Kathryn A. Watt

Maternal Transfer of Strain-Specific Immunity in an Invertebrate

The most celebrated component of the vertebrate immune system is the acquired response in which memory cells established during primary infection enhance the proliferation of antibodies during secondary infection. Additionally, the strength of vertebrate acquired immune responses varies dramatically depending on the infecting pathogen species or on the pathogen genotype within species. Because invertebrates lack the T-cell receptors and Major Histocompatibility Complex (MHC) molecules that mediate vertebrate adaptive immune responses, they are thought to lack adaptive immunity and be relatively unspecific in their interactions with pathogens. With only innate immunity, invertebrate hosts are believed to be nai;ve at each new encounter with pathogens. Nevertheless, some forms of facultative immunity appear to be important in insects; some individuals have enhanced immunity due to population density, and some social insects benefit when their nest-mates have been exposed to a pathogen or pathogen mimic (; see for a predation example.) Here we provide evidence for acquired strain-specific immunity in the crustacean Daphnia magna infected with the pathogenic bacteria Pasteuria ramosa. Specifically, the fitness of hosts was enhanced when challenged with a bacterial strain their mother had experienced relative to cases when mother and offspring were challenged with different strains.

Fitness Correlates of Heritable Variation in Antibody Responsiveness in a Wild Mammal

Self-Recognition and Survival Soay sheep are a remnant of an ancient breed of sheep that, although intensively studied for many years, live unmanaged on the remote Scottish island of St. Kilda. Life is harsh on the island, and the numbers of sheep show cycles of winter population crashes and high exposure to infection. Graham et al. (p. 662; see the Perspective by Martin and Coon) measured levels of self-reactive antibodies in the sheep called antinuclear antibodies (ANA). Having high ANA levels was a heritable trait that reflected generally high levels of immunoglobulin in individuals and of specific antibodies to parasitic worms. Female sheep with high levels of ANAs survived better during crash years, but had fewer births. If these sheep did reproduce, although the lambs tended to be small, they tended to have higher rates of early survival. Thus, maintaining high antibody levels apparently reflected investment in immunity and greater survival, but doing so was also associated with reduced reproductive success. In Soay sheep, self-reactive antibodies are indicators of an evolutionary trade-off between survival and reproduction. A functional immune system is important for survival in natural environments, where individuals are frequently exposed to parasites. Yet strong immune responses may have fitness costs if they deplete limited energetic resources or cause autoimmune disease. We have found associations between fitness and heritable self-reactive antibody responsiveness in a wild population of Soay sheep. The occurrence of self-reactive antibodies correlated with overall antibody responsiveness and was associated with reduced reproduction in adults of both sexes. However, in females, the presence of self-reactive antibodies was positively associated with adult survival during harsh winters. Our results highlight the complex effects of natural selection on immune responsiveness and suggest that fitness trade-offs may maintain immunoheterogeneity, including genetic variation in autoimmune susceptibility.

Host–parasite interactions for virulence and resistance in a malaria model system

A rich body of theory on the evolution of virulence (disease severity) attempts to predict the conditions that cause parasites to harm their hosts, and a central assumption to many of these models is that the relative virulence of pathogen strains is stable across a range of host types. In contrast, a largely nonoverlapping body of theory on coevolution assumes that the fitness effects of parasites on hosts is not stable across host genotype, but instead depends on host genotype by parasite genotype interactions. If such genetic interactions largely determine virulence, it becomes difficult to predict the strength and direction of selection on virulence. In this study, we tested for host‐by‐parasite interactions in a medically relevant vertebrate disease model: the rodent malaria parasite Plasmodium chabaudi in laboratory mice. We found that parasite and particularly host main effects explained most of the variance in virulence (anaemia and weight loss), resistance (parasite burden) and transmission potential. Host‐by‐parasite interactions were of limited influence, but nevertheless had significant effects. This raises the possibility that host heterogeneity may affect the rate of any parasite response to selection on virulence. This study of rodent malaria is one of the first tests for host‐by‐parasite interactions in any vertebrate disease; host‐by‐parasite interactions typical of those assumed in coevolutionary models were present, but were by no means pervasive.

Natural selection on individual variation in tolerance of gastrointestinal nematode infection.

A 25-year study of wild sheep shows that individuals vary in how quickly they lose weight as parasite infections increase, and that those who lose the least weight when heavily infected produce more offspring.

Age-related variation in immunity in a wild mammal population

Age‐related changes in immunity are well documented in humans and laboratory mammals. Using blood samples collected from wild Soay sheep, we show that pronounced differences in T‐cell subsets and inflammatory markers amongst age classes are also evident under natural conditions. These shifts parallel those observed in mammals experiencing protected environments. We found progressive declines in the proportion of naïve CD4 T cells with age, a precipitous drop in γδ T cells after the second year of life and an increase in acute phase protein levels amongst geriatric sheep. Our findings suggest immune aging patterns observed in laboratory and domestic mammals may generalize to more complex, challenging environments and could have fitness costs under natural conditions.

Multivariate immune defences and fitness in the wild: complex but ecologically important associations among plasma antibodies, health and survival

Despite our rapidly advancing mechanistic understanding of vertebrate immunity under controlled laboratory conditions, the links between immunity, infection and fitness under natural conditions remain poorly understood. Antibodies are central to acquired immune responses, and antibody levels circulating in vivo reflect a composite of constitutive and induced functional variants of diverse specificities (e.g. binding antigens from prevalent parasites, self tissues or novel non-self sources). Here, we measured plasma concentrations of 11 different antibody types in adult females from an unmanaged population of Soay sheep on St Kilda. Correlations among antibody measures were generally positive but weak, and eight of the measures independently predicted body mass, strongyle parasite egg count or survival over the subsequent winter. These independent and, in some cases, antagonistic relationships point to important multivariate immunological heterogeneities affecting organismal health and fitness in natural systems. Notably, we identified a strong positive association between anti-nematode immunoglobulin (Ig) G antibodies in summer and subsequent over-winter survival, providing rare evidence for a fitness benefit of helminth-specific immunity under natural conditions. Our results highlight both the evolutionary and ecological importance and the complex nature of the immune phenotype in the wild.

