The genus Carex is one of the most widespread and ecologically important of all plant genera, yet the evolutionary and biogeographic history of most of its approximately 2,500 species remains unclear. Convergence and homoplasy have made difficult elucidating phylogenetic relationships among taxa using traditional morphology. The Carex acuta complex, as defined by Standley, comprises 12 species from the largest section of Carex, section Phacocystis. The species of the complex were originally distinguished by their nerved, stipitate perigynia with torulose bases adnate to iridescent achenes. ITS sequence data was used to test the phylogenetic integrity of the C. acuta complex, to establish its position within section Phacocystis, to identify any biogeographic pattern indicated by the phylogeny, and to examine infraspecific variation within two of its polymorphic species, C. nigra and C. lenticularis. The C. acuta complex was found to be paraphyletic as circumscribed. However, a monophyletic group that included all of the C. acuta complex was identified. Within this group, two distinct clades were found, one comprising the Eurasian species of the complex, the other comprising the North American species of the complex plus three other North American species of section Phacocystis. Little sequence divergence among European and American accessions of C. nigra was found. In contrast, the two varieties of C. lenticularis examined were more closely related to other species within the North American clade than to each other. When chromosome numbers were mapped onto a strict consensus tree the data show the Eurasian and the North American clades to have unique aneuploid series, 2n = 72 - 84 and 2n = 66 - 92 respectively. These numbers failed to support to the theory that chromosome numbers have consistently increased during the evolution of Carex.

Key words: Carex acuta complex, Carex, monophyletic groups, section Phacocystis