Carex section Acrocystis (Cyperaceae) traditionally has included ca. 45 species. Recent studies have suggested that a “core” group of Acrocystis species forms a monophyletic group, while other species are more closely related to other sections. This core group includes all North American Acrocystis species and some but not all Eurasian members. Taxonomists have recognized several species complexes in North American Acrocystis, with several “orphan” species not delt with in these species complexes. The complexes that have been defined include the “Carex pensylvanica complex,” the “Carex nigromarginata complex,” the “Carex deflexa complex,” and the “Carex umbellata complex”. Taxa not included in these complexes are Carex novae-angliae, C. turbinata, C. leucodonta, C. communis, C. globosa, C. brainerdii, and C. serpenticola. How these complexes and orphan species are related has not been addressed explicitly, and none of the North American species have been explicitly compared with Eurasian or South American members of Acrocystis, with few exceptions. The study presented here explores relationships within North American members of Carex section Acrocystis and related Eurasian species and addresses biogeographic patterns of diversification using nuclear Adh genic regions and the nrDNA ITS region.

Key words: Acrocystis, Adh, biogeography, Carex, Cyperaceae, ITS