Cell-mediated immunity predicts the probability of local recruitment in nestling blue tits

We investigated whether the variation in T-cell-mediated immune function of blue tit nestlings affected their fledgling success and the probability of local survival. We studied the relationship between immune function and survival under two rearing conditions: control, unmanipulated, and experimentally enlarged broods. Brood enlargement had negative effects on nestling immune response. Immune response was positively related to fledgling success and it predicted the probability of local recruitment. However, the relationship between immune response and the probability of recruitment was significantly positive only among control broods and nonsignificant among enlarged broods. The effect of immune response on the recruitment probability was not affected by variation in body mass. Our study suggests that selection for immune responsiveness seems to be weak or even absent under unfavourable rearing conditions as simulated by brood size enlargement. Therefore, year-to-year environmental variation and environmental heterogeneity may constrain evolution towards higher immune responsiveness.     

Immune function and survival of great tit nestlings in relation to growth conditions

Life history theory predicts a trade-off between number and quality of offspring. Reduced quality with increasing brood size may arise from a decrease in body condition or in immunocompetence that would be important in fighting off virulent parasites by immunologically naive offspring. We tested the effect of rearing conditions on immune function of nestling great tits (Parus major) by reducing or increasing broods by two hatchlings. In the middle of the nestling period (on day 8), nestlings from enlarged broods developed lower T cell responses [as measured from the cutaneous swelling reaction to injection with phytohaemagglutinin (PHA)] and tended to have lower total leukocyte and lymphocyte concentrations in their peripheral blood than nestlings from reduced broods. Brood size manipulation affected the PHA response of nestlings most strongly in small clutches, suggesting that nestling immune function was dependent on their parents’ condition, as estimated by original clutch size. Intra-brood differences in nestling mortality were unrelated to immune parameters, but nestlings in broods without mortality had a stronger PHA response, higher concentration of lymphocytes and higher body mass on day 15 than nestlings in broods with mortality. These results support the prediction that the immune function of altricial birds is affected by rearing conditions, and that growth and immune parameters are related to inter-brood differences in nestling survival. Received: 1 February 1999 / Accepted: 19. July 1999     

Female house wrens (Troglodytes aedon) increase the size, but not immunocompetence, of their offspring through extra-pair mating

House wrens are typically socially monogamous, but frequently engage in extra-pair matings leading to multisired broods. Because females do not appear to acquire direct material benefits from their extra-pair mates, we tested the hypothesis that female house wrens derive indirect genetic benefits, such as enhanced immunocompetence (cutaneous immune activity, humoral immunity, and plasma bactericidal activity) and condition (size and haematoserological traits) for their offspring, by mating polyandrously. We predicted that extra-pair young (EPY) should show greater immune responsiveness and better body condition than their within-pair maternal half-siblings (WPY). Contrary to our prediction, WPY had higher cutaneous immune activity than their EPY brood-mates in two of three years, and EPY and WPY did not differ in measures of innate and humoral immunity. WPY also had higher albumin to γ-globulin ratios than EPY; however, they were not in better condition based on other measures. EPY had consistently longer tarsi (a measure of long-bone size) than their WPY half-siblings, suggesting that females engage in extra-pair copulations with larger males. The benefits of large structural size in the study population is unknown, but based on evidence from other passerines, we suggest that structural size may be an important fitness-related trait in house wrens. We conclude that our results are not consistent with the hypothesis that females gain immune-related benefits for their offspring by engaging in extra-pair matings. Further study of the fitness consequences of differences in tarsus length is needed to determine whether females acquire size-related benefits for their offspring from extra-pair mates.     

