The Xerula/Oudemansiella Complex (Agaricales)

"The complex of the agaricoid genera Xerula and Oudemansiella (Physalacriaceae, Agaricales) is unraveled in great detail in this taxonomic treatment by Ron Petersen and Karen Hughes. The 625-page thick book reveals a much greater complexity than ever imagined. The complex is morphologically studied, and ITS and LSU phylogenies are constructed.

Let us first look at the contents of the book. After a general introduction with a history of the genera/genus and its classifications, material and methods for the research are given, followed by a chapter on the DNA-based phylogenies. 330 pages are devoted to genus and species descriptions, keys to the species, line drawings, and photographs. The next 200 or so pages contain the type studies, and finally a list of new taxa and new combinations, indices, and literature references fill the rest of the pages.

A big problem faced by the authors was how to name the supraspecific taxa, and whether to recognize one genus or name the separate clades. The choice was made to split the group and to recognize seven genera, four of them newly described here, some of them distinctly not monophyletic, but morphologically distinct and homogeneous. The two genera with non-rooting fruitbodies that grow directly on wood are Oudemansiella, restricted to tropical species without a persistent annulus, and Mucidula as the temperate counterpart with a persistent annulus. Although the two look very much alike, they are not sister groups. The other five genera all have a ‘rooting’ stipe connected to subterraneous wood or tree roots. The old Xerula is redistributed into Xerula (in the strict sense) for species with thick-walled setae on the pileus; Paraxerula harbours species with thin-walled setae on the pileus; Hymenopellis, with the highest number of species, is characterized by a moist to glutinous pileus; Protoxerula species, also with a sticky pileus, occur in Australia and have green colours; species with spiny spores are accommodated in the genus Dactylosporina; and Ponticulomyces (which did not make it into the general key) is an Asian clade of two species with characters in between Hymenopellis and Oudemansiella. Hymenopellis is not a monophyletic unit, and several other genera are nested within it; which genera depend on which gene region the phylogeny is based. The position of Mucidula in the middle of Hymenopellis was not expected. It is surprising that the authors have not tried to show more support for these decisions by either analyzing the data with topological constraints (such as a monophyletic Hymenopellis) or adding data from protein coding genes. Another solution might be to recognize three genera – Xerula s. str. and Paraxerula as defined above plus Oudemansiella containing all other taxa, including the secotioid genus Cribbea. All three form well supported monophyletic clades in the ITS and the LSU phylogenies. Personally, I find the recognition of nonmonophyletic genera very problematic, and this is my main critique on this book.

Besides the four new genera, four new species are described, one from Guyana, one from the USA, a third from India, and the fourth from eastern Russia.

The value of this monograph lies in the very thorough descriptions, not only of all accepted taxa, but also and especially of all the type specimens that could be studied. It is also extremely pleasant to have all this information in one place, and not scattered over various publications in a diverse set of journals. However, the information on the type collections should be searchable on the web, ideally linked to nomenclatural data, such as in Index Fungorum or Mycobank. On the negative side is of course the cost of this book, a high price that will certainly deter people in less developed countries from purchasing. This is very infelicitous, as the highest diversity of these taxa is in Asia.

The quality of the photos is variable, and some have been reproduced in a strange way. Unfortunately, but understandably, not all taxa are depicted with a colour plate.

With a book of this size it is inevitable that details have been overlooked; one Latin description never got beyond the first phase of some jotted down characteristics, the epithet ‘kuehneri’ is consistently misspelled as ‘kuehnerii’, and diacritical signs in non-English article titles and publications are not or wrongly applied.

This book should nonetheless find a wide audience due to its thorough descriptions and worldwide coverage."

– Mycotaxon 114 (2011)
 

"This fantastic book which serves as an active monographic source deals with a small group of mushrooms genera (Homobasidiomycota, Agaricales) centered in Xerula s.l. and Oudemansiella. The book is nicely arranged in to sixteen chapters. Chapter (1) starts with a general introduction that includes the history of this group. The second chapter looks at the materials and methods and briefly explains how to work on the mushrooms i.e. on molecular methodology, basidiome observations and lamellae edge extension in KOH. The third chapter focuses on the molecular phylogenies of the Xerula/Oudemansiella complex which is a very good written and extremely attractive chapter. All students who are doing molecular phylogenetic studies should go through these chapters. The chapters (4) to (11) describe sixty-eight new taxa including four new genera (Hymenopelllis, Paraxerula, Pointiculomyes and Protoxerula) and these chapters nicely key out the agaricoid genera, Dactyosporina, Hymenopellis, Mucidula, Ouemensiella, Paraxerula, Ponticulomyces, Protoxerula and Xerula. The descriptions for each genus are in detail with type specimen information, macro and micro descriptions, morphotaxonomy, monography, molecular applications and phylogenetics. There are also a list of references, subject index and taxonomic index. This book is fantastically illustrated with 625 pages, 576 figures, 31 color plates and 8 tables, numerous pages of line drawings and phylogenic trees.

Recommendation: should be available in all good schools, community and university libraries and any research laboratories dealing those with mushrooms especially in agaric groups."

– Fungal Diversity April 2012