Exploring the causes of heterozygosity-fitness correlations in the blue tit

Abstract

For this thesis I studied heterozygosity-fitness correlations (HFC) in a natural population of a passerine bird, the blue tit (Cyanistes caeruleus), that was carefully monitored during nine years. In a first step, I characterized a set of at least 95 polymorphic microsatellite loci with utility for population genetic studies in the blue tit (Chapter 1). I classified the markers as presumably functional or neutral by considering whether the genomic region where the markers are located is transcribed to RNA and therefore expressed. This classification captured differences in the characteristics of the loci (e.g. number of alleles, expected heterozygosity, distance to genes) that would be expected to occur between markers with genuine differences in functionality (Chapter 2). In a first approach for investigating HFC in the blue tit population, I used a sample of breeding birds with varying levels of inbreeding. With this approach, I found positive correlations between multilocus heterozygsity at neutral markers and three reproductive and one survival traits (Chapters 2 and 3). Supporting a role of inbreeding on the detected HFC, I documented 1) some consanguineous matings, 2) a particular type of population structure that can contribute to the occurrence of inbreeding events, and 3) identity disequilibrium (i.e. the correlation between heterozygosity or homozygosity across loci within individuals, caused by inbreeding) with all markers and with the group of neutral loci (Chapter 2). In a second approach for investigating HFC, I controlled for the level of inbreeding by using groups of full-siblings (Chapter 3). In this case, the correlations between multilocus heterozygosity and two estimates of early survival were detected only with the group of functional markers and were negative. These correlations are better explained by localized effects at single loci. Generally, heterozygosity at single loci did not show disproportionately large and consistent effects on the phenotypic traits considered. The results of this thesis highlight the importance of considering the putative functionality of the markers when interpreting the results of HFC studies, the use and limitations of HFC to reveal genotype – phenotype associations, and the value of a thorough consideration of the population processes causing HFC.