Parasite variation and the evolution of virulence in a Daphnia-microparasite system.

Understanding genetic relationships amongst the life-history traits of parasites is crucial for testing hypotheses on the evolution of virulence. This study therefore examined variation between parasite isolates (the bacterium Pasteuria ramosa) from the crustacean Daphnia magna. From a single wild-caught infected host we obtained 2 P. ramosa isolates that differed substantially in the mortality they caused. Surprisingly, the isolate causing higher early mortality was, on average, less successful at establishing infections and had a slower growth rate within hosts. The observation that within-host replication rate was negatively correlated with mortality could violate a central assumption of the trade-off hypothesis for the evolution of virulence, but we discuss a number of caveats which caution against premature rejection of the trade-off hypothesis. We sought to test if the characteristics of these parasite isolates were constant across host genotypes in a second experiment that included 2 Daphnia host clones. The relative growth rates of the two parasite isolates did indeed depend on the host genotype (although the rank order did not change). We suggest that testing evolutionary hypotheses for virulence may require substantial sampling of both host and parasite genetic variation, and discuss how selection for virulence may change with the epidemiological state of natural populations and how this can promote genetic variation for virulence.

Detecting genes for variation in parasite burden and immunological traits in a wild population: testing the candidate gene approach

Identifying the genes underlying phenotypic variation in natural populations can provide novel insight into the evolutionary process. The candidate gene approach has been applied to studies of a number of traits in various species, in an attempt to elucidate their genetic basis. Here, we test the application of the candidate gene approach to identify the loci involved in variation in gastrointestinal parasite burden, a complex trait likely to be controlled by many loci, in a wild population of Soay sheep. A comprehensive literature review, Gene Ontology databases, and comparative genomics resources between cattle and sheep were used to generate a list of candidate genes. In a pilot study, these candidates, along with 50 random genes, were then sequenced in two pools of Soay sheep; one with low gastrointestinal nematode burden and the other high, using a NimbleGen sequence capture experiment. Further candidates were identified from single nucleotide polymorphisms (SNPs) that were highly differentiated between high‐ and low‐resistance sheep breeds. A panel of 192 candidate and control SNPs were then typed in 960 individual Soay sheep to examine whether they individually explained variation in parasite burden, as measured as faecal egg count, as well as two immune measures (Teladorsagia circumcincta‐specific antibodies and antinuclear antibodies). The cumulative effect of the candidate and control SNPs were estimated by fitting genetic relationship matrices (GRMs) as random effects in animal models of the three traits. No more significant SNPs were identified in the pilot sequencing experiment and association study than expected by chance. Furthermore, no significant difference was found between the proportions of candidate or control SNPs that were found to be significantly associated with parasite burden/immune measures. No significant effect of the candidate or control gene GRMs was found. There is thus little support for the candidate gene approach to the identification of loci explaining variation in parasitological and immunological traits in this population. However, a number of SNPs explained significant variation in multiple traits and significant correlations were found between the proportions of variance explained by individual SNPs across multiple traits. The significant SNPs identified in this study may still, therefore, merit further investigation.

Heritable, Heterogeneous, and Costly Resistance of Sheep against Nematodes and Potential Feedbacks to Epidemiological Dynamics*

Infected hosts may preserve fitness by resisting parasites (reducing parasite burden) and/or tolerating them (preventing or repairing infection-induced damage). Theory predicts that these individual-level defense strategies generate divergent population-level feedbacks that would maintain genetic heterogeneity for resistance but purge heterogeneity for tolerance. Because resistance reduces parasite abundance, selection for costly resistance traits will weaken as resistance becomes common. Such negative frequency-dependent selection contrasts with predictions for tolerance, which maintains parasite abundance and so is expected to generate positive frequency-dependent selection, unless, for example, tolerance trades off with resistance. Thus far, there have been few tests of this theory in natural systems. Here, we begin testing the predictions in a mammalian field system, using data on individual gastrointestinal nematode burdens, nematode-specific antibody titers (as a resistance metric), the slope of body weight on parasite burden (as a tolerance metric), and fitness from an unmanaged population of Soay sheep. We find that nematode resistance is costly to fitness and underpinned by genetic heterogeneity, and that resistance is independent of tolerance. Drawing upon empirical metrics such as developed here, future work will elucidate how resistance and tolerance feedbacks interact to generate population-scale patterns in the Soay sheep and other field systems.

Sex differences in leukocyte telomere length in a free-living mammal

Mounting evidence suggests that average telomere length reflects previous stress and predicts subsequent survival across vertebrate species. In humans, leucocyte telomere length (LTL) is consistently shorter during adulthood in males than in females, although the causes of this sex difference and its generality to other mammals remain unknown. Here, we measured LTL in a cross‐sectional sample of free‐living Soay sheep and found shorter telomeres in males than in females in later adulthood (>3 years of age), but not in early life. This observation was not related to sex differences in growth or parasite burden, but we did find evidence for reduced LTL associated with increased horn growth in early life in males. Variation in LTL was independent of variation in the proportions of different leucocyte cell types, which are known to differ in telomere length. Our results provide the first evidence of sex differences in LTL from a wild mammal, but longitudinal studies are now required to determine whether telomere attrition rates or selective disappearance are responsible for these observed differences.