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.