Effects of inbreeding on individual levels of immunocompetence within bottlenecked bird populations that have been exposed to introduced pathogens

Inbreeding and loss of genetic diversity are believed to reduce the ability of natural populations to cope with disease, presenting important implications for the conservation of small, bottlenecked populations. This prediction has been shown experimentally in Drosophila populations (reduced resistance in inbred lines) and in house finches (genetically-diverse individuals showed stronger immune responses and less severe infection). However, few studies have identified the effect of inbreeding on immune function in free-living populations. A recent study demonstrated reduced immunocompetence in a severely bottlenecked robin population in comparison to their out-bred source. Whilst there is little doubt that inbred populations are more likely to be susceptible to disease, it is remains unclear where, when and to what extent this occurs within populations. This project will examine interactions between immune function and inbreeding in two bottlenecked bird species, where level of inbreeding and known extent of pathogenic infection vary between individuals. In avian field studies, immune function in birds has most commonly been measured using the Phytohaemaglutinin (PHA) skin test. PHA response has been shown to be significantly reduced in a severely bottlenecked robin population and to be significantly elevated in hybridised versus pure-bred parakeets. The technique has recently been histologically validated in birds. This project will measure individual levels of immunocompetence amongst two endemic bird species whose restored populations have been intensively-monitored for the past 25 years. The endangered Mauritius parakeet (Psittacula echo) and endangered Mauritius pink pigeon (Columba mayeri) have both experienced a severe population bottleneck over the past 30 years. Long-term management has produced detailed pedigree records and complete life histories for all individuals since the bottleneck. Both bottlenecked populations are infected with introduced pathogens. The Mauritius parakeet suffered a recent outbreak of Psittacene Beak and Feather Virus (PBFV). The pink pigeon is host to two blood pathogens; Trichomonas gallinae and Leucocytozoon marchouxi, whose prevalence has been intensively monitored. PBFV was introduced by feral ringneck parakeets and T. Gallinae/L. Marchouxi by feral ground dove populations on Mauritius. Genetic and pedigree information has confirmed considerable variation in inbreeding within each endangered bird population. Sympatric populations of the introduced Indian ringneck parakeet and barred ground dove will provide out-bred 'control' populations. Each population is closely monitored each year as part of the ongoing recovery programme. Field aviaries on Mauritius will enable replicate measurement of an individuals' immunocompetence across breeding and non-breeding seasons and provide estimates of heritability of immune response. Existing studies assume low immunocompetence to reflect reduced resistance to infection. However, on Mauritius, immunocompetence can be re-measured in free-living infected birds to evaluate change in immune function as infection progresses. The project will use confirmed pedigrees to identify individual inbreeding coefficients; validate measures of immunocompetence for known inbreeding coefficients and observed level of infection in the free-living populations to identify interactions between inbreeding and immune function. Genetically confirmed pedigrees already exist for the Mauritius parakeet population, and pigeon pedigrees will be confirmed using archived samples. Immunocompetence will be measured in ~180 birds from each of the four populations using the PHA skin test and hemolysis-haemagglutination assays. PHA methods will be calibrated in captivity before use on free-living populations. General linear models will identify effects of inbreeding on immunocompetence from effects of inbreeding on immunocompetence from effects of season/life-history.