Dual strategy for immune defense in the land snail Cornu aspersum (Gastropoda, Pulmonata)

Immune defenses have been shown to be heavily involved in the evolution of physiological trade-offs. In this study, we compared the internal defense systems in two subspecies of the land snail Cornu aspersum that exhibit contrasting life-history strategies. The "fast-living" Cornu aspersum subsp. aspersa is widespread throughout the world, especially in ecosystems disturbed by man, whereas natural populations of the giant Cornu aspersum subsp. maxima, characterized by a longer life span, are present only in north Africa. Snails were experimentally challenged with Escherichia coli; the measurements used to assess their internal defense for cell- and humoral-mediated immune responses were bacterial clearance, hemocyte density, reactive oxygen species (ROS) production, and plasma antibacterial activity. Both subspecies showed a similar ability to clear bacteria from their hemolymph; however, they varied in the robustness of different individual immune components. Cornu aspersum aspersa had higher ROS activity than did C. a. maxima and lower plasma bactericidal activity. These results suggest that ecological factors can sculpt the immune response. One interpretation is that shorter life span selects for immune defenses such as ROS that, although effective, can cause long-term damage. Such different immune patterns obviously entail various costs involved in the strong intraspecific variation of life-history trade-offs we previously observed. We also have to consider that such variation might be related to intraspecific differences in the relative strength of resistance and tolerance mechanisms.     

Constitutive innate immunity is a component of the pace-of-life syndrome in tropical birds

We studied the relationship between one component of immune function and basal metabolic rate (BMR), an indicator of the 'pace-of-life syndrome', among 12 tropical bird species and among individuals of the tropical house wren (Troglodytes aedon), to gain insights into functional connections between life history and physiology. To assess constitutive innate immunity we introduced a new technique in the field of ecological and evolutionary immunology that quantifies the bactericidal activity of whole blood. This in vitro assay utilises a single blood sample to provide a functional, integrated measure of constitutive innate immunity. We found that the bactericidal activity of whole blood varied considerably among species and among individuals within a species. This variation was not correlated with body mass or whole-organism BMR. However, among species, bacteria killing activity was negatively correlated with mass-adjusted BMR, suggesting that species with a slower pace-of-life have evolved a more robust constitutive innate immune capability. Among individuals of a single species, the house wren, bacteria killing activity was positively correlated with mass-adjusted BMR, pointing to physiological differences in individual quality on which natural selection potentially could act.     

Endocrine-reproductive-immune interactions in female and male Galápagos marine iguanas

Endocrine-immune interactions are variable across species and contexts making it difficult to discern consistent patterns. There is a paucity of data in non-model systems making these relationships even more nebulous, particularly in reptiles. In the present study, we have completed a more comprehensive test of the relationship among steroid hormones and ecologically relevant immune measures. We tested the relationship between baseline and stress-induced levels of sex and adrenal steroid hormones and standard ecoimmunological metrics in both female and male Galápagos marine iguanas (Amblyrhynchus cristatus). We found significant associations between adrenal activity and immunity, whereby females that mounted greater corticosterone responses to stress had lower basal and stress-induced immunity (i.e., bactericidal ability). Males showed the opposite relationship, suggesting sex-specific immunomodulatory actions of corticosterone. In both sexes, we observed a stress-induced increase in corticosterone, and in females a stress-induced increase in bactericidal ability. Consistent with other taxa, we also found that baseline corticosterone and testosterone in males was inversely related to baseline bactericidal ability. However, in females, we found a positive relationship between both testosterone and progesterone and bactericidal ability. Multivariate analysis did not discern any further endocrine-immune relationships, suggesting that interactions between adrenal, sex steroid hormones, and the immune system may not be direct and instead may be responding to other common stimuli, (i.e., reproductive status, energy). Taken together, these data illustrate significant endocrine-immune interactions that are highly dependent on sex and the stress state of the animal.     

Sex‐specific heritability of cell‐mediated immune response in the blue tit nestlings (Cyanistes caeruleus)

Here, we aimed at estimating sex‐specific heritabilities of cell‐mediated immune response (CMI) in the blue tit nestlings (Cyanistes caeruleus). To separate genetic and environmental components of the phenotypic variance in CMI (measured using phytohaemagglutinin assay), we performed a cross‐fostering experiment. Additionally, controlled environmental variation was introduced by enlarging some broods. Our analyses revealed a significant genetic component (as approximated by the nest‐of‐origin term) of the phenotypic variance in immune response. More importantly, these genetic effects differed between sexes and experimentally manipulated brood sizes, as indicated by significant genotype‐by‐sex and genotype‐by‐environment interactions. We discuss possible causes of such sexual dimorphism in gene expression and suggest that sex‐ and environment‐specific genetic interactions may contribute to the maintenance of genetic variability in traits related to immune functions.     

