Disentangling Taxonomic Complexity in the Native Range: Morphological and Genetic Differentiation Among Subspecies of Taeniatherum caput-medusae (Poaceae)

The timely and accurate identification of invasive species is a critical first step in recognizing the threats that they present in their new habitats. The accurate identification of an invasive species, however, can prove difficult if that species displays taxonomic complexity in its native range, i.e., it consists of morphologically similar subspecies. Across its native range, the grass Taeniatherum caput-medusae (medusahead) exhibits taxonomic complexity: three subspecies have been recognized. As part of our ongoing research to better understand the invasion of T. caput-medusae in the western United States, the accurate identification of these three subspecies is a requisite first step. Plants from each native population were grown in a greenhouse common garden, harvested at maturity, and measured using five previously described morphological characteristics. Three characteristics, glume length, glume angle, and palea length, were found to be statistically significant, and are diagnostic in differentiating the three subspecies. The results for the two other characteristics were not significantly different, although conical cell prominence was only slightly non-significant (p = 0.0532). Genetic differentiation among native populations of T. caput-medusae was assessed using allozymes as molecular markers. Results of an UPGMA cluster diagram based on allozyme data indicate that subspecies crinitum is genetically differentiated from the other two, some populations of subspecies caput-medusae and asperum co-occur within a cluster, and subspecies asperum is the most variable. Results of the analysis of multilocus genotypes are generally consistent with the UPGMA diagram (e.g., subspecies caput-medusae and asperum share six multilocus genotypes). Our findings confirm the need for a better understanding of the taxonomic complexity that can be found within the native ranges of invasive species.