The MHC genes: variation and impact on life-history traits in house sparrwos
Sammanfattning: In birds the major histocompatibility complex (MHC) organization differs both among and within orders. Galliformes often have simple arrangements with few transcribed MHC genes whilst many songbirds (Passeriformes) have a complex arrangement with many MHC genes and also presence of pseudo-genes. In this thesis I have partly characterized MHC class I in house sparrows (Passer domesticus), sequencing the α1 to α3 domains. In sparrows alleles with a 6bp deletion form a distinct phylogenetic cluster in a tree based on the α1 and α2 domains. These alleles have many characteristics suggesting that they have a non-classical function (e.g. highly conserved amino acid positions were substituted compared with the other alleles, low nucleotide diversity and only a single site subject to positive selection). The alleles without a 6bp deletion show classical MHC characteristics (e.g. high nucleotide diversity and many positively selected sites). Sparrows can have at least four genes that hold alleles without the 6bp deletion and at least eight genes that hold alleles with the 6bp deletion. To understand more of the function of these genes we analyzed binding properties using in-silico modeling. This modeling was done based on full-length transcripts (α1 to α3 domains) from three different passerine birds, the great reed warbler (Acrocephalus arundinaceus), the house sparrow and the tree sparrow (Passer montanus). The phylogenetic analysis revealed not only genetic differences but also trans-species evolution within the sparrows. The in silico peptide-binding predictions reveal separate functional clusters of MHC-I allomorphs based on their peptide-binding specificities. A handful of studies in birds have reported association between MHC and survival. In my study population of house sparrows I have investigated if MHC is important for nestling survival, juvenile survival and the probability to become recruited into the breeding population. My findings indicate that specific MHC alleles are more important for survival than the absolute number of alleles per se, suggesting that negative frequency dependent selection or fluctuating selection is more likely to maintain the MHC diversity than heterozygote advantage. In birds there have been reports indicating a role for that MHC may be involved in mate-choice. I have investigated whether the social mate-choice in house sparrows is MHC-based. Despite using several different approaches there was no evidence for an MHC based female mate-choice regarding MHC diversity in this house sparrow population. Investigating MHC genes in the wild is necessary for the understanding of which selection mechanisms that operates at MHC and how pathogens interact and shape the variation observed at MHC genes.
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