Adaptations to Southern Conditions
The CANCOR analysis revealed a southern range of Sitka that contains loci that correlate strongly with increases in height. From previous studies, Sitka spruce has been shown to recolonise, post glaciation, from Kodiak Island, Haidi Gwaii and the southern range (Byrne et al., 2022). Here we have identified southern range specific adaptions that allow for optimal growth in this range. This southern range has implications for the adaptive evolution of Sitka, showing that southern recolonisation led to adaptive loci which spread through the populations to the 50th latitude. The traits identified from the co-association study show that cluster 3 can be strongly linked to good growing conditions, with cluster 1 being mildly linked. Minimum, maximum and mean solar radiation are all associated with loci involved with increased height. This is common as an increase in solar radiation allows for increased tree growth (Strand et al., 2006). Increases in the diurnal range allow greater amounts of solar radiation, increasing growth. The temperate climate of the southern range is optimal for growth, with freezing temperatures causing growth cessation, as seen in the northern range being less likely. Mean vapour is grouped with these traits as Sitka is well adapted to receiving moisture through sea fog. Forest fire risk is seen as linked to the same cluster of traits as it is based on the aridity index. This may indicate some adaptations in this loci cluster to aridity in these conditions. It may also be linked to forest fires themselves, as known adaptations in other conifers have been found. This would take additional phenotyping to elucidate. For a breeding context we have defined a cluster of loci that should be included for better growing trees in ideal conditions. We have also identified a range of trees that could be included in breeding programs to increase height of trees. These loci could be included through genomic selection models by adding weight to selected loci.
The methods employed here have allowed for the merging of phenotype, genotype and environmental data to elucidate the relationships between the three. Here we have grouped loci into three clusters which relate to wintering traits, like snowfall, and southern growth conditions which seem optimal for maximising tree size. We have traced the spread of these loci and discovered a north south divide on loci involved with height with the southern population having a higher proportion of loci positively correlated to height. Many of these adaptions have occurred recently and are not conserved throughout the population, as shown by the high MAF of adaptive loci compared to non-adaptive loci. We have also shown that subtle shifts in allele frequency between northern and southern populations are responsible for adaption to certain traits in Sitka spruce. With improvement of genome resources discovered loci can be anchored to chromosomes.