolation is complete there is sometimes a phase of heterosis in which hybrid fitness exceeds that of the two parental species. The genetics and evolution of heterosis and postzygotic isolation have typically been studied in isolation, precluding the development of a unified theory of speciation. Here, we develop a model that incorporates both positive and negative gene interactions, and accounts for the evolution of both heterosis and postzygotic isolation. We parameterize the model with recent data on the fitness effects of 10,000 mutations in yeast, singly and in pairwise epistatic combinations. The model makes novel predictions about the types of interactions that contribute to declining hybrid fitness. We reproduce patterns familiar from earlier models of speciation (e.g. Haldane's Rule and Darwin's Corollary) and identify new mechanisms that may underlie these patterns. Our approach provides a general framework for integrating experimental data from gene interaction networks into speciation theory and makes new predictions about the genetic mechanisms of speciation.

The genomic signature of ecological divergence along the benthic-limnetic axis in allopatric and sympatric threespine stickleback.

The repeated occurrence of similar phenotypes in independent lineages (i.e., parallel evolution) in response to similar ecological conditions can provide compelling insights into the process of adaptive evolution. An intriguing question is to what extent repeated phenotypic changes are underlain by repeated changes at the genomic level and whether patterns of genomic divergence differ with the geographic context in which populations evolve. Here, we combined genomic, morphological and ecological data sets to investigate the genomic signatures of divergence across populations of threespine stickleback (Gasterosteus aculeatus) that adapted to contrasting ecological niches (benthic or limnetic) in either sympatry or allopatry. We found that genome-wide differentiation (FST ) was an order of magnitude higher and substantially more repeatable for sympatric benthic and limnetic specialists compared to allopatric populations with similar levels of ecological divergence. We identified genomic regions consistently differentiated between sympatric ecotypes that were also differentiated between or associated with benthic vs. limnetic niche in allopatric populations. These candidate regions were enriched on three chromosomes known to be involved in the benthic-limnetic divergence of threespine stickleback. Some candidate regions overlapped with QTL for body shape and trophic traits such as gill raker number, traits that strongly differ between benthic and limnetic ecotypes. In summary, our study shows that magnitude and repeatability of genomic signatures of ecological divergence in threespine stickleback highly depend on the geographic context. The identified candidate regions provide starting points to identify functionally important genes for the adaptation to benthic and limnetic niches.

What evolutionary processes maintain MHC IIꞵ diversity within and among populations of stickleback?

Major Histocompatibility Complex (MHC) genes code for proteins that recognize foreign protein antigens to initiate T-cell-mediated adaptive immune responses. They are often the most polymorphic genes in vertebrate genomes. How evolution maintains this diversity remains of debate. Three main hypotheses seek to explain the maintenance of MHC diversity by invoking pathogen-mediated selection: heterozygote advantage, frequency-dependent selection, and fluctuating selection across landscapes or through time. Here, we use a large-scale field parasite survey in a stickleback metapopulation to test predictions derived from each of these hypotheses. We identify over 1000 MHC IIß variants (alleles spanning paralogous genes) and find that many of them covary positively or negatively with parasite load, suggesting that these genes contribute to resistance or susceptibility. However, despite our large sample-size, we find no evidence for the widely cited stabilizing selection on MHC heterozygosity, in which individuals with an intermediate number of MHC variants have the lowest parasite burden. Nor do we observe a rare-variant advantage, or widespread fluctuating selection across populations. In contrast, we find that MHC diversity is best predicted by neutral genome-wide heterozygosity and between-population genomic divergence, suggesting neutral processes are important in shaping the pattern of metapopulation MHC diversity. Thus, although MHC IIß is highly diverse and relevant to the type and intensity of macroparasite infection in these populations of stickleback, the main models of MHC evolution still provide little explanatory power in this system.

Between-population differences in constitutive and infection-induced gene expression in threespine stickleback.

Vertebrate immunity is a complex system consisting of a mix of constitutive and inducible defences. Furthermore, host immunity is subject to selective pressure from a range of parasites and pathogens which can produce variation in these defences across populations. As populations evolve immune responses to parasites, they may adapt via a combination of (1) constitutive differences, (2) shared inducible responses, or (3) divergent inducible responses. Here, we leverage a powerful natural host-parasite model system (Gasterosteus aculeatus and Schistochephalus solidus) to tease apart the relative contributions of these three types of adaptations to among-population divergence in response to parasites. Gene expression analyses revealed limited evidence of significant divergence in constitutive expression of immune defence, and strong signatures of conserved inducible responses to the parasite. Furthermore, our results highlight a handful of immune-related genes which show divergent inducible responses which may contribute disproportionately to functional differences in infection success or failure. In addition to investigating variation in evolutionary adaptation to parasite selection, we also leverage this unique data set to improve understanding of cellular mechanisms underlying a putative resistance phenotype (fibrosis). Combined, our results provide a case study in evolutionary immunology showing that a very small number of genes may contribute to genotype differences in infection response.

Nothing in Evolution Makes Sense Except in the Light of Biology.

A key question in biology is the predictability of the evolutionary process. If we can correctly predict the outcome of evolution, we may be better equipped to anticipate and manage species' adaptation to climate change, habitat loss, invasive species, or emerging infectious diseases, as well as improve our basic understanding of the history of life on Earth. In the present article, we ask the questions when, why, and if the outcome of future evolution is predictable. We first define predictable and then discuss two conflicting views: that evolution is inherently unpredictable and that evolution is predictable given the ability to collect the right data. We identify factors that generate unpredictability, the data that might be required to make predictions at some level of precision or at a specific timescale, and the intellectual and translational value of understanding when prediction is or is not possible.