Specific alterations in complement protein activity of little brown myotis (Myotis lucifugus) hibernating in white-nose syndrome affected sites

White-nose syndrome (WNS) is the most devastating condition ever reported for hibernating bats, causing widespread mortality in the northeastern United States. The syndrome is characterized by cutaneous lesions caused by a recently identified psychrophilic and keratinophylic fungus (Geomyces destructans), depleted fat reserves, atypical behavior, and damage to wings; however, the proximate cause of mortality is still uncertain. To assess relative levels of immunocompetence in bats hibernating in WNS-affected sites compared with levels in unaffected bats, we describe blood plasma complement protein activity in hibernating little brown myotis (Myotis lucifugus) based on microbicidal competence assays using Escherichia coli, Staphylococcus aureus and Candida albicans. Blood plasma from bats collected during mid-hibernation at WNS-affected sites had higher bactericidal ability against E. coli and S. aureus, but lower fungicidal ability against C. albicans when compared with blood plasma from bats collected at unaffected sites. Within affected sites during mid-hibernation, we observed no difference in microbicidal ability between bats displaying obvious fungal infections compared to those without. Bactericidal ability against E. coli decreased significantly as hibernation progressed in bats collected from an affected site. Bactericidal ability against E. coli and fungicidal ability against C. albicans were positively correlated with body mass index (BMI) during late hibernation. We also compared complement activity against the three microbes within individuals and found that the ability of blood plasma from hibernating M. lucifugus to lyse microbial cells differed as follows: E. coli>S. aureus>C. albicans. Overall, bats affected by WNS experience both relatively elevated and reduced innate immune responses depending on the microbe tested, although the cause of observed immunological changes remains unknown. Additionally, considerable trade-offs may exist between energy conservation and immunological responses. Relationships between immune activity and torpor, including associated energy expenditure, are likely critical components in the development of WNS.     

Relative importance of factors affecting nestling immune response differs between junior and senior nestlings within broods of hoopoes Upupa epops

Animals should invest in the immune system to protect themselves from parasites, but the cost of immune responses may limit investment depending on resource availability. In birds' broods, senior and junior chicks in size hierarchies face different rearing conditions, and thus we predicted that factors affecting immune response should differ between them. In asynchronously hatched hoopoe Upupa epops broods, we found that the immune response of senior nestlings was not related to their body condition, but positively related to risk of parasitism (which was indirectly estimated by laying date). This suggests that their immunocompetence is not limited by access to resources, and they can differentially invest in immune response with increasing risk of parasitism. On the other hand, immune response of junior nestlings was related to their body condition, but secondarily also to risk of parasitism. Our results agree with previous studies that have found significant influence of nutritional status and risk of parasitism on nestlings immune defence, but show that the effects of these environmental factors on nestling immunocompetence differ between nestlings occupying high and low rank positions in size hierarchies. The possible influence of maternal effects on the results found is also discussed.     

Differential mechanisms for regulation of the stress response across latitudinal gradients

We examined plasticity of the stress response among three populations of the white-crowned sparrow (Zonotrichia leucophrys). These populations breed at different elevations and latitudes and thus have breeding seasons that differ markedly in length. We hypothesize that in populations where birds raise only one or rarely two broods in a season, the fitness costs of abandoning a nest are substantially larger than in closely related populations that raise up to three broods per season. Thus individuals with short breeding seasons should be less responsive to stressors and therefore less likely to abandon their young. In our study, baseline and handling-induced corticosterone levels were similar among populations, but corticosteroid-binding globulins differed, leading to a direct relationship between stress-induced free corticosteroid levels and length of breeding season. There were also population-specific differences in intracellular low-affinity (glucocorticoid-like) receptors in both liver and brain tissue. Although investigations of population-based differences in glucocorticoid secretion are common, this is the first study to demonstrate population-level differences in binding globulins. These differences could lead to dramatically different physiological and behavioral responses to stress.     

