Phylogeography of Icelandic Agrostis (Poaceae)

Geothermal areas create living conditions strikingly different from their surroundings, providing opportunities for studying organisms thermal adaptations and responses to climate warming. However, the origins of geothermal organisms are poorly known, limiting our ability to design and interpret experimental studies. To address this, we tested whether geothermal populations of Agrostis (Poaceae) in Iceland represent a single lineage (clade) and, if so, if that lineage originated in adjacent, non-thermal areas or elsewhere.We studied the phylogeography of Agrostis stolonifera and A. vinealis from geothermally heated and non-heated areas in Iceland, using whole plastome data and Bayesian and Likelihood phylogenetic analyses. Having identified an exclusively geothermal clade for A. stolonifera, we performed a common garden experiment to test for heritable phenotypic differences between geothermal and non-thermal plants.At least two lineages of A. stolonifera have colonised Iceland independently, one being exclusively geothermal and the other exclusively non-thermal. The geothermal lineage is phylogenetically isolated and its geographical origin could not be determined. Grown in a common garden, geothermal A. stolonifera produced significantly shorter stolons than non-thermal plants. In contrast, geothermal A. vinealis did not form a clade. Instead, all Icelandic A. vinealis formed a clade with non-thermal A. vinealis from outside of Iceland, plus geothermal A. rossiae and A. scabra.We demonstrate the existence of an exclusively geothermal lineage of A. stolonifera, which also shows heritable phenotypic differences, but not of A. vinealis. These contrasting findings possibly reflect different growth forms and life histories. Our results provide a phylogenetic hypothesis to be tested with nuclear data and a framework for future experimental studies, highlighting the importance of a historical perspective for understanding the peculiar plant communities that thrive in geothermal areas. Finally, our results suggest A. vinealis is in need of taxonomic revision.