Host-Parasite Interactions in Natural Populations

Sammanfattning: Parasitism is one of the most common ways of living and it has arised in many taxa. Parasites feed and live inside or on their hosts resulting in both long and short term consequences for the host. This thesis is exploring the phenotypic and genotypic effects of animals living with parasitic infections. I have been studying three different parasite groups and their associated host species: the great snipe, a lekking freshwater wader bird that migrates between Africa and Northern Europe; the tree sparrow, a stationary passerine found close to human settlements and lastly the water vole, a large rodent living in riparian habitats.Avian malaria is one of the most commonly studied parasites affecting birds. Atoxoplasma, an intestinal protozoan parasite is less studied but is thought to be endemic in free-ranging birds. Given the freshwater habitat great snipes inhabit, a prevalence of 30% avian malaria infections is not high and that the prevalence fluctuated among years. Sequencing of the avian malaria cytochrome b gene revealed that parasites are similar to avian malaria parasites found in African birds suggesting that they were infected on the wintering grounds in Africa. Tree sparrows had few malaria infected individuals, a result that is consistent with other studies of stationary birds at high latitudes. Atoxoplasma infections were common in tree sparrows and capture-recapture analyses show decreased survival in infected compared to uninfected birds and signs of lower mating success among infected.Genetic analyses comparing the transcriptome between mated and unmated great snipe males revealed that the genotype is important for mating success and health status for some of the expressed genes. That variations in some of these genes are involved in maintaining a good health status and mating success supports handicap models for sexual selection in this lek mating system.The major histocompatibility complex (MHC) is a part of the immune system and it contains genes involved in immune response. In water voles, a number of new MHC alleles were identified. Based on their in silico phenotype they were grouped into supertypes to facilitate studies on how helminth infections affect the MHC diversity in the water voles. Some of these MHC supertypes provided resistance to one helminth species, but the same supertype caused the opposite effect for other helminth parasites. Overall, parasites are a driving force for maintaining genetic diversity and parasite infections lowers survival rate, which would lead to a lower lifetime breeding success.