A new phylogeny-based generic classification of Costaceae (Zingiberales)

Costaceae is one of the most easily recognizable groups within the Zingiberales, distinguished from other families within the order by well-developed and sometimes branched aerial shoots that have a characteristic monistichous (one-sided) spiral phyllotaxy (e.g., Kirchoff & Rutishauser, 1990). Its close relationship with Zingiberaceae is evidenced by its former placement as a subfamily within the larger Zingiberaceae family. The placement of Costaceae within Zingiberaceae was largely based on broad similarities of inflorescence and floral characters. Tomlinson (1962) suggested that, although these types of characters may indicate common ancestry, they are not sufficient to overcome the morphological and anatomical differences that warrant independent familial rank of the two lineages which had been proposed by Nakai (1941). Costaceae sensu Tomlinson (1962) consisted of four genera: Costus, Monocostus, Dimerocostus and Tapeinochilos. Costus, which contains the majority of the species and maintains the greatest morphological diversity, is pantropical with its greatest diversity centered in the neotropics (c. 40 spp.); 25 species occur in tropical Africa and about five species in southeastern Asia. A separate genus, Cadalvena, was recognized when Costoideae was part of Zingiberaceae (Scitamineae) (Fenzl, 1865) and was maintained in some subsequent treatments of the family (e.g., Thiselton-Dyer, 1898), but in later treatments was reduced to subgeneric status within Costus (Schumann, 1904). Floristic treatments that included species of Cadalvena either did not distinguish subgenera (Koechlin, 1964, 1965; Hepper, 1968) or treated Costus and Cadalvena as subgenera within Costus (Maas, 1972, 1977). Maas (1972) described several new species from South America which he included in C. subgenus Cadalvena. No new species from Africa have been added to Cadalvena since Schumann (1904) and, in fact, many of the species included in Schumann’s treatment have been subsequently synonymized with the generic type, Costus spectabilis. In addition to Cadalvena, Schumann (1904) recognized four other subgenera in Costus in his treatment of global Zingiberaceae (including the Costoideae): Eucostus (= C. subg. Costus according to the current International Code of Botanical Nomenclature, ICBN, Greuter & al., 2000), Metacostus, Epicostus and Paracostus. These same subgenera were maintained by Loesener (1930). In Maas’ treatments of neotropical Costaceae (1972, 1977), a formal division between the two subgenera that are found in South America (Costus, Cadalvena) was maintained. In addition, subgenus Costus was divided into two separate sections: Costus sect. Ornithophilus and Costus sect. Costus. These sections were based upon characters of the labellum and reflected two distinct floral forms associated with polli-