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the revision of species-rich genera - American Journal of Botany

the revision of species-rich genera - American Journal of Botany

the revision of species-rich genera - American Journal of

American Journal of Botany 93(3): 426–441. 2006. THE REVISION OF SPECIES-RICH GENERA: A PHYLOGENETIC FRAMEWORK FOR THE STRATEGIC REVISION OF PILEA (URTICACEAE) BASED ON CPDNA, NRDNA, AND MORPHOLOGY 1 ALEX K. MONRO 2 Department of Botany, Natural History Museum, London SW7 5BD, UK The revision of species-rich genera underpins research and supports the sustainable use and monitoring of biological diversity. One fifth to one quarter of the diversity of all seed plant species occurs in such genera, but difficulties with the revision of speciesrich genera has resulted in many of them being ignored since the late 1800s. Pilea, with 600–715 species is in need of revision. The only realistic approach is in manageable subunits, which requires confirmation of monophyly and identification of monophyletic subdivisions. Parsimony analyses of trnL-F, ITS, and morphology data were used to test the monophyly of, and explore intrageneric relationships within, Pilea. Analysis of trnL-F data confirms and recovers two morphologically diagnosable monophyletic clades that include all of the taxa within Pilea. Overlaying geographic distribution on a most parsimonious tree indicates a strong association between geography and phylogenetic relatedness. It is suggested that a strategic revision within the framework of morphologically and geographically diagnosable units might enable the revision of the group using an iterative approach. Analysis of the outgroup taxa supports the inclusion of Poikilospermum within the Urticaceae and suggests that the Urticaceae tribes could be placed into two clades that are supported by floral morphology. Key words: Achudemia; ITS; Pilea; Poikilospermum; species-rich genera; trnL-F; Urticaceae. Pilea, with 600–715 taxa (Adams, 1970; Burger, 1977; Monro, 2004) is the largest genus in the Urticaceae and one of the larger genera in the Urticales and Eudicot Rosids. It is distributed throughout the tropics, subtropics, and warm temperate regions (with the exception of Australia and New Zealand). The majority of taxa are succulent shade-loving herbs or shrubs, which are easily distinguished from other Urticaceae by the combination of opposite leaves (with rare exceptions) with a single ligulate, intrapetiolar stipule in each leaf axil and cymose or paniculate inflorescences (again with rare exceptions). Pilea is of little economic importance; four species are of minor horticultural importance (P. cadierei, P. involucrata, P. microphylla, and P. peperomioides) and one species is used in Chinese traditional medicine (P. plataniflora). The genus has attracted little monographic attention since Weddell (1869), and the majority of taxonomic contributions have come from floristic treatments (Killip, 1939; Standley and Steyermark, 1952; Adams, 1972; Burger, 1977; Van Royen, 1982; Chen, 1982, 1995; Monro, 2001, in press; Chen and Monro, 2003; Pool, 2001; Hopkins and Monro, in press). To date, 787 species names have been published (International Plant Names Index, 2003) and estimates for the species number range from 250 (Friis, 1989), 715 (Monro, 2004) to 1000 (C. D. Adams, BM, 1 Manuscript received 15 October 2005; revision accepted 1 December 2005. The author thanks M. Carine for support and help with the manuscript; S. Russell for help in the laboratory; the Side, Bonhote, Omer-Cooper Westwood Fund of The Linnean Society of London; The Natural History Museum’s Special Fund; Chen J.-R. and D. Adams for their considerable work on Pilea and their advice; H. Fortune-Hopkins and B. Johns at K for duplicates from Mount Jaya; N. Garwood for sequence data for Trema integerrima; the curators of KMG, P, PE, and PMA for their hospitality and of BM, PE, PMA, and MO for allowing the sampling of their herbarium collections for DNA; and T. Pennington, C. Jarvis, S. Knapp, and two anonymous reviewers for comments on earlier drafts of this manuscript. 2 Author for correspondence (e-mail: alm@nhm.ac.uk) 426 personal communication). Based on previous floristic treatments (Table 1), ca. 30% of the species from regions not yet covered by contemporary floristic treatments may be undescribed. As more species are discovered, the logistical problems of undertaking a monographic revision will grow. One fifth to one quarter of all diversity in seed plant species is contained in species-rich genera (Frodin, 2004). Difficulties associated with the revision of such large genera have resulted in many of these groups being ignored since the 19th century. This has impeded research into broad-ranging questions in systematics such as the reasons for the radiation in species-rich genera and the accumulation of species richness in the tropics (Axelrod, 1983; Pelser et al., 2002; Bramley et al., 2004; Frodin, 2004). Robust and stable classifications of such species-rich groups also underpin the effective assessment, monitoring, and conservation of biodiversity within the context of the Convention on Biological Diversity (Secretariat of the Convention on Biological Diversity, 2005b). Given the number of taxa, that almost all are nondescript understory herbs of minimal economic use, and the current global taxonomic capacity (Secretariat of the Convention on Biological Diversity, 2005a), it is unlikely that Pilea will be the subject of a global monographic revision in the near future. One approach to tackling taxonomic problems in Pilea would be to revise geographically diagnosable units, as happens in floristic treatments, or morphologically diagnosable units, as happened in the classical works of the 19th and 20th centuries. Under this approach, pragmatically defined units could be revised by different authors at different times on a pragmatic basis. The limitations of such an approach would be that it is unlikely to result in monophyletic groupings that would represent best estimates of evolutionary relationships between taxa. They would not therefore result in a classification that underpins both broader research in systematics and science and the conservation of biological diversity. An alternative approach would be to confirm generic monophyly first and then identify manageable monophyletic infrageneric groupings that could be revised independently. However, the identifica-

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