Systematics of the Carex aquatilis and C. lenticularis lineages: Geographically and ecologically divergent sister clades of Carex section Phacocystis (Cyperaceae).

Carex aquatilis is a highly diverse and geographically widespread member of one of the largest genera of flowering plants, Carex, and is ideally suited for the study of the role of hybridization and niche partitioning in ecological speciation. Phylogenetic analyses of nuclear ITS and ETS 1f and chloroplast psbA-trnH DNA sequences support the monophyly of a broadly defined Carex aquatilis-Carex lenticularis lineage, which includes C. aquatilis and C. lenticularis and their allies within section Phacocystis. However, neither taxon is monophyletic as currently circumscribed. The C. aquatilis lineage includes C. aquatilis and four morphologically and molecularly distinct salt-tolerant maritime taxa with which C. aquatilis s.s. is reported to form stabilized homoploid hybrids. The C. lenticularis lineage includes a paraphyletic C. lenticularis and seven allied species from both the New and Old World. The data provided here allow recognition of four species within the North American endemic C. lenticularis and suggest a neotropical origin for the C. lenticularis lineage with subsequent radiation and divergence through northwestern North America to Asia and via northeastern North America to Europe and southern South America. Evolutionary rate analyses indicate an origin for the C. aquatilis-C. lenticularis group around 1.89 million years ago during the early Pleistocene.