The recognition of taxonomic ranks in the Linnean classification system is largely arbitrary. Some authors have proposed the use of divergence time as a universally standardized criterion. Agaricus (Agaricaceae, Agaricales) is a mushroom genus that contains many species of high commercial value. Recent studies using ITS sequence data discovered 11 new phylogenetic lineages within the genus, however their taxonomic ranks were uncertain due to the lack of criteria to define them within traditional taxonomy. In this study, we analyzed ITS sequence data from 745 collections (nearly 600 being newly generated) including 86 from type specimens of previously recognized subgenera and sections. Many monophyletic groups were recognized, but most basal relationships were unresolved. One hundred and fourteen representatives of the identified ITS clades were selected in order to produce a multi-gene phylogeny based on combined LSU, tef-1α, and rpb2 sequence data. Divergence times within the multi-gene phylogeny were estimated using BEAST v1.8. Based on phylogenetic relationships and with respect to morphology, we propose a revised taxonomic system for Agaricus that considers divergence time as a standardized criterion for establishing taxonomic ranks. We propose to segregate Agaricus into five subgenera and 20 sections. Subgenus Pseudochitonia is substantially emended; circumscription of the subgenera Agaricus and Flavoagaricus is restricted to taxa of sections Agaricus and Arvenses, respectively; and two new subgenera (Minores and Spissicaules) are introduced. Within Pseudochitonia, sections Bivelares, Brunneopicti, Chitonioides, Nigrobrunnescentes, Sanguinolenti and Xanthodermatei are maintained, but the latter two are reduced because we raise subsection Bohusia to sectional rank and a clade within section Xanthodermatei is formally introduced as section Hondenses; and sections Rubricosi, Crassispori, Flocculenti, and Amoeni are introduced. Section Laeticolores is placed in the subgenus Minores and sections Rarolentes and Subrutilescentes are placed in the subgenus Spissicaules. Twenty-two new species belonging to various sections are described. This work exemplifies that ITS data, while useful at lower taxonomic levels (i.e., detection of species and species groups), are of limited value for inferring deeper phylogenetic relationships. Finally, we suggest that the establishment of a standardized taxonomic system based on divergence times could result in a more objective, and biologically more meaningful, taxonomic ranking of fungi.

Agaricus (Basidiomycota) is a genus of saprobic fungi that includes edible cultivated species such as Agaricus bisporus, the button mushroom. There has been considerable ecological, nutritional and medicinal interest in the genus, yet the extent of its diversity remains poorly known, particularly in subtropical and tropical areas. Classification of tropical species has for a large part followed the classification of temperate species. The objective of our study was to examine to what extent this system of classification is appropriate for tropical Agaricus species. Species from temperate sections were therefore compared to the major clades of tropical species using a phylogenetic approach. ITS1 + 2 sequence data from 128 species were used in the phylogenetic analysis. Specimens included four species of genera closely related to Agaricus, 38 temperate species representing the eight classical sections of the genus, and 86 putative species of Agaricus from tropical areas of Africa, Asia and the Americas. Bayesian and maximum likelihood analyses produced relatively congruent trees and almost identical clades. Our data show that (i) only about one-third of tropical species belong to the classical sections based on temperate species; the systematics of the genus therefore needs to be expanded; (ii) among the remaining two-thirds of tropical species, those from the Americas and those from Africa and/or Asia group in distinct clades, suggesting that secondary diversification occurred in these two areas; (iii) in contrast, several clades of classical sections contain American and African + Asian species along with temperate species. In this study, we used approximately 50 distinct species from a small area of northern Thailand, most probably being novel species. This diversity indicates that Agaricus is a species-rich genus in the tropics as well as in temperate regions. The number of species and the hypothetical paleotropical origin of the genus are discussed.

In eukaryotes, introns are located in nuclear and organelle genes from several kingdoms. Large introns (up to 5 kbp) are frequent in mitochondrial genomes of plant and fungi but scarce in Metazoa, even if these organisms are grouped with fungi among the Opisthokonts. Mitochondrial introns are classified in two groups (I and II) according to their RNA secondary structure involved in the intron self-splicing mechanism. Most of these mitochondrial group I introns carry a “Homing Endonuclease Gene” (heg) encoding a DNA endonuclease acting in transfer and site-specific integration (“homing”) and allowing intron spreading and gain after lateral transfer even between species from different kingdoms. Opposed to this gain mechanism, is another which implies that introns, which would have been abundant in the ancestral genes, would mainly evolve by loss. The importance of both mechanisms (loss and gain) is matter of debate. Here we report the sequence of the cox1 gene of the button mushroom Agaricus bisporus, the most widely cultivated mushroom in the world. This gene is both the longest mitochondrial gene (29,902 nt) and the largest group I intron reservoir reported to date with 18 group I and 1 group II. An exhaustive analysis of the group I introns available in cox1 genes shows that they are mobile genetic elements whose numerous events of loss and gain by lateral transfer combine to explain their wide and patchy distribution extending over several kingdoms. An overview of intron distribution, together with the high frequency of eroded heg, suggests that they are evolving towards loss. In this landscape of eroded and lost intron sequences, the A. bisporus cox1 gene exhibits a peculiar dynamics of intron keeping and catching, leading to the largest collection of mitochondrial group I introns reported to date in a Eukaryote.

Medicinal mushrooms have currently become a hot issue due to their various therapeutic properties. Of these, Agaricus subrufescens, also known as the "almond mushroom", has long been valued by many societies (i.e., Brazil, China, France, and USA). Since its discovery in 1893, this mushroom has been cultivated throughout the world, especially in Brazil where several strains of A. subrufescens have been developed and used as health food and alternative medicine. This article presents up-to-date information on this mushroom including its taxonomy and health promoting benefits. Medicinal properties of A. subrufescens are emphasized in several studies which are reviewed here. In addition, safety issues concerning the use of this fungus will be discussed.

Agaricus bisporus (Lange) Imbach, the familiar button mushroom of commerce, is a major vegetable crop around the world. In the past 10 years a significant worldwide effort has been made to collect Agaricus germ plasm from the wild. Here we report the genetic analysis of a collection of 342 isolates from 12 locations. For 10 nuclear toci marked by RFLPs, we found high genetic diversity in all geographic populations. Among the 342 isolates, three different analyses of genetic diversity were carried out: the first on the total sample of 342 isolates, the second on a subset of 108 “cultivar-like” isolates shown previously to carry either of the two mitochondrial DNA haplotypes found in cultivated strains, and the third on the other 234 isolates that carry a diversity of mtDNA haplotypes not found in cultivated strains. We found that the samples of cultivar-like isolates from various locations were genetically more similar among themselves and to the cultivars than to the samples of other isolates from the same locations. Furthermore, the cuitivar-like samples showed no evidence of genetic differentiation between continents and between regions within a continent. in contrast, samples of other isolates showed significant differentiation at the same levels of the