Understanding the genetic basis of adaptation is of primary interest in evolutionary biology. The classical theory of quantitative genetics predicts that adaptive variation is mostly based on a large number of loci with small effects (Fisher 1930). Studies in domesticated species have shown that few genes with major effects are responsible for the adaptation to human cultivation (Doebleyet al. 1990). But the situation in natural populations has been much less studied. Both the number of loci and the effects of the alleles are interesting evolutionary issues. It is also of interest to find out whether the same loci are responsible for the adaptive differences between different pairs of populations.
Northern environments are characterized by steep latitudinal gradients in the length of growing season. In these gradients we expect genetic variation and evolution of phenotypic traits that are responsible for adaptation to these environments such as flowering time, bud-setting and frost-hardiness. The species currently studied in the laboratory are Scots Pine (Pinus sylvestris),Arabidopsis thalianaandArabidopsis lyrata.
We are studying the genetic basis of adaptation in Scots pine,Pinus sylvestris. Scots pine has a large distribution and it is locally adapted to different photoperiod and temperature conditions. A steep cline in the timing of budset and cold hardiness is observed that extends from northern to southern areas of the distribution. QTL mapping between northern and southern Finnish populations has demonstrated several genomic areas for cold tolerance and timing of budset (Hurmeet al. 2000). Most neutral molecular markers do not show clinal variation, but have similar allelic frequencies across the range. Against this homogeneous background we expect that the loci governing the climatic adaptation will be discernible by their higher differentiation.
We have characterized the quantitative genetics of variation of timing of budset and cold tolerance, and measured the strength of natural selection. Next step is conducting association studies of individual polymorphisms at candidate loci. These SNPs (Single Nucleotide Polymorphisms) are chosen based on the studies of candidate gene variation (below). Also SNPs from other genomic areas are included. Association studies are made in collaboration with the Finnish Forest Research Institute, in the confines of two EU projects (EVOLTREE, NOVELTREE) (Sonja Kujala, Timo Knürr, Outi Savolainen, Katri Kärkkäinen from Finnish Forest Research Institute and Mikko Sillanpää from the University of Helsinki).
We are studying sequence variation at candidate genes for cold hardiness and timing of growth to search for traces of natural selection (e.g. Wachowiak et al. submitted). We are also examining other parts of the genome of Scots pine for evidence of selection (Pyhäjärvi et al. submitted, Palmé et al. in press). Further, we are using nucleotide sequence variation to examine the demographic history of Scots pine (Pyhäjärvi et al. 2008).
Current work on sequence variation is extending the number and coverage of candidate loci (Sonja Kujala, Janita Ranta and Katja Salminen).
Arabidopsis lyrata is one of the closest relative of Arabidopsis thaliana, but it is an outcrossing species. Arabidopsis lyratacomprises a number of subspecies including ssp. lyrata distributed in North America and ssp. petraeawhich has a patchy distribution in Europe from latitude 36 N to 70 N. Complementary approaches are combined to understand the natural variation for flowering time in this species: ecological field studies, quantitative genetics, QTL mapping, molecular population genetics, and molecular biology. They provide a description of the natural variation, its interaction with environmental factors such as photoperiod or winter (exposure to low temperatures), and an understanding of its genetic basis at the molecular level (genes involved and their effects). Importantly, the genome of the species has just been sequenced.
We are particularly interested in describing inter- and intrapopulation variation in flowering time in response to day length and vernalization. Flowering time of Arabidopsis lyrata varies with latitude so that southern populations flower earlier than northern populations. Environmental conditions - photoperiod and vernalization (cold treatment) - affect the probability and the time to flowering (Riihimäki and Savolainen 2004). We have also demonstrated that the FRI gene has an important role in governing flowering time variation in A. lyrata even if the patterns of variation differ very much from those in A. thaliana. In collaboration with Swedish researchers from Uppsala University we have also examined patterns of natural selection for flowering time in different populations (Sandring et al. 2007). Currently field studies are going on the genetics of adaptive variation with the aim to map fitness QTLs in natural populations (Päivi Leinonen and Elisa Vilhunen).
The genetic basis of quantitative variation in natural populations is largely unknown. With QTL mapping one can obtain information on the number of loci and the distribution of the allelic effects. We are currently conducting QTL-mapping experiments using a cross between two A. lyrata populations that have different flowering times, different trichome numbers, or other interesting phenotypic traits (Helmi Kuittinen, Päivi Leinonen and Johanna Leppälä). This and related projects involve a collaboration with David Remington from University of North Carolina at Greensboro.