Gene flow and local adaptation along elevation gradients in an invasive plant (Solidago canadensis)
Solidago canadensis was introduced to Europe from North America in the 18th century, and has since become invasive. In Switzerland, it is common in disturbed habitats at low elevation, but is rarely found above 1200 m. We combined population genetics analyses with a common garden experiment to investigate a) whether the genetic diversity of Swiss Solidago populations could be explained by landscape structure (eg. human or road density) b) whether genetically diverse populations perform better in marginal environments and c) whether there is evidence of local adaptation along climate-elevation gradients to local conditions. To investigate genetic diversity and isolation by distance, we genotyped individuals from 44 populations at nuclear microsatellite and chloroplast loci. We then planted replicate genotypes (clones) from 13 populations at 650 m, 1250 m, and 1680 m and measured their growth, phenology, and survival.
Solidago populations exhibited weak isolation by distance at neutral loci, and were generally not well differentiated by allele frequency, suggesting extensive dispersal. Patterns of genetic diversity could not be explained by elevation or by human or road density. Within the common gardens, there was no evidence of heterozygote advantage. In the first year, survival was higher at higher elevations, but only plants in the lowest garden produced seed before winter, suggesting that reproductive failure may limit spread above 1200 m. However, plants from colder home locations tended to grow faster at the highest site, and sometimes produced flower heads, while those from warmer sites did not, suggesting that such population have adapted to colder conditions despite likely high gene flow across elevation gradients. Even within populations, clones differed in their performance. High relative growth rate at 650 m predicted high growth at the other sites, but flower production was only correlated between the 1250 and 1680 m sites – again suggesting that there are particular genotypes that perform better in cold conditions. This has implications for future spread, as edge populations (the most likely source of colonists) contain more cold-adapted individuals than do low-elevation populations.