ABSTRACTClosely related populations often differ in resistance to a given parasite, as measured by infection success or failure. Yet, the immunological mechanisms of these evolved differences are rarely specified. Does resistance evolve via changes to the host’s ability to recognize that an infection exists, actuate an effective immune response, or attenuate that response? We tested whether each of these phases of the host response contributed to threespine sticklebacks’ recently evolved resistance to their tapeworm Schistocephalus solidus. While marine stickleback and some susceptible lake fish permit fast-growing tapeworms, other lake populations are resistant and suppress tapeworm growth via a fibrosis response. We subjected lab-raised fish from three populations (susceptible marine ‘ancestors’, a susceptible lake, a resistant lake), to a novel immune challenge (injection of: 1) a saline control, 2) alum, a generalized pro-inflammatory adjuvant that causes fibrosis, 3) a tapeworm protein extract, and 4) a combination of alum and tapeworm protein). All three populations were capable of a robust fibrosis response to the alum treatments (but not the saline control). Yet, only the resistant population exhibited a fibrosis response to the tapeworm protein alone. Thus, these populations differed in their ability to recognize the tapeworm but shared an intact fibrosis pathway. However, the resistant population also initiated fibrosis faster, and was able to attenuate fibrosis, unlike the susceptible populations slow but longer-lasting response to alum. As fibrosis has presumed pathological side-effects, this difference may reflect adaptions to mitigate costs of immunity in the resistant population. Broadly, our results confirm that parasite detection, activation speed, and immune attenuation simultaneously contribute to adaptations to parasite infection in natural populations.IMPACT SUMMARYDramatic variation in parasite resistance is common in nature, even to the same parasite, yet we are still working to understand the mechanisms of how such differences evolve. Many evolution studies focus on the broad outcomes of infection (infected or not) when studying this question, without specifying what part of the immune response has evolved. Here, we experimentally partition different sequential stages in the host immune response (recognition, actuation, attenuation), to evaluate which stage(s) underly the evolution of host resistance to infection. This study compares three populations of threespine stickleback that naturally differ in their exposure and their ability to resist infections of a freshwater tapeworm. These include a “resistant” lake population, a “susceptible” lake population, and an ancestral marine population that is rarely exposed to the tapeworm in nature, but is susceptible when exposed in the lab. The resistant population exhibits a fibrosis immune response to infection, which has previously been linked to suppressed tapeworm growth and viability. We injected different immune challenges directly into the site of infection (peritoneal cavity) and measured the subsequent fibrosis response through time. We found that all populations were capable of producing fibrosis in response to a general immune stimulant (alum). But, only the resistant population was able to recognize and respond to tapeworm protein alone. This population also responded faster than the others, within 24 hours, and attenuated its fibrosis by 90 days post-injections whereas the other populations exhibited a slower response that did not attenuate in the study time-frame. We concluded that variation in parasite recognition, an early phase in the host response, shapes the evolution of the initiation and resolution of the physical response to infection. Broadly, our results support that parasite detection mechanisms could play a key role in the rapid evolution of parasite resistance.

Closely related populations often differ in resistance to a given parasite, as measured by infection success or failure. Yet, the immunological mechanisms of these evolved differences are rarely specified. Does resistance evolve via changes to the host's ability to recognize that an infection exists, actuate an effective immune response, or attenuate that response? We tested whether each of these phases of the host response contributed to threespine sticklebacks’ recently evolved resistance to their tapeworm Schistocephalus solidus. Although marine stickleback and some susceptible lake fish permit fast‐growing tapeworms, other lake populations are resistant and suppress tapeworm growth via a fibrosis response. We subjected lab‐raised fish from three populations (susceptible marine “ancestors,” a susceptible lake population, and a resistant lake population) to a novel immune challenge using an injection of (1) a saline control, (2) alum, a generalized pro‐inflammatory adjuvant that causes fibrosis, (3) a tapeworm protein extract, or (4) a combination of alum and tapeworm protein. With enough time, all three populations generated a robust fibrosis response to the alum treatments. Yet, only the resistant population exhibited a fibrosis response to the tapeworm protein alone. Thus, these populations differed in their ability to respond to the tapeworm protein but shared an intact fibrosis pathway. The resistant population also initiated fibrosis faster in response to alum, and was able to attenuate fibrosis, unlike the susceptible populations’ slow but longer lasting response to alum. As fibrosis has pathological side effects that reduce fecundity, the faster recovery by the resistant population may reflect an adaptation to mitigate the costs of immunity. Broadly, our results confirm that parasite detection and immune initiation, activation speed, and immune attenuation simultaneously contribute to the evolution of parasite resistance and adaptations to infection in natural populations.

Since the New Synthesis, most migration-selection balance theory has predicted that there should be negligible differentiation over small spatial scales (relative to dispersal), because gene flow should erode any effect of divergent selection. Nevertheless, there are classic examples of microgeographic divergence, which theory suggests can arise under specific conditions: exceptionally strong selection, phenotypic plasticity in philopatric individuals, or nonrandom dispersal. Here, we present evidence of microgeographic morphological variation within lake and stream populations of threespine stickleback (Gasterosteus aculeatus). It seems reasonable to assume that a given lake or stream population of fish is well-mixed. However, we found this assumption to be untenable. We examined trap-to-trap variation in 34 morphological traits measured on stickleback from 16 lakes and 16 streams. Most traits varied appreciably among traps within populations. Both between-trap distance and microhabitat characteristics such as depth and substrate explained some of the within-population morphological variance. Microhabitat was also associated with genotype at particular loci but there was no genetic isolation by distance, implying that heritable habitat preferences may contribute to microgeographic variation. Our study adds to growing evidence that microgeographic divergence can occur across small spatial scales within individuals' daily dispersal neighborhood where gene flow is expected to be strong. K E Y W O R D S : dispersal neighborhood, Gasterosteus aculeatus, migration-selection balance.

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 in threespine stickleback (Gasterosteus aculeatus) has collateral effects on their locomotion. When freshwater stickleback are infected with the tapeworm parasite Schistocephalus solidus, they face an array of fitness consequences ranging from impaired body condition and fertility to an increased risk of mortality. To fight the infection, some stickleback will initiate a fibrosis immune response in which they produce excess collagenous tissue in their coelom. 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. Here we quantify the locomotor effects of the fibrosis immune response in the absence of parasites to investigate whether there are collateral costs of fibrosis that could help explain why some fish forego this effective defense. To do this, we induce fibrosis in stickleback and then test their C-start escape performance. Additionally, we measure 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 reveals 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.

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