Trees know: winter is coming

Two related species of spruce share very similar genetic adaptations to their local conditions, according to this paper from scientists supported by the FORGER project. The team, led by Martin Lascoux of Uppsala University in Sweden, looked in detail at the genetic make-up of Siberian spruce (Picea obovata), a close relative of Norway spruce (P. abies). Indeed, one of the mutations associated with adaptation to climate is identical in the two species.
New growth, Picea abies, Western Alps, Italy. Photo: B.Vinceti
One of the ways to study the genetic basis of adaptation is to look at the differences between populations that are growing under different environmental conditions. This approach showed two genes that vary in populations of Norway spruce as one moves north in Scandinavia. One of the genes is part of the system that enables the plant to sense the length of light and dark, the photoperiod system. It’s easy to speculate that this could be linked to predicting the onset of winter. The other is part of the circadian clock machinery, which links to the photoperiod system but whose role is more difficult to guess.

In Scandinavia, the frequency of some alleles (different forms of a gene) of these two genes increases from south to north, while the date on which the tree stops growing and shuts down before winter hits hard becomes earlier and earlier. Trees in the south grow for a much longer period than trees in the north, so it is tempting to assume that the genetic differences are responsible for the observed changes in growth patterns. The trouble is that as you move from south to north in Scandinavia, and the frequency of these alleles change, you also move from one genetically distinct population of trees to others. The different populations arose when the trees expanded from the refugia in which they survived the cold of the most recent ice age. So although on the face of it the variation in the different genes could be the result of adaptation through selection, it could equally have already been different in the different ancestral populations, and play no part in adaptation to present-day conditions.

To overcome this possibility, Lascoux’s team studied samples of Siberian spruce taken from seven locations spread along about 1200 km of the Yenisei river, one of the massive Russian rivers that flow almost due north across Siberia to empty into the Arctic ocean. Looking at the genes from the seven locations along the river, the team first confirmed that the entire sample did indeed belong to one genetic population. This means that any difference that turns out to be associated with how far north the trees probably is a genetic adaptation.

To look for differences, the scientists carried out two sets of experiments. In one, seeds from the different locations were grown under tightly controlled conditions in a growth chamber. Each week, the seedlings were measured and samples collected for genetic analysis. And each week, the length of the dark night was increased by 90 minutes. Growth ceased after less than 20 days in the northernmost population, while seedlings from the southernmost populations kept growing for more than 60 days. In the other set of experiments seeds from the seven locations were taken to a greenhouse in Finland, where they experienced natural changes in the length of the night. Again, the expected differences manifested themselves; seeds from the north stopped growing after less than 70 days, while those from the south kept going for more than 90 days. In both sets of experiments the seedlings from the different locations were growing under identical conditions, so the differences among them show clearly that there is a genetic basis to when the tree shuts down for winter.

The specific genes associated with adaptation in Siberian spruce are the same as those in Norway spruce, notably one from the photoperiod system and one from the circadian clock. The photoperiod gene has a mutation strongly associated with latitude. This mutation increases the expression of the gene, a finding supported by the analysis of DNA from the sample seedlings. The circadian clock gene is in many respects more intriguing. This gene, called Gigantea, or GI, is found in many plant species and has an effect on a large range of physiological processes in the plant. In Siberian spruce there are very few mutations in GI and they are overwhelmingly non-synonymous, meaning that the mutation affects the protein for which the gene codes. But there is no association between different versions of the gene and latitude. The protein the GI gene codes for is different, and that difference has been strongly selected – presumably because it confers some kind of benefit – hence the lack of synonymous mutations.

One mutation of the GI gene is, in fact, identical in Norway spruce and Siberian spruce, but is not found in Alpine populations of Norway spruce nor in other spruce species studied to date. The suggestion is that it was present in the ancestral species shared by Norway and Siberian spruce, and has subsequently been lost from the Alpine populations, perhaps because it conveys no benefit there.

As foresters grapple with the need to conserve forest genetic resources in the face of climate change, this research will help them to understand the genetic basis of adaptation and also how population structure and adaptation affect one another.

Read the full article (requires a subscription to the journal)