Plants have spread to almost everywhere in the world. As they disperse, they meet many different environments to which they may be able to adapt. For a plant species to adapt to a new environment, genetic variation is needed. The individuals differ from each other in their genetic composition, which often means differences in phenotypes. Those individuals that manage to reproduce will form the next generation. With different conditions in different environments, it will not be the same phenotypes that reproduce everywhere. In that way, plant species will form into a mosaic of locally adapted populations varying genetically as the species disperses.
After the last ice age plants have started to disperse away from the equators. With increasing latitudes come increasing challenges to migrating plants. As plant species disperse northwards along this gradient of varying conditions individuals are selected for cold adaptive traits like flowering time and freezing tolerance, acquired by cold acclimation. In this way, genetic variation from the original populations for these traits becomes sorted out along a latitudinal cline.
The aim of this thesis was to understand how selection along a latitudinal gradient has shaped natural variation in cold adaptive traits in plants dispersing northwards, and specifically, to investigate what variation can be observed in phenotypes for these traits and how these traits correlate with genetic variation in genes known to be involved in cold acclimation.
In this study significant variation was found in a sample of the model plan Arabidopsis thaliana accessions in cold adaptive traits flowering time and freezing tolerance. A clear latitudinal cline in the cold adaptive traits freezing tolerance for A. thaliana was observed.
Analysis of nucleotide polymorphism for the cold responsive ICE1 (inducer of CBF expression 1) transcription factor revealed a haplotype structure with two allelic clades as well as unusually high levels of synonymous polymorphism.
Nucleotide polymorphism analysis for the transcription factors CBF1, CBF2 and CBF3 (C-repeat binding factors) that play a key role in regulating the expression of a group of target genes known as the “CBF regulon” showed a distinct geographical haplotype structure. One haplotype was dominant in southern accessions while in the other northern accessions overrepresented. There was a significant effect of CBF haplotype on both freezing tolerance and flowering time even after correcting for latitude.
Significant differences in CBF expression levels were found between the different CBF genes as well as between different accessions. Sequence variation at CBF was shown to have a significant effect on expression levels of CBF2. No clear correlations were found between CBF gene expression and freezing tolerance or temperature sensitivity for any of the accessions used in the study. This highlights the complex relationship between sequence variation in candidate genes and gene expression, and the problems associated with unraveling the genetic basis of ecologically important traits.