Hatching asynchrony and brood reduction influence immune response in Common Kestrel Falco tinnunculus nestlings

The onset of incubation before the end of laying imposes asynchrony at hatching and, therefore, a size hierarchy in the brood. It has been argued that hatching asynchrony might be a strategy to improve reproductive output in terms of quality or quantity of offspring. However, little is known about the mediating effect of hatching asynchrony on offspring quality when brood reduction occurs. Here, we investigate the relationship between phenotypic quality and hatching asynchrony in Common Kestrel Falco tinnunculus nestlings in Spain. Hatching asynchrony did not increase breeding success or nestling quality. Furthermore, hatching asynchrony and brood reduction had different effects on nestlings’ phytohaematogglutinin (PHA)‐mediated immune response and nestling growth. In asynchronous and reduced broods (in which at least one nestling died), nestlings showed a stronger PHA‐mediated immune response and tended to have a smaller body size compared with nestlings raised in synchronous and reduced broods. When brood reduction occurred in broods hatched synchronously, there was no effect on nestling size, but nestlings had a relatively poor PHA‐mediated immune response compared with nestlings raised in asynchronous and reduced broods. We suggest that resources for growth can be directed to immune function only in asynchronously hatched broods, resulting in improved nestling quality, as suggested by their immune response. We also found that males produced a greater PHA‐mediated immune response than females only in brood‐reduced nests without any effect on nestling size or condition, suggesting that females may trade off immune activities and body condition, size or weight. Overall, our results suggest that hatching pattern and brood reduction may mediate resource allocation to different fitness traits. They also highlight that the resolution of immune‐related trade‐offs when brood reduction occurs may differ between male and female nestlings.     

Bactericidal Activity of Lachrymal Secretion and Complement System in Copper Deficient Bovines

Copper deficiency in humans and animals has been related to increased susceptibility to infections. Neutrophils are one of the most studied components of the immune response; however, to the best of our knowledge, other defenses of the innate immune system have not been analyzed in copper-deficient animals. Our previous studies in copper-deficient bovines have shown increased susceptibility to infectious keratoconjunctivitis, an ocular disease caused by Moraxella bovis. The objective of this work was to evaluate the bactericidal activity of lachrymal secretion and complement system, two main mechanisms of the innate immune response against M. bovis, in copper-deficient cattle. Our results indicate that copper deficiency has no effect on bactericidal activity of complement system and lachrymal secretion against M. bovis in calves. Other components of local and systemic ocular defense mechanisms that could explain the increased susceptibility to infectious keratoconjunctivitis observed in copper-deficient bovines should be investigated.     

Genetic and environmental variation in immune response of collared flycatcher nestlings

This paper aims at partitioning genetic and environmental contribution to the phenotypic variance in nestling immune function measured with the hypersensitivity test after inoculation with phytohaemagglutinin. A cross-fostering experiment with artificial enlargement of some broods was conducted. Variation in nestling immune response was related to their common origin, which suggests heritable component of cell-mediated immunity. A common rearing environment also explained a significant part of variation. However, deterioration of rearing conditions as simulated by enlargement of brood size did not affect nestling immunocompetence, although it affected nestling body mass. Variation in body mass explained some of the variation in immune response related to rearing environment, which means that growth is more sensitive to the shifts in rearing conditions than the development of immune function. Heritable variation in immune response suggests that there should be potential for selection to operate and the micro evolutionary changes in immunity of flycatcher nestlings are possible.     

