Above: A. arenosa plants growing on scree in the upper Danube Valley in Southwestern Germany.
Other types of local adaptation
Most populations of A. arenosa are found in sheltered sites in hills or mountains that are generally well-watered and shaded. Some populations, however, have colonized railways. From our as yet limited sampling of railway plants, it seems the most abundant railway lineage likely arose just once. This is based on the observation that populations sampled in Sweden, Southwestern Germany, and Central Poland are all extremely closely related. We discovered previously that the railway lineage shows constitutive expression of genes associated with flowering, as well as heat shock and cold tolerance that are all inducible by cold in the mountain lineage. Railway plants are accordingly early flowering, and have undetectable levels of expression of FLC, a key floral repressor. We scanned the genomes of plants from one railway population, contrasting to a mountain lineage, to ask whether any genes showed evidence of selection associated with local adaptation. From this, we obtained 14 genes, but one is a particularly good candidate, based on its known targeting of many of the genes we see differentially expressed between mountain and railway types when grown in common garden: LHY. This gene has 18 polymorphisms that are at higher frequency in the railway population than the mountain population, nine of which encode amino acid changes; all are clustered tightly in one small region of the gene. Thus we hypothesize that these mutations may alter the regulation of downstream cold and heat responsive genes, but perhaps do not affect the central role of LHY in regulating the circadian clock. Alternatively, it may be that the clock itself is adjusted to tune gene expression more appropriately to the locally hotter and drier habitat found in railway settings.
Berchtesgaden: We have identified one isolated perhaps independent railway colonization that has a distinct phenotype with respect to flowering gene expression (high expression of FLC, despite early flowering). This population is clearly a hybrid between the abundant railway type, and the adjacent mountain type. How it became early flowering despite its high FLC expression, however, remains mysterious.
Gulsen: Another early flowering A. arenosa population was isolated from a serpentine site in Austria. These plants are phenotypically indistinguishable (with respect to flowering) from the railway lineage, but genetically they are clearly a mountain lineage related to plants from other mountain sites around them. How Gulsen plants became early flowering remains unknown, and their FLC expression status is also unknown.
Key questions moving forward:
- Is LHY responsible for phenotypic differences between weedy and mountain arenosa types? And is this effect specific to the cold and heat response or does it affect circadian regulation as well?
- What causes the independent evolution of early flowering in Berchtesgaden and Gulsen populations?
- Did hybridization play a role in adaptation to railways in the case of the Berchtesgaden population? And if so, to what extent? All traits or just some?
Below: The "Zinnen" or Battlements, one of our A. arenosa sites in the Upper Danube Valley, Germany.