With about 350 species, Paspalum is one of the richest genera within the Poaceae. Its species inhabit ecologically diverse areas along the Americas and they are largely responsible for the biodiversity of grassland ecosystems in South America. Despite its size and relevance, no phylogeny of the genus as a whole is currently available and infrageneric relationships remain uncertain. Many Paspalum species consist of sexual-diploid and apomictic-polyploid cytotypes, and several have arisen through hybridization. In this paper we explore the phylogenetic structure of Paspalum using sequence data of four non-coding cpDNA fragments from a wide array of species which were combined with morphological data for a subset of diploid taxa. Our results confirmed the general monophyly of Paspalum if P. inaequivalve is excluded and the small genus Thrasyopsis is included. Only one of the four currently recognized subgenera was monophyletic but nested within the remainder of the genus. Some informal morphological groups were found to be polyphyletic. The placement of known allopolyploid groups is generally congruent with previously stated hypotheses although some species with shared genomic formulae formed paraphyletic arrangements. Other species formed a basal grade including mostly umbrophilous or hygrophilous species. It is hypothesized that the genus may have diversified as a consequence of the expansion of C4 grass-dominated grasslands in South America.
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We are indebted to Pamela Soltis for laboratory facilities at the University of Florida, USA, to the Willi Hennig Society for making available a sponsored version of TNT, and to J. Marcos Mirande for kindly making available his unpublished TNT script. The following persons collaborated with us during field collection work and/or made available plant material from their own collections: Lone Aagesen, Alejandro Asenjo, Isabel B. Boccaloni, M. Eugenia Buela, Irene Caponio, Ana M. Carrión, Julio Daviña, Juan R. Fernández, Luis Frank, M. Cecilia Gróttola, Diego H. Hojsgaard, Ana I. Honfi, Graciela Lavia, Regina C. Oliveira, Florencia Perret, Camilo L. Quarin, Yamil Ramírez, Germán G. Roitman, José L. Rosa, G. Mónica Tourn, Nicolás Trillo, José F. M. Valls. This work received financial support through grants PIP 6568 (CONICET, Argentina), UBACyT JG21, and G806 (University of Buenos Aires, Argentina), and to GHR from the “Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)”, Brazil. GHR is a member of the “Carrera del Investigador” of the “Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)”, Argentina.