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Immune responses vary greatly depending on physiological and ecological circumstances, but the causes of this variation and its consequences for host–parasite interactions are not clear. One problem is whether resource trade-offs within hosts affect host choice by macroparasites. We aimed experimentally to disentangle the effects of the nutritional and immunological parameters of the host on the choice of host by a blood-feeding ectoparasitic fly, Carnus hemapterus. We used micronutrient supplementation to enhance host immunity and antioxidant capacity and balance the resource allocation of the host from growth to immune function in nestlings of the European roller, Coracias garrulus. Micronutrient-supplemented nestlings showed lower growth rate and body condition than sham-supplemented nestlings. Nestlings receiving micronutrients also had a higher albumin-to-globulin ratio and β- and γ-globulin concentrations, but lower α1- and α2-globulin concentrations, total protein level and cutaneous response to phytohaemagglutinin. Our results revealed that the phytohaemagglutinin response and β- and γ-globulin concentrations reflect body condition, whereas the strength of the acute phase response (α1- and α2-globulin concentrations) reflects the protein status in nestling European rollers. Ectoparasitic C. hemapterus flies generally chose hosts according to their body condition. However, this choice was moderated by host immunity when the host had poor body condition or posed a high risk for parasites in terms of the acute phase immune response. [ABSTRACT FROM AUTHOR]

Resource availability, predation, and sexual selection have all been shown to play an important role in the ecology and evolution of guppies, Poecilia reticulata, but the role of parasitism has received comparatively little attention. In the present study, we examined natural infection levels of wild-caught guppies from the Upper and Lower Aripo River in Trinidad (UA and LA, respectively) and experimentally infected a subsample of fish with an isogenic line of the ectoparasite Gyrodactylus turnbulli. LA fish showed a superior resistance compared to UA guppies during the primary, but not secondary infection 53 days later. Resistance of LA individuals was consistent across experiments, suggesting immunocompetence has a heritable genetic basis. The efficiency of the immune response of UA fish was not correlated across infections. During primary infections, UA fish were highly susceptible, but their resistance was significantly improved during a secondary infection, highlighting the importance of acquired resistance. We discuss the role of innate and acquired resistance, and place our findings in the context of natural gyrodactylid infections. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 647–655. [ABSTRACT FROM AUTHOR]

Knowledge of the role of origin-related, environmental, sex, and age factors on host defence mechanisms is important to understand variation in parasite intensity. Because alternative components of parasite defence may be differently sensitive to various factors, they may not necessarily covary. Many components should therefore be considered to tackle the evolution of host–parasite interactions. In a population of barn owls ( Tyto alba), we investigated the role of origin-related, environmental (i.e. year, season, nest of rearing, and body condition), sex, and age factors on 12 traits linked to immune responses [humoral immune responses towards sheep red blood cells (SRBC), human serum albumin (HSA) and toxoid toxin TT, T-cell mediated immune response towards the mitogen phytohemagglutinin (PHA)], susceptibility to ectoparasites (number and fecundity of Carnus haemapterus, number of Ixodes ricinus), and disease symptoms (size of the bursa of Fabricius and spleen, proportion of proteins that are immunoglobulins, haematocrit and blood concentration in leucocytes). Cross-fostering experiments allowed us to detect a heritable component of variation in only four out of nine immune and parasitic parameters (i.e. SRBC- and HSA-responses, haematocrit, and number of C. haemapterus). However, because nestlings were not always cross-fostered just after hatching, the finding that 44% of the immune and parasitic parameters were heritable is probably an overestimation. These experiments also showed that five out of these nine parameters were sensitive to the nest environment (i.e. SRBC- and PHA-responses, number of C. haemapterus, haematocrit and blood concentration in leucocytes). Female nestlings were more infested by the blood-sucking fly C. haemapterus than their male nestmates, and their blood was less concentrated in leucocytes. The effect of year, season, age (i.e. reflecting the degree of maturation of the immune system), brood size, position in the within-brood age hierarchy, and body mass strongly differed between the 12 parameters. Different components of host defence mechanisms are therefore not equally heritable and sensitive to environmental, sex, and age factors, potentially explaining why most of these components did not covary. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 703–718. [ABSTRACT FROM AUTHOR]

The major histocompatibility complex (MHC) presents a group of genes with highly polymorphic loci involved in specific immune responses. The factors maintaining extensive MHC polymorphism have been questioned, considering three possible hypotheses of parasite-mediated selection driving an extensive MHC diversity (i.e. heterozygote advantage, rare-allele advantage, and favouring optimal MHC diversity). The patterns of MHC diversity of class IIB genes were investigated following two noncontradicting hypotheses, parasite-driven selection and MHC-based mating preferences, using males of common bream collected in the spawning period. Two allelic groups DAB1 and DAB3 were recognized from the phylogenetic analyses. Individuals expressed one or two alleles of the same or different allelic groups. Several individuals shared identical alleles; however, the presence of parasite species was not associated with the occurrence of a particular allele. The presence of different allelic groups (only DAB1, only DAB3, or both DAB1 and DAB3) in individuals was not associated with parasite presence or diversity. The expression of two DAB1 alleles was associated with higher endoparasite abundance. Moreover, nucleotide diversity in individuals expressing a single type of alleles ( DAB1 or DAB3) increased with the abundance of ectoparasitic Dactylogyrus spp. (Monogenea) and Ergasilus sp. (Crustacea). This suggests that the expression of two alleles of a single allelic type is related to high metazoan parasite infection whereas no significant influence of parasitism on the combined allelic form (the presence of both DAB1 and DAB3 alleles) was found. Moreover, the expression of two alleles of a single allelic type was related to decreased immunocompetence measured by spleen size. The condition factor was higher in fish expressing the combined allelic type. Thus, the presence of alleles of different lineages in individuals appears to be advantageous for individual male fitness. The expression of a single allelic type was related to higher sexual ornamentation, which could support the role of MHC in the hypothesis of the sexual selection of ‘good genes’. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 525–538. [ABSTRACT FROM AUTHOR]