The genetic architecture of a reproductive life-history trait in a wild passerine

Understanding the capacity for species to respond to changes in their environment and the rate at which they are able to do so is a key topic in evolutionary biology and of increasing importance in wildlife conservation and management. However the mechanisms involved in mediating these responses are poorly understood. Specifically, while reactive responses may be advantageous in the short term persistent directional changes in environmental conditions may require a more profound response in order for organisms to adapt and persist successfully. Here I use data from two long-term studies of the great tit Parus major and apply a range of statistical techniques to dissect the genetic architecture of laying date, a reproductive life-history trait, to discern the extent to which a genetic component of variation contributes to observed phenotypic variation. A heritable component of variation exists in both populations, but specific regions of the genome contributing to trait variation could not be detected by quantitative trait loci mapping (Chapters 2 & 3), genome-wide association (Chapters 2 & 3) or chromosome partitioning (Chapter 4) analyses. These findings are consistent with a highly polygenic basis for variation in laying date, variation maintained by many genes of small effect. Attempts to increase the statistical power by combining two phenotypic datasets to increase overall sample size (Chapter 3) and increasing marker density (Chapter 5) drew similar conclusions, with an absence of genome-wide significant QTL. Despite evidence of a strong association on chromosome 3 (Chapter 5), an overall lack of consistency between analyses and datasets on regions exhibiting the highest associations suggests that power to detect genomic regions, particularly when variation may be determined by many variants of small effect, is low. I conclude that while genetic variation exists, environmental factors and phenotypic plasticity likely account for much of the variation in laying date.