Reconstructing the introduction history of the invasive grass Taeniatherum caput-medusae subsp. asperum in the western United States: low within-population genetic diversity does not preclude invasion

Authors: MARSCH, DEAN - Boise State University; DEINES, LYNELL - Boise State University; RAUSCH, JOSEPH - Boise State University; TINDON, YVES - Montpellier Supagro – International Center For High Education In Agricultural Sciences; SFORZA, RENE - European Biological Control Laboratory (EBCL); MELTON, ANTHONY - Boise State University; NOVAK, STEPHEN - Boise State University

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2024
Publication Date: 2/15/2025
Citation: Marsch, D., Deines, L., Rausch, J.L., Tindon, Y., Sforza, R., Melton, A.E., Novak, S.J. 2025. Reconstructing the introduction history of the invasive grass Taeniatherum caput-medusae subsp. asperum in the western United States: low within-population genetic diversity does not preclude invasion. American Journal of Botany. e70001. https://doi.org/10.1002/ajb2.70001.
DOI: https://doi.org/10.1002/ajb2.70001

Interpretive Summary: Knowing the patterns of range expansion and the history of introduction of an invasive alien plant species help to understand the types of management one can propse to fight the invasion. Grasses are a big component of alien plant species in the USA. One of the major threat is medusahead, an annual grass highly invasive on rangelands of the western US, in the Great basin, from Colorado to Washington State. The purpose of this study was to reconstructing the introduction history and pattern of range expansion of medusahead. This approach requires multiple sources of information. Validated herbarium specimens provide reliable information concerning the occurrence of a plant at a certain place and time. The collection history of medusahead in the western U.S. is consistent with the pattern associated with multiple introduction events. Based on herbarium specimens and report records from the western U.S., for the years pre-1900 to circa 1960, we show that medusahead occurred in at least 26 California, eight Idaho, one Nevada, 13 Oregon, and three Washington counties. The simultaneous consideration of historical and genetic data (using allozymes) provides detailed insights into the introduction and spread of medusahead in the western U.S. We analyzed a total of 1700 individuals from the western US. Despite multiple (seven) introductions, invasive populations of medusahead exhibit low levels of genetic admixture, low levels of genetic diversity within populations, and high genetic structure. This putatively low evolutionary potential did not however preclude the initial establishment of medusahead and has not limited its spread across its new range.

Technical Abstract: The genetic diversity and structure of invasive populations is influenced by multiple factors, including the details of a species’ introduction, its pattern of range expansion, and its mating system. Using historical and genetic data, we reconstructed the introduction history and pattern of range expansion of the primarily self-pollinating invasive annual grass Taeniatherum caput-medusae subsp. asperum (medusahead), in the western United States (U.S.). Collection and report records of medusahead in the western U.S. indicate that the grass was first collected near Roseburg, Oregon in 1884 and then at Steptoe Butte, Washington in 1901. Genetic analysis of 46 invasive populations (1700 individuals) of medusahead detected seven homozygous multilocus genotypes (MLGs) across the western U.S., with several MLGs found in localities associated with early collection sites. Our data are consistent with the pattern associated with multiple introductions and local or regional range expansion. Despite multiple introductions, invasive populations of medusahead exhibit low levels of within-population genetic diversity and high genetic structure. Only three of the 1700 individuals we analyzed were heterozygous. High rates of self-pollination likely preserved the MLGs introduced into the western U.S. and contributed to our ability to reconstruction the invasion history of this grass in this region.