Evolution of genetic diversity at two hypervariable markers: a microsatellite and the MHC

Abstract

Genetic variation is a prerequisite for evolution. The present thesis examines how genetic diversity arises and is maintained. The thesis can be divided into two subprojects, addressing neutral and functional genetic variation, respectively. In the first subproject, germline mutation patterns on the presumably non-functional microsatellite locus HrU10 were investigated. By sequencing observed mutations uncovered by microsatellite length typing, a significant association between microsatellite length and number of interrupting motifs (sequence motifs deviating from the repeated core unit) was found. Moreover, a majority of the mutations revealed homoplasic alleles. This result may imply a general caution in population genetic studies where genotypes are based on allele sizes only. In a comparative analysis of HrU10 mutations in six species of swallows, the following sex specific patterns in HrU10 mutability were found: (i) Mutations in long alleles were typically attributed to male germlines and mutations in short alleles were most pronounced in female germlines. (ii) Mutations in the male germlines were significantly biased for expansions while no such effects were uncovered in female germlines. These results indicate fundamental dimorphism in male and female gametogenesis with respect to slippage mutations. The aim of the second subproject was to study the polymorphism at genes of the Major Histocompatibility Complex (MHC). The gene products of the MHC are important for immune response and subsequently individual fitness. First, the polymorphism at exon 2 in bluethroat (Luscinia svecica) MHC class II genes was assessed. The results revealed at least eleven different loci, and the amino acids involved with antigen recognition were subjected to significant diversifying selection. MHC class IIB (MHCIIB) exon 2 sequences from the thrush nightingale (L. luscinia) were also analyzed for comparison. Phylogenetic analyses indicate an ancestral origin of some of the polymorphism uncovered at the MHCIIB exon 2. Secondly, a standardized multispecies genotyping strategy was developed to estimate MHC diversity at eight closely related passerine species with known levels of promiscuity. Interestingly, an association between MHCIIB exon 2 diversity and promiscuity levels was identified. This result may indicate that promiscuity in passerine birds is a mechanism creating MHC diversity, and consequently promoting offspring immunological quality.