Genetics of complex traits
Natural selection act to differentiate populations for adaptive traits even in the presence of substantial migration. A classic example in forest trees is adaptations to the steep latitudinal gradient in the length of growing season that characterizes northern environments. Trees often show latitudinal clines in important traits such as the timing of bud set, the onset of flowering and frost hardening even through population differentiation at neutral molecular markers is generally low. Comparisons between genetic differentiation at quantitative traits, measured as QST, and genetic differentiation at neutral marker loci, measured as FST, can thus yield important insight into which quantitative traits that are under diversifying selection.
It is less clear how patterns of genetic differentiation at the loci directly controlling the quantitative trait (quantitative trait loci or QTLs) are affected by natural selection. Theory models suggest that genetic differentiation at QTLs is better predicted by FST that QST, because selection generates covariance between individual QTLs that act to reinforce the total effect of the QTLs. Diversifying selection will also result in substantial heterogeneity among loci, with a few QTLs showing strong allelic differentiation among populations while the bulk of QTLs appear no more differentiated than neutral markers.
Actual patterns of differentiation at quantitative traits, QTLs and neutral markers is largely unknown, and few studies have to date examined genetic differentiation across all three types of markers. Variation in several phenology traits, such as the timing of bud flush, bud set, leaf senescence in the fall and frost tolerance are known to have a large heritable components. Variation among populations in the timing of these traits also shows excellent correlation with latitude, suggesting that different populations are locally adapted to the seasonal variation at their latitude of origin. We are investigating the genetic basis of several phenology traits that are responsible for climatic adaptations in European aspen (Populus tremula). We have established two common gardens of P. tremula clones collected from twelve populations across Sweden (the SwAsp collection). We are currently investigating levels of nucleotide polymorphism and linkage disequilibrium in trees from the SwAsp collection to infer patterns of haplotype structure along a latitudinal gradient across Sweden. We are also using association mapping (LD mapping) to dissect the genetic architecture of quantitative traits and to link variation in putative candidate genes to phenotypic variation in several phenology traits.
Ingvarsson, P.K., Garcia, M.V., .Luquez, V, Hall, D. and Jansson, S. (2008) Nucleotide polymorphism and phenotypic associations within and around the phytochromeB2 locus in European aspen (Populus tremula, Salicaeae). Genetics 178, 2217-2226.
Luquez, V., Hall, D., Albrectsen, B., Karlsson, J., Ingvarsson, P. K., and Jansson, S. (2007) Natural Phenological Variation in Aspen (Populus tremula): The Swedish Aspen Collection. Tree Genetics and Genomes, 4, 279-292.
Hall, D., Luquez, V., St. Onge, K.R., Garcia, M.V., Jansson, S. and Ingvarsson, P.K. (2007) Adaptive population differentiation in bud phenology across a latitudinal gradient in European aspen (Populus tremula, L., Salicaeae): a comparison of neutral markers, candidate genes and quantitative traits. Evolution, 61, 2849-2860.
Ingvarsson, P. K., Garcia, M. V., Hall, D., Luquez, V. and Jansson, S. 2006. Clinal variation in phyB2, a candidate gene for day-length induced growth cessation and bud set, across a latitudinal gradient in European aspen (Populus tremula). Genetics, 172, 1845-1855.
Ingvarsson, P. K. 2005. Nucleotide polymorphism and linkage disequilibrium within and among natural populations of European aspen (Populus tremula L., Salicaceae). Genetics, 169, 945-953.