No effect of carotenoid supplementation on phytohemagglutinin response or body condition of nestling house wrens

Carotenoids are an essential and often limiting resource in animals and play important roles in immune system function. In birds, the period shortly after hatching is an energetically demanding stage characterized by rapid growth in body size and organ systems, including the immune system. Availability of carotenoids for the growing nestlings may be of particular importance and potentially limiting at this stage of development. We tested the hypothesis that the availability of carotenoids for the embryo in the egg and in the diet of nestlings limits the condition and immune responses of nestling house wrens (Troglodytes aedon Vieillot 1809), a species with melanin-based plumage pigments. In one experiment, nestlings within females' second broods were randomly assigned to receive either a control or a lutein supplement (2008); in a second experiment, females, before their first broods, were either induced to lay additional eggs or not induced, and nestlings within both kinds of broods were supplemented as in the first experiment (2009). There were no significant effects of lutein supplementation on nestling condition or phytohemagglutinin response. There was a significant effect of lutein supplementation on nestling mass in 2008, but the difference was opposite to that predicted. Moreover, even when breeding females were stressed by inducing them to lay supernumerary eggs, lutein supplementation of nestlings had no effect on the size or condition of nestlings hatching from these eggs. These results suggest that maternally derived lutein in the egg and that provided in the diet of nestlings are not limiting to normal development and to the components of the immune system involved in the phytohemagglutinin response of nestling house wrens.     

Genetic and environmental components of growth in nestling blue tits (Parus caeruleus)

We investigated the effect of brood‐size mediated food availability on the genetic and environmental components of nestling growth in the blue tit (Parus caeruleus), using a cross‐fostering technique. We found genetic variation for body size at most nestling ages, and for duration of mass increase, but not of tarsus growth. Hence, nestling growth in our study population seems to have the potential to evolve further. Furthermore, significant genotype–environment interactions indicated heritable variation in reaction norms of growth rates and growth periods, i.e. that our study population had a heritable plasticity in the growth response to environmental conditions. The decreasing phenotypic variance with nestling age indicated compensatory growth in all body traits. Furthermore, the period of weight increase was longer for nestlings growing up in enlarged broods, while there was no difference to reduced broods in the period of tarsus growth. At fledging, birds in enlarged broods had shorter tarsi and lower weights than birds in reduced broods, but there was no difference in wing length or body condition between the two experimental groups. The observed flexibility in nestling growth suggests that growing nestlings are able to respond adaptively to food constraint by protecting the growth of ecologically important traits.     

Allocation of Male Parental Care in Relation to Paternity Within and Among Broods of the Common Yellowthroat (Geothlypis trichas)

The relationship between male parental care and paternity has been investigated in a number of avian species, but in many cases the influences of confounding factors, such as variation in male and territory quality, were not addressed. These sources of variation can be controlled for by making within-male comparisons between successive broods or within-brood comparisons between groups of fledglings in a divided brood. We studied the relationship between male parental care and paternity in the common yellowthroat ( Geothlypis trichas ) at three levels: between groups of fledglings in divided broods, between first and second broods of the same pair, and among all broods in the population. In this study we proposed three hypotheses: first, males in double-brooded pairs should provide relatively more parental care to broods in which they have higher paternity; secondly, after fledging and brood division, males should provide more care to related offspring; and finally, among all broods in the population, paternity should be related positively to male parental care. Brood division occurred in many of the broods studied; however, broods were not divided according to fledgling size or paternity. Furthermore, within divided broods, males fed within-pair and extra-pair fledglings at similar rates. For sequential broods of the same pair, male feeding rates were not associated with differences in paternity between broods. Among all broods in the population, males did not provide relatively less care to broods containing unrelated young. The lack of a relationship between male parental care and paternity suggests that either males cannot assess their paternity or the costs of reducing male parental care outweigh the benefits.     

