Ecology and evolution of geothermally adapted Agrostis (bentgrass) of North America and the Kamchatka Peninsula.

摘要

Agrostis species have been known to rapidly evolve ecotypes in response to geographically localized variations in soil chemistry. Two Agrostis taxa occur in the geothermal habitats of Yellowstone National Park: Agrostis rossiae is endemic to Yellowstone, and Agrostis scabra occurs both in the thermal areas and in a variety of non-thermal habitats. I noticed that every thermal population of Agrostis is surrounded by a non-thermal population of A. scabra that is reproductively isolated from the thermal area by its later flowering time. This suggested that ecotype evolution had happened twice, producing thermal A. scabra and A. rossiae in separate events. I used randomly amplified polymorphic DNA (RAPD) markers to resolve the historical relationships among the Yellowstone thermal populations, the non-thermal A. scabra populations, seven other non-thermal Agrostis species that occur in Wyoming, and thermal Agrostis populations in California and Kamchatka. Contrary to my original hypothesis, I found that none of the populations of the thermal taxa are ecotypes of non-thermal Agrostis scabra, but instead constitute a single, previously unrecognized endemic taxon that currently has four names. A UPGMA phenogram showed that while the thermal populations form geographically distinct subclusters, populations of the two morphologically distinct thermal taxa (A. rossiae and thermal A. scabra) do not cluster independently. Even though currently confused with the thermal populations, non-thermal A. scabra does not appear to be closely related. I used a series of field and laboratory experiments, as well as extensive field monitoring, to show that there are no important ecological differences between A. rossiae and thermal A. scabra. The ecological factor primarily responsible for the geographic separation of the thermal and non-thermal populations is the seasonal fluctuation of soil temperature relative to the life cycle of the plants. Lethal summer soil temperatures in the thermal areas are selecting for precocious flowering and an annual habit, which excludes the more slowly maturing non-thermal plants. The thermal plants, in contrast, do not grow in the non-thermal matrix in prevailing cool field temperatures, but do grow in these habitats under artificially elevated temperatures. The thermal taxa may have reduced competitive ability at cooler temperatures.