Curcuma comprises 120 species that occur throughout tropical and subtropical Asia. The taxonomy of the genus is haunted by polyploid speciation and homoploid hybridization, making it the most challenging genus in Zingibereae (Zingiberaceae). Curcuma is best known for turmeric (C. longa), but numerous species are extensively used as medicinal plants, ornamentals, and sources of starch, among many other uses. The delimitation of the genus has been a matter of dispute since its establishment by Linnaeus (1753), and further conflict has arisen from recent molecular and morphological studies suggesting either paraphyly of Curcuma or the necessity to broaden the genus to include four small genera (Laosanthus, Paracautleya, Stahlianthus, Smithatris) as well as several species currently placed in Kaempferia and Hitchenia. All previous infrageneric classifications were based on limited material that did not include species from the Indochinese floristic region, and these classifications are unable to unequivocally accommodate all currently known members of the genus. To test the monophyly and delimitation of Curcuma and to gain more insight into infrageneric relationships, three plastid regions (trnL-trnF, psbA-trnH, matK) and the internal transcribed spacer (ITS) of nuclear ribosomal DNA were sequenced. Fifty Curcumaspecies cover- ing the morphological and geographic variation of the genus and 12 Curcuma-like species currently or previously treated as members of other genera were included in this study. In addition, four Zingibereae and three other Zingiberaceae species were used as outgroups. The results of maximum parsimony and Bayesian analyses clearly support a broad generic boundary for Curcuma, with inclusion of Laosanthus, Paracautleya, Stahlianthus, Smithatris and some species of Kaempferia and Hitchenia (K. scaposa, K. candida, H. caulina, H. glauca). Four main groups in Curcuma s.l. were detected, and their importance for classification at the subgenus level is discussed. A new infrageneric classification is proposed here with a formal description of a new subgenus. Cloning uncovered a broad range of variation of ITS sequences within individuals, particularly in the terminal 'Curcuma' group containing representatives of the nominal subgenus Curcuma. This 'intra-individual ITS polymorphism' increases with ploidy level and is coupled with preferred vegetative reproduction. Additional studies are needed to further uncover highly complex relationships in this subgenus.
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Publication date: 17 August 2012