Copy number variation of a fatty acid desaturase gene Fads2 associated with ecological divergence in freshwater stickleback populations.

Fitness of aquatic animals can be limited by the scarcity of nutrients such as long-chain polyunsaturated fatty acids, especially docosahexaenoic acid (DHA). DHA availability from diet varies among aquatic habitats, imposing different selective pressures on resident animals to optimize DHA acquisition and synthesis. For example, DHA is generally poor in freshwater ecosystems compared to marine ecosystems. Our previous work revealed that, relative to marine fishes, several freshwater fishes evolved higher copy numbers of the fatty acid desaturase2 (Fads2) gene, which encodes essential enzymes for DHA biosynthesis, likely compensating for the limited availability of DHA in freshwater. Here, we demonstrate that Fads2 copy number also varies between freshwater sticklebacks inhabiting lakes and streams with stream fish having higher Fads2 copy number. Additionally, populations with benthic-like morphology possessed higher Fads2 copy number than those with planktivore-like morphology. This may be because benthic-like fish mainly feed on DHA-deficient prey such as macroinvertebrates whereas planktivore-like fish forage more regularly on DHA-rich prey, like copepods. Our results suggest that Fads2 copy number variation arises from ecological divergence not only between organisms exploiting marine and freshwater habitats but also between freshwater organisms exploiting divergent resources.

Resource diversity promotes among-individual diet variation, but not genomic diversity, in lake stickleback.

Many generalist species consist of specialised individuals that use different resources. This within-population niche variation can stabilise population and community dynamics. Consequently, ecologists wish to identify environmental settings that promote such variation. Theory predicts that environments with greater resource diversity favour ecological diversity among consumers (via disruptive selection or plasticity). Alternatively, niche variation might be a side-effect of neutral genomic diversity in larger populations. We tested these alternatives in a metapopulation of threespine stickleback. Stickleback consume benthic and limnetic invertebrates, focusing on the former in small lakes, the latter in large lakes. Intermediate-sized lakes support generalist stickleback populations using an even mixture of the two prey types, and exhibit greater among-individual variation in diet and morphology. In contrast, genomic diversity increases with lake size. Thus, phenotypic diversity and neutral genetic polymorphism are decoupled: trophic diversity being greatest in intermediate-sized lakes with high resource diversity, whereas neutral genetic diversity is greatest in the largest lakes.

Trends and Transitions in 150 Years of The American Naturalist.

The American Naturalist recently passed its sesquicentennial. Throughout this long history, it regularly encountered moments of introspection and debate over its goals, mission, identity, and audience. Here, we chronicle the history of those debates and transitions at critical moments. The Naturalist began as a popular magazine for amateur naturalists in the late 1860s. In the late 1870s, it transitioned to an increasingly academic journal for professional scientists from all branches of the natural sciences. By the turn of the century, academic specialization led to increasing fragmentation of the sciences into a multitude of societies and journals, creating an identity crisis for the once-broad-reaching American Naturalist. This identity crisis was resolved when the journal pivoted around 1910 to focus on fundamental advances in the newly emerging field of genetics. In the 1960s, the journal underwent a remarkably rapid transition to its present focus on evolution and ecology. The profound shifts in the journal's contents over this time are a reflection of the historical changes in science as a whole: from amateur naturalists, to polymath professionals, to increasingly specialized academics. This chronicle reveals the ways in which The American Naturalist has left its mark on many disciplines, many of which are today only loosely affiliated with the journal, if at all.

Sickness effects on social interactions depend on the type of behaviour and relationship.

Infections can change social behaviour in multiple ways, with profound impacts on pathogen transmission. However, these impacts might depend on the type of behaviour, how sociality as a biological trait is defined (e.g. network degree vs. mean edge strength) and the type of social relationship between the interacting individuals. We used the highly social common vampire bat Desmodus rotundus to test how an immune challenge by lipopolysaccharide (LPS) injections affects two different social behaviours and three alternate measures of sociality, and whether the LPS effect differs by kinship relationship. Effects of sickness should be lower for social behaviours that bestow greater benefits to inclusive fitness, such as food sharing. As predicted, immune-challenged bats experienced a greater reduction in allogrooming received than food sharing received. Sickness effects might also depend on how a social interaction is defined (e.g. the number of grooming partners vs. the duration of grooming events). We predicted that sickness would impact both the number and duration of social encounters, but we only detected a decrease in the number of grooming partners. Finally, sickness effects might vary with social relationship type. We predicted that sickness effects should be smaller for interactions among close kin. As expected, the immune challenge had smaller effects on mother-offspring interactions. In conclusion, our results highlight the need to explicitly consider how the effects of sickness on social network structure can differ depending on the 'who, what, and how' of social interactions, because these factors are likely to influence how sickness behaviour alters pathogen transmission.

Immune-challenged vampire bats produce fewer contact calls.

Vocalizations are an important means to facilitate social interactions, but vocal communication may be affected by infections. While such effects have been shown for mate-attraction calls, other vocalizations that facilitate social contact have received less attention. When isolated, vampire bats produce contact calls that attract highly associated groupmates. Here, we test the effect of an immune challenge on contact calling rates of individually isolated vampire bats. Sickness behaviour did not appear to change call structure, but it decreased the number of contact calls produced. This effect could decrease contact with groupmates and augment other established mechanisms by which sickness reduces social encounters (e.g. mortality, lethargy and social withdrawal or disinterest).