Naturally occurring genetic variation in the age-specific immune response of Drosophila melanogaster

Immunosenescence, the age‐related decline in immune response, is a well‐known consequence of aging. To date, most studies of age‐related changes in immune response focused on the cellular and physiological bases of this decline; we have virtually no understanding of the genetic basis of age‐related changes in the immune system or if indeed such control exists. We used 25 chromosome substitution lines of Drosophila melanogaster derived from a natural population to address three questions: (i) How is the function of the innate immune system influenced by age? (ii) Is there a genetic basis for phenotypic variation in immune response at different ages? (iii) Is there a genetic basis for differences in the way that age influences the immune function? Virgin females from each line were assayed for immune response using clearance of infection with Escherichia coli at 1 and 4 weeks of age. We found significant genetic variation among lines in immune response at each age. Unexpectedly, when averaged across all lines, the immune response actually improved with age. However, there was significant variation in the effect of age on immune response with 11 lines showing improvement, nine lines showing no change and five exhibiting a decline with age. There was no genetic correlation of immune response across ages suggesting that different loci contribute to variation in immune response at each age. The genetic component of the variation in immune response increased with age, a pattern predicted by the mutation accumulation model of senescence. However, this increase in variation resulted in part from the improvement of the immune response in some lines with age. Thus the observed changes in genetic variation in immune function with age are not entirely explained by the mutation accumulation model.     

Heterozygosity is linked to the costs of immunity in nestling great tits (Parus major)

There is growing evidence that heterozygosity–fitness correlations (HFCs) are more pronounced under harsh conditions. Empirical evidence suggests a mediating effect of parasite infestation on the occurrence of HFCs. Parasites have the potential to mediate HFCs not only by generally causing high stress levels but also by inducing resource allocation tradeoffs between the necessary investments in immunity and other costly functions. To investigate the relative importance of these two mechanisms, we manipulated growth conditions of great tit nestlings by brood size manipulation, which modifies nestling competition, and simultaneously infested broods with ectoparasites. We investigated under which treatment conditions HFCs arise and, second, whether heterozygosity is linked to tradeoff decisions between immunity and growth. We classified microsatellites as neutral or presumed functional and analyzed these effects separately. Neutral heterozygosity was positively related to the immune response to a novel antigen in parasite‐free nests, but not in infested nests. For nestlings with lower heterozygosity levels, the investments in immunity under parasite pressure came at the expenses of reduced feather growth, survival, and female body condition. Functional heterozygosity was negatively related to nestling immune response regardless of the growth conditions. These contrasting effects of functional and neutral markers might indicate different underlying mechanisms causing the HFCs. Our results confirm the importance of considering marker functionality in HFC studies and indicate that parasites mediate HFCs by influencing the costs of immune defense rather than by a general increase in environmental harshness levels.     

Interactions between Mytilus haemocytes and different strains of Escherichia coli and Vibrio cholerae O1 El Tor: role of kinase-mediated signalling

Marine bivalves accumulate large amounts of bacteria from the environment (mainly Vibrionaceae and coliforms). Although persistence of different bacteria in bivalve tissues largely depends on their sensitivity to the bactericidal activity of circulating haemocytes and haemolymph soluble factors, the mechanisms involved in bacteria-host cell interactions in these invertebrates are largely unknown. In the mussel Mytilus, differences in interactions between haemocytes and different Escherichia coli and Vibrio cholerae strains [E. coli MG155, a wild-type strain carrying type 1 fimbriae, and its unfimbriated derivative, AAEC072 Deltafim; V. cholerae O1 El Tor biotype strain N16961, carrying the mannose-sensitive haemagglutinin (MSHA), and its MSHA mutant] lead to differences in bactericidal activity in the presence of serum. Here we show that different bacteria induced distinct patterns of phosphorylation of mitogen-activated protein kinases (MAPKs), in particular of the stress-activated MAPKs involved in the immune response. Differences in phosphorylation of PKC-like proteins were also observed. The results support the hypothesis that, like in mammalian host cells, different bacteria can modulate the signalling pathways of mussel haemocytes. The lower anti-bacterial activity towards the mutant E. coli strain and wild-type V. cholerae compared with wild E. coli may result from a reduced capacity of activating MAPKs. Moreover, the mutant V. cholerae strain that was the most resistant to the haemocyte bactericidal activity induced downregulation of cell signalling and showed the strongest effect on lysosomal membrane stability, evaluated as a marker of bivalve cell stress. These data suggest that certain bacteria could evade the bactericidal activity of mussel haemocytes through disruption of the host signalling pathways.