Phylogeography of Icelandic Agrostis (Poaceae) Reveals a New Geothermal Lineage

Premise of research. 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, whether that lineage originated in adjacent, nonthermal areas or elsewhere. Methodology. We studied the phylogeography of A. stolonifera and A. vinealis from geothermally heated and nonheated 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 nonthermal plants. Pivotal results. At least two lineages of A. stolonifera have colonized Iceland independently, one being exclusively geothermal and the other exclusively nonthermal. 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 nonthermal plants. In contrast, geothermal A. vinealis did not form a clade. Instead, all Icelandic A. vinealis formed a clade with nonthermal A. vinealis from outside of Iceland, plus geothermal A. rossiae and A. scabra. Conclusions. 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.