Agaricus section Xanthodermatei: a phylogenetic reconstruction with commentary on taxa Richard W. Kerrigan 1 Sylvan Research, 198 Nolte Dr., Kittanning, Pennsylvania 16201, USA Philippe Callac Jacques Guinberteau INRA, MYCSA (Mycologie et se ´curite ´ des aliments) BP 81, 33883 Villenave d’Ornon cedex, France Michael P. Challen Warwick HRI, University of Warwick, Wellesbourne, Warwickshire, CV35 9EF, UK Luis A. Parra Avda. Padre Claret nu 7, 5u G, 09400 Aranda de Duero, Burgos, Spain Abstract: Agaricus section Xanthodermatei comprises a group of species allied to A. xanthodermus and generally characterized by basidiomata having phe- nolic odors, transiently yellowing discolorations in some parts of the basidiome, Schaeffer’s reaction negative, and mild to substantial toxicity. The section has a global distribution, while most included species have distributions restricted to regions of single continents. Using specimens and cultures from Europe, North America, and Hawaii, we analyzed DNA sequences from the ITS1+2 region of the nuclear rDNA to identify and characterize phyloge- netically distinct entities and to construct a hypothesis of relationships, both among members of the section and with representative taxa from other sections of the genus. 61 sequences from affiliated taxa, plus 20 from six (or seven) other sections of Agaricus, and one Micropsalliota sequence, were evaluated under distance, maximum parsimony and maximum likeli- hood methods. We recognized 21 discrete entities in Xanthodermatei, including 14 established species and 7 new ones, three of which are described elsewhere. Four species from California, New Mexico, and France deserve further study before they are de- scribed. Type studies of American taxa are particularly emphasized, and a lectotype is designated for A. californicus. Section Xanthodermatei formed a single clade in most analyses, indicating that the traditional sectional characters noted above are good unifying characters that appear to have arisen only once within Agaricus. Deep divisions within the sequence-derived structure of the section could be interpreted as subsections in Xanthodermatei; however, various con- siderations led us to refrain from proposing new supraspecific taxa. The nearest neighbors of section Xanthodermatei are putatively in section Duploannu- lati. Key words: Mushroom poisoning, nomenclature, phylogeny, systematics, toxicology INTRODUCTION Agaricus L. section Xanthodermatei was created by Singer (1948) to include a distinctive group of species sharing characters with the type species, A. xantho- dermus Genev. Singer in his original diagnosis characterized this section as follows: ‘‘A sectione Arvensibus Konr. & Maubl. (1924) reactione anilinica in pileo et carne intense laeteque lutea vel aurantiaca et reactione ‘‘cruciformi’’ (anilino-nitrico) negativa differt. Species typica: A. xanthoderma Genevier.’’ At present, members of the group are commonly recognized by generalized biochemical characteristics of their basidiomata: (1) ‘chemical’ odors, of phenol, ink, library paste, (collectively called ‘phenolic’ or ‘carbolic’ odors) and/or of iodine or ammonia (‘iodoform’ odors), (2) the presence of context (often in the stipe base), surfaces, and/or veils that become transiently yellow when damaged by cutting, bruising or (rarely) desiccation, and more distinctly by application of KOH, (3) Schaeffer’s reaction negative (i.e. no red or orange color appears when aniline plus nitric acid are applied), and (4), typically, the presence of one or more toxic compounds such as phenol (Gill and Strauch 1984, Wood et al 1998) and/or hydroquinone (Jovel et al 1996) that can cause gastrointestinal distress progressing, in severe cases, to violent vomiting (see also De Haro et al 1999, Singer 1961, Cleland and Harris 1948). Note that a useful substitution of glacial acetic acid for nitric acid in a simplified Scha ¨ffer’s reaction has been proposed by Frank (1988). There are some caveats to this generalization. Both odors and intensity of yellowing vary and may be quite faint in some taxa and some specimens. Rufescence, which we define as a rapid (1–5 min) development of a reddish color upon exposure or damage to some or all parts of the basidiome, in a reaction typically catalyzed by tyrosinase, is absent from most Xantho- dermatei but is present in A. pseudopratensis, A. freirei, 1 Corresponding author. E-mail: rwk@sylvaninc.com Mycologia, 97(6), 2005, pp. 1292–1315. # 2005 by The Mycological Society of America, Lawrence, KS 66044-8897 1292