‘Out of Iberia’: How Bristol rock cress got to where it is today

While conservation of animal distribution and biodiversity is a topic of much popular and scientific attention, conservation of plant diversity is comparatively underappreciated. Key to conserving the diversity and distribution of plants on the planet today is understanding how this distribution arose in the first place. One of the key historical events likely to have had a significant impact on modern plant distribution in Europe is glaciation, which last peaked around 20,000 to 26,000 years ago. A key question when trying to understand how many European plants acquired the distribution and diversity they currently have is how did this and other glaciations affect their distribution? Did plants somehow persist in areas of extensive glaciation to give rise to the populations present today, or did areas spared glaciation (known as refugia) provide sources for subsequent population spread? In a recent Annals of Botany paper, Marcus Koch and colleagues from Heidelberg University examine the evolutionary history of Arabis scabra (Bristol rock cress), a plant with a peculiar distribution pattern, to see what light it can shed on how plants reached the distribution and diversity they have today.

A.scabra is found in Iberian mountain ranges and also in mountainous areas of France. Interestingly, there is also a small existing population around the Bristol area in the UK including in the Avon Gorge. To understand how this disjunct distribution pattern arose, the authors used DNA sequences from herbaria specimens collection across its distribution range. The first interesting finding that arose from this is that around 10% of the specimens they investigated turned out to not be A.scabra at all, but were actually other Arabis species. This highlights an increasingly appreciated point that many herbaria specimens may not actually be what they are annotated as, and that we are only beginning to appreciate the possible extent of this in the light of now widely-available DNA sequencing technology. By comparing the DNA sequences obtained from the A. scabra specimens, Koch and colleagues find that some of the genetic diversity between the different specimens from different locations is relatively low in the grand scheme of things. This is consistent with population isolation during the last glaciation period and subsequent spreading from refugia.

The question then arises where this refuge area was. The sequence data collected by Koch and colleagues support that this was at least partially an area in the Iberian Peninsula, as the proportion of rare DNA sequences is highest in the samples obtained from this region. This and other data from the DNA sequences obtained allow the authors to arrive at the most likely scenario for how modern A.scabra distribution arose. This is that glaciation forced A.scabra distribution to be limited to areas of the Iberian Peninsula and France, with its distribution expanding after the end of the last glacial period including to the UK. Interestingly, Koch and colleagues find genetic diversity within the current Bristol population to be high, indicating either a larger past population that has subsequently shrunk, or multiple past colonisation events of the UK. They speculate that increasing temperatures after the last glaciation period may have been a factor that could shrink populations again after an initial period of expansion. The timelines involved in this study are also consistent with a period in which the Celtic Sea was dry, providing a possible crossing point to the UK for A.scabra before sea levels rose.

The ‘Out of Iberia’ hypothesis proposed by Koch and colleagues therefore manages to link genetic information from A.scabra specimens to both historic events and current distribution patterns. The authors note that the limestone-rich regions around Bristol are also home to other disjunctly-distributed plant species. Whether this hypothesis for how A.scabra distribution arose may apply to such other species too remains to be seen, but it does serve to highlight the likely high interest this region is of in conservation work to maintain plant biodiversity and distribution.