Seasonal changes in immune response and parasite impact on hosts

Seasonal changes in the impact of parasites on hosts should result in seasonal changes in immune function. Since both ectoparasites and endoparasites time their reproduction to that of their hosts, we can predict that hosts have been selected to show an annual peak in their ability to raise an immune response during the reproductive season. We found large seasonal changes in immune function between the breeding and the nonbreeding season for a sample of temperate bird species. These changes amounted to a decrease in spleen mass from the breeding to the nonbreeding season by on average 18% across 71 species and a seasonal decrease in T-cell-mediated immunity by on average 33% across 13 species. These seasonal changes in immune function differed significantly among species. The condition dependence of immune function also differed between the breeding and the nonbreeding season, with individuals in prime condition particularly having greater immune responses during breeding. Analyses of ecological factors associated with interspecific differences in seasonal change of immune function revealed that hole-nesting species had a larger increase in immune function during the breeding season than did open nesters. Since hole nesters suffer greater reduction in breeding success because of virulent parasites than do open nesters, this seasonal change in immune function is suggested to have arisen as a response to the increased virulence of parasites attacking hole-nesting birds.     

Phylogeographic structure,demographic history and morph composition in a colour polymorphic lizard

In polymorphic species, population divergence in morph composition and frequency has the potential to promote speciation. We assessed the relationship between geographic variation in male throat colour polymorphism and phylogeographic structure in the tawny dragon lizard, Ctenophorus decresii. We identified four genetically distinct lineages, corresponding to two polymorphic lineages in the Northern Flinders Ranges and Southern Flinders Ranges/Olary Ranges regions respectively, and a monomorphic lineage in the Mt Lofty Ranges/Kangaroo Island region. The degree of divergence between these three lineages was consistent with isolation to multiple refugia during Pleistocene glacial cycles, whereas a fourth, deeply divergent (at the interspecific level) and monomorphic lineage was restricted to western New South Wales. The same four morphs occurred in both polymorphic lineages, although populations exhibited considerable variation in the frequency of morphs. By contrast, male throat coloration in the monomorphic lineages differed from each other and from the polymorphic lineages. Our results suggest that colour polymorphism has evolved once in the C. decresii species complex, with subsequent loss of polymorphism in the Mt Lofty Ranges/Kangaroo Island lineage. However, an equally parsimonious scenario, that polymorphism arose independently twice within C. decresii, could not be ruled out. We also detected evidence of a narrow contact zone with limited genotypic admixture between the polymorphic Olary Ranges and monomorphic Mt Lofty Ranges regions, yet no individuals of intermediate colour phenotype. Such genetic divergence and evidence for barriers to gene flow between lineages suggest incipient speciation between populations that differ in morph composition.     

Local adaptation and divergence in colour signal conspicuousness between monomorphic and polymorphic lineages in a lizard

Population differences in visual environment can lead to divergence in multiple components of animal coloration including signalling traits and colour patterns important for camouflage. Divergence may reflect selection imposed by different receivers (conspecifics, predators), which depends in turn on the location of the colour patch. We tested for local adaptation of two genetically and phenotypically divergent lineages of a rock‐inhabiting lizard, Ctenophorus decresii, by comparing the visual contrast of colour patches to different receivers in native and non‐native environments. The lineages differ most notably in male throat coloration, which is polymorphic in the northern lineage and monomorphic in the southern lineage, but also differ in dorsal and lateral coloration, which is visible to both conspecifics and potential predators. Using models of animal colour vision, we assessed whether lineage‐specific throat, dorsal and lateral coloration enhanced conspicuousness to conspecifics, increased crypsis to birds or both, respectively, when viewed against the predominant backgrounds from each lineage. Throat colours were no more conspicuous against native than non‐native rock but contrasted more strongly with native lichen, which occurs patchily on rocks inhabited by C. decresii. Conversely, neck coloration (lateral) more closely matched native lichen. Furthermore, although dorsal coloration of southern males was consistently more conspicuous to birds than that of northern males, both lineages had similar absolute conspicuousness against their native backgrounds. Combined, our results are consistent with local adaptation of multiple colour traits in relation to multiple receivers, suggesting that geographic variation in background colour has influenced the evolution of lineage‐specific coloration in C. decresii.     

Developmental instability and immune function in colour polymorphic pygmy grasshoppers

Random left minus right deviations from symmetry in otherwise bilaterally symmetric traits may arise due to developmental instability in response to environmental stress. Here we test for variation in developmental instability, measured as asymmetry of (femur) size, among individuals belonging to four different genetically encoded colour morphs of the pygmy grasshopper Tetrix undulata (Sow.) (Orthoptera: Tetrigidae). Such a difference is expected under the hypotheses that perturbed growth and development and concomitant morphological asymmetry may result from exposure to unfavourable temperatures or costs associated with melanization, or reflect a by-product of past disease in individuals with poor immune responsiveness. Results from mixed model two-way ANOVAs uncovered no statistically significant directional asymmetry in femur size, whereas non-directional asymmetry was significant in each of the four different colour morphs. The degree of intraindividual, interlateral variance in femur size varied significantly among individuals belonging to different colour morphs, being considerably higher in the black morph, which also suffers the greatest risk of overheating, compared to the brown, striped and grey morphs. Asymmetry in femur size was not associated with one measure of immune responsiveness, the encapsulation response of individuals experimentally implanted with a novel antigen consisting of a nylon mono-filament. These results are consistent with the notion that individual pigmentation pattern may indirectly influence developmental instability and morphological asymmetry, via the effects of coloration on body temperature.Co-ordinating editor: Hurst     

Seasonal colour changes and shifting antipredator tactics in darters

Dull coloured, non-breeding males of the darter Etheostoma flabellare 'freeze' in response to simulated predatory threat. As they enter breeding condition, colour changes are minor and antipredator behaviour does not change. Male E. blennioides , which develop bright green colours in the breeding season, also freeze in response to threats in both breeding and non-breeding states. Male E. spectabile change colours dramatically when breeding, developing intense and contrasting orange, blue, yellow and red colours on their sides, fins and throats. In non-breeding colouration, they freeze when threatened, but in breeding colouration they become less likely to freeze and more likely to flee. These results are discussed both with respect to the hypothesis that changing colours may require changing antipredator behaviour, and in terms of the probable degree of crypticity of these species. Details of the duration of immobility of 'freezing' fish and the pattern of subsequent recovery of activity level are also presented.     

Setaria digitata infections in cattle: parasite load, microfilaraemia status and relationship to immune response

A total of 110 cattle were examined in an area endemic for Bancroftian filariasis for the prevalence of infection of the bovine filarial parasite Setaria digitata. About 12.5% of cattle were found to harbour both adult worms in the peritoneum and microfilariae (mf) in circulation; 70% of the cattle were amicrofilaraemic but with an adult worm infection. A third group of cattle (16.5%) was free of detectable mf and adult worms. The presence of adult worms and/or mf did not influence the antibody levels to any of the four antigen preparations of S. digitata. However, there was a significant inverse relationship between the presence of antibodies to microfilarial sheaths and the absence of circulating mf as shown by the immunoperoxidase assay. Cattle immunoglobulin containing high titres of anti-sheath antibodies cleared circulating microfilariae very effectively in Mastomys coucha thus demonstrating the protective nature of anti-sheath antibodies in eliminating circulating microfilariae in vivo.     

Seasonal changes in the relationship between ornamentation and immune response in red jungle fowl

Resistance to disease is frequently suggested to be important in mate choice, but information about how immune status can be conveyed is lacking. During the breeding season, male red jungle fowl with large combs, a sexually selected trait, have lower levels of lymphocytes, but greater cell-mediated immunity, indicated by a cutaneous hypersensitivity response. Before the breeding season, however, both cell-mediated immunity and proportion of lymphocytes are positively correlated with comb length. Cell-mediated immunity is particularly important to jungle fowl during the breeding season, because the likelihood of injury during sexual competition is high and cell-mediated immunity is essential for healing wounds and resisting infection. This seasonal change in one aspect of immunity but not another suggests that the birds adaptively maintain certain immune system abilities, and that it can be misleading to use a single aspect of immune response in evaluating immunocompetence.     

Stress‐induced changes in color expression mediated by iridophores in a polymorphic lizard

Stress is an important potential factor mediating a broad range of cellular pathways, including those involved in condition‐dependent (i.e., honest) color signal expression. However, the cellular mechanisms underlying the relationship between stress and color expression are largely unknown. We artificially elevated circulating corticosterone levels in male tawny dragon lizards, Ctenophorus decresii, to assess the effect of stress on the throat color signal. Corticosterone treatment increased luminance (paler throat coloration) and decreased the proportion of gray, thereby influencing the gray reticulations that produce unique patterning. The magnitude of change in luminance for corticosterone‐treated individuals in our study was around 6 “just noticeable differences” to the tawny dragon visual system, suggesting that lizards are likely to be able to perceive the measured variation. Transmission electron microscopy (TEM) of iridophore cells indicated that luminance increased with increasing density of iridophore cells and increased spacing (and/or reduced size) of crystalline guanine platelets within them. Crystal spacing within iridophores also differed between skin colors, being greater in cream than either gray or yellow skin and greater in orange than yellow skin. Our results demonstrate that stress detectably impacts signal expression (luminance and patterning), which may provide information on individual condition. This effect is likely to be mediated, at least in part, by structural coloration produced by iridophore cells.     

Mating system variation and morph fluctuations in a polymorphic lizard

In polymorphic male painted dragon lizards (Ctenophorus pictus), red males win staged contests for females over yellow males, and yellow males have greater success in staged sperm competition trials than red males. This predicts different reproductive strategies in the wild with red males being more coercive or better mate guarders than yellow males. Yellow males would be expected to sire more offspring per copulation and have a greater proportion of offspring from clutches with mixed paternity. However, here we show using microsatellites that the frequency of mixed paternity in the wild is low (< 20% on average across years), that all morphs on average have the same number of offspring sired per year, and that mating system variation (polyandry vs. monandry) is strongly correlated with perch density on male territories. Furthermore, a logistic regression on male successful vs. unsuccessful mate acquisition showed that red males were under negative selection when they dominated the population, which suggests ongoing frequency dependent selection on male colouration.     

Seasonal variation and the immune response: a fish perspective

The environment in which an animal lives affects the physiology and psychology of that animal. The greater the distance from the equator the more profound this influence becomes, as the environment becomes more variable over the years. Temperature, photoperiod, precipitation and other environmental conditions, which are directly or indirectly controlled by the season, can affect an animal. It is becoming apparent that these conditions may impact on the immune system, and this can affect animal health. This review looks at the known mechanisms for transducing environmental cues and how these can affect immune parameters and function. The main focus is fish, especially in relation to aquaculture and the associated disease risks. Work on other animal classes is included for comparison.     

Convergence and divergence in lizard colour polymorphisms

Colour polymorphic species are model systems for examining the evolutionary processes that generate and maintain discrete phenotypic variation in natural populations. Lizards have repeatedly evolved strikingly similar polymorphic sexual signals in distantly related lineages, providing an opportunity to examine convergence and divergence in colour polymorphism, correlated traits and associated evolutionary processes. Herein, we synthesise the extensive literature on lizard colour polymorphisms in both sexes, including recent advances in understanding of the underlying biochemical, cellular and genetic mechanisms, and correlated behavioural, physiological and life-history traits. Male throat, head or ventral colour morphs generally consist of red/orange, yellow and white/blue morphs, and sometimes mixed morphs with combinations of two colours. Despite these convergent phenotypes, there is marked divergence in correlated behavioural, physiological and life-history traits. We discuss the need for coherence in morph classification, particularly in relation to ‘mixed’ morphs. We highlight future research directions such as the genetic basis of convergent phenotypes and the role of environmental variation in the maintenance of polymorphism. Research in this very active field promises to continue to provide novel insights with broad significance to evolutionary biologists.     

Dietary modulation of the mucosal immune response to a parasite

This study investigated the hypothesis that dietary deficiency of readily available carbohydrate (raCHO) modifies the immune response of lambs to the gastrointestinal nematode Trichostrongylus colubriformis. Sixty helminthologically naive Merino lambs were fed throughout the experiment diets containing three levels of raCHO that provided adequate, moderate or low intakes according to recommended standards and were given primary or both primary and secondary infections of T. colubriformis. A further 20 uninfected lambs received the low diet for 9 weeks, after which they were returned to the standard diet. Immune status was assessed by the measurement of plasma and jejunal antibody concentrations and blood and jejunal cell numbers and function. Diets low in carbohydrate resulted in a failure of the lambs to gain weight and decreases in plasma glucose concentration, blood lymphocytes expressing CD8 or Tcrgammadelta, monocytes, eosinophils, platelets and red blood cells, jejunal and plasma antibody concentrations, lymphocyte proliferation to worm antigen and numbers of jejunal CD8(+) and Tcrdeltagamma(+) lymphocytes, eosinophils and CD1b(+) dendritic cells. Thus, a low dietary concentration of raCHOs impaired the constitutive availability of lymphocytes and antigen-presenting cells, and the cellular and humoral immunological responses. A hypothesis is suggested for the mechanism and for the possible wider implications.     

The insect cellular immune response

The innate immune system of insects is divided into humoral defenses that include the production of soluble effector molecules and cellular defenses like phagocytosis and encapsulation that are mediated by hemocytes. This review summarizes current understand- ing of the cellular immune response. Insects produce several terminally differentiated types of hemocytes that are distinguished by morphology, molecular and antigenic markers, and function. The differentiated hemocytes that circulate in larval or nymphal stage insects arise from two sources： progenitor cells produced during embryogenesis and mesodermally derived hematopoietic organs. Regulation of hematopoiesis and hemocyte differentiation also involves several different signaling pathways. Phagocytosis and encapsulation require that hemocytes first recognize a given target as foreign followed by activation of downstream signaling and effector responses. A number of humoral and cellular receptors have been identified that recognize different microbes and multicellular parasites. In turn, activation of these receptors stimulates a number of signaling pathways that regulate different hemocyte functions. Recent studies also identify hemocytes as important sources Of a number of humoral effector molecules required for killing different foreign invaders.     

Seasonal changes in a ultraviolet structural colour signal in blue tits, Parus caeruleus

Recent studies of blue tits, Parus caeruleus , have found sexual selection and a viability-indicating function of the structural ultraviolet and blue crown plumage, but the reasons for this signal variation are not understood. Furthermore, studies in England and Sweden have yielded somewhat different results (particularly with regard to the spectral position of the reflectance peak). Here we investigate whether the blue tit UV/blue ornament varies with time of year since such variation might be relevant to the signalling function as well as the apparent difference between populations. From 400 blue tits captured at two different localities in Sweden, we found that objective measures of 'hue' (spectral location), 'chroma' (spectral purity) and 'brightness' (spectral intensity), varied substantially with season. Just after moult (October), crown 'hue' is maximally UV-shifted (359 nm for males and 373 nm for females). Thereafter the peak drifts upwards and by the time of nestling feeding (June) male reflectance peaks at 404 nm and female at 413 nm. This change is probably due to feather wear as well as fat and dirt accumulation, which might constitute an additional male quality cue. Our results suggest that it is important to consider plumage age when exploring variation in structural plumage coloration, and that it can largely explain the difference between the British and Swedish studies. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 237–245.     

Environment-contingent sexual selection in a colour polymorphic fish

Sexual selection could be a driving force in the maintenance of intraspecific variation, but supporting observations from nature are limited. Here, we test the hypothesis that spatial heterogeneity of the visual environment can influence sexual selection on colourful male secondary traits such that selective advantage is environment contingent. Using a small fish endemic to Sulawesi, Indonesia (Telmatherina sarasinorum) that has five male colour morphs varying in frequency between two visually distinct mating habitats, we used direct behavioural observations to test the environment-contingent selection hypothesis. These observations were combined with measurements of the visual environment, fish coloration and the sensitivity of visual photopigments to determine whether differential morph conspicuousness was associated with reproductive success across habitats. We found that blue and yellow males are most conspicuous in different habitats, where they also have the highest reproductive fitness. A less conspicuous grey morph also gained high reproductive success in both habitats, raising the possibility that alternative behaviours may also contribute to reproductive success. In a comprehensive analysis, conspicuousness was strongly correlated with reproductive success across morphs and environments. Our results suggest an important role for spatially heterogeneous environments in the maintenance of male colour polymorphism.     

Seasonal changes in human immune responses to malaria

Cellular as well as humorol immune responses to malaria antigens fluctuate in time in individuals living in molono-endemic areas, particularly where malaria transmission is seasonal. The most pronounced changes are seen in association with clinical attacks, but osymptomatic infection can also lead to apparent immune depression. However, recent data have shown that seasonal variation in cellular immune responses may occur even in the absence of detectable porositaemia. Here, Lars Hviid and Thor G. Theonder review the seasonal variation in human immune responses to malaria, and discuss its possible causes and implications.     

Reproductive effort reduces specific immune response and parasite resistance

If a trade-off exists between reproductive effort and immune function, life-history decisions may have important implications for parasite resistance. Here, we report effects of experimental manipulation of reproductive effort on subsequent specific immune function and parasite resistance in the collared flycatcher, Ficedula albicollis. Our results show that increased reproductive effort of females immunized with Newcastle disease virus (NDV) vaccine negatively affected the ability to respond with NDV-specific antibodies. We further show that increased reproductive effort increased the intensity of Haemoproteus infections and that such infections are associated with higher mortality. Our results thus provide support for the hypothesis that immune suppression caused by reproductive effort may be an important mechanism mediating the life-history cost of reproduction.     

Territory acquisition in a polymorphic lizard: An experimental approach

Males of the colour polymorphic Australian painted dragon lizard Ctenophorus pictus occur in red or yellow head colouration. In a previous experiment, we showed that red is associated with a higher probability of winning staged contests for resources (females or space) and that manipulation of male colouration by painting males in the opposing morph changed the dynamics of staged interactions by prolonging them 30‐fold. Thus, colour is linked to behavioural differences between males and is involved in information transfer between competing males. This inherent red dominance could result in yellow males being marginalized to poorer quality territories in terms of access to females, food, perch sites or shade. With an experiment in the wild, we test to what extent this prediction is upheld, and how colour manipulation affects morph‐specific success in territory acquisition when male body size, territory quality and emergence time from hibernation are controlled through manipulation or randomization. There was no significant effect of colour category per se, although on average red males remained closer to the release sites (our proxy for territory acquisition ability) than yellow males and artificially altered morphs moved the furthest away. There was a significant interaction effect between colour category and experimental release position, which may be linked to differences in how exposed (or not) these positions were and morph‐specific ability to cope with such exposure (e.g. ‘boldness’). Our data show that territory acquisition success is not merely a function of competitive ability but a composite outcome of a suite of factors, including signal perception.