All plants studied in natural ecosystems are symbiotic with fungi (1), which obtain nutrients while either positively, negatively, or neutrally affecting host fitness (2). Plant adaptation to selective pressures is considered to be regulated by the plant genome (3). To test whether mutualistic fungi contribute to plant adaptation, we collected 200 Dichanthelium lanuginosum plants from geothermal soils at 10 sites in Lassen Volcanic (LVNP) and Yellowstone (YNP) National Parks. These soils have annual temperature fluctuations ranging from...