The genetic composition of thale cress populations changes within just a few years under the influence of climate. A large-scale field experiment shows that plants can adapt rapidly – but also that this adaptation has its limits.
Climate change can be read directly in the DNA of plants: in a global field experiment, the genetic composition of thale cress populations shifted within a few years towards variants better adapted to local climate conditions. This is the finding of a large-scale study, led in part by Dutch evolutionary biologist Niek Scheepens (Goethe University Frankfurt), in which researchers followed populations at dozens of sites across the Northern Hemisphere (Science, 26 March).
Botanical Gardens
In that experiment, biologists sowed identical seed mixtures of Arabidopsis thaliana – collected from 231 natural populations – at a range of sites, from the subtropics to near the Arctic Circle. Experimental plots were also established at the Botanical Gardens of Utrecht University, where molecular plant physiologist Martijn van Zanten and bioinformatician Basten Snoek carried out the fieldwork for the international consortium. By sampling flowering plants every two weeks and sequencing their genomes, the researchers tracked precisely which genetic variants increased or decreased in frequency.
According to Van Zanten, the strength of the study lies in that combination of scale and detail. ‘Around 70,000 individuals were sampled over three years’, he says. ‘The sheer volume of data is already unique, but you can also see that selection occurs very consistently under comparable climate conditions.’
Patterns
That consistency gives rise to parallel patterns. Not only were similar changes in genetic composition observed between replicates within a single site, but also between sites with comparable climates. In Spain and Greece, for example, the same genetic variants persisted over time. In more northerly areas, the genetic composition was likewise similar between sites, but the northern sites differed markedly from those in more southerly regions. Temperature in particular appears to be an important selective pressure.
The optimal conditions for the rapidly evolved plants turn out to correspond to a climate approximately 1.5 degrees cooler than the current climate at their sites of origin. ‘That optimum now lies in a different place from where those Arabidopsis variants originally came from’, says Van Zanten. ‘That difference corresponds to current climate change, and makes its effect very tangible.’
Limits
The genetic shifts were observed within just three generations. According to Van Zanten, this shows that natural selection can act rapidly, provided there is sufficient genetic variation in the founding population. At the same time, this adaptation has its limits: at the warmest sites, genetic change was initially random and populations subsequently went extinct, presumably because the selective pressure was too great to allow rapid adaptation under extreme conditions.
A step further
The study builds on earlier analyses based on distribution data, but takes an important step beyond them, according to Wieger Wamelink – an ecologist at Wageningen University who was not involved in the research. ‘These are genuine experiments, not conclusions drawn from field distribution data’, he says. ‘They show very clearly that genetic variation is being drawn upon and shifts under the influence of climate.’
According to Wamelink, that genetic variation is crucial for the resilience of species. ‘If it diminishes, you lose flexibility. You become less able to respond to change.’ At the same time, Wamelink adds a caveat to the interpretation of the results. ‘I would not use the word evolution here. What they are showing above all is that something is shifting within existing variation, not that new genetic changes are arising.’
Van Zanten points out that this study is indeed about shifts in genetic composition, but that these occur in a highly consistent and even predictable manner. ‘Whether the shifts are truly adaptive has yet to be established. In follow-up experiments we are currently testing whether and how the shifts increase fitness.’
Migration
Wamelink also emphasises that the experimental set-up is more favourable than real-world conditions. In the field, populations typically have less genetic variation and migration is limited. ‘That genetic information has to reach the right place at the right time. That is not a given.’
Van Zanten acknowledges that point: in the experiment, the populations started with an exceptionally broad genetic mix. ‘We introduced an above-average level of genetic variation into the field here. Whether natural populations of, for example, endangered plants can adapt as rapidly as our experimental Arabidopsis populations will have to emerge from follow-up research.’