Genetic and Morphological Variation in Populations of Taeniatherum caput-medusae (medusahead) from the Vicinity of Yakima, Washington: Evidence for the Cryptic Invasion of a Second Subspecies of Medusahead in the Western United States
Taeniatherum caput-medusae (medusa head) is a self-pollinating grass native to Eurasia. Across its native range, three subspecies have been recognized: T. caput-medusae ssp. caput-medusae, T. caput-medusae ssp. asperum, and T. caput-medusae ssp. crinitum. Only ssp. asperum is believe to occur in the United States (US), where it is invasive in portions of California, Idaho, Nevada, Oregon, Utah and Washington. The recent discovery of a specimen in the herbarium at the University of Nevda, Reno indicates that medusahead was collected in Yakima, WA, in 1899. This represents the second earliest record of medusahead in the western US. In 2010, we collected samples from nine populations of medusahead in and near Yakima, and the morphological variation of these samples was used to assess the identity of this putative early introduction site. Based on our morphological data, plants from three populations south of Yakima (along Ahtanum Rd.) were identified as being ssp. crinitum. This represents the first report of this second subspecies of medusahead in the US, and indicates the cryptic invasion of this subspecies in the US. Morphological analysis of all individuals in the other six populations in this region indicates that are identified as ssp. asperum. All individuals identified as being ssp. crinitum shared the same multilocus genotype, suggesting a single introduction of this subspecies in this locality. The six populations of ssp. asperum we analyzed exhibited four multilocus genotypes, including a genotype that has never been detected in the US. These data suggest multiple introduction of ssp. asperum into the area around Yakima. Results of this study reveal the importance of combining historical information (herbarium specimens) and other sources of data (morphological and genetic analyses) in reconstructing the introduction history of a non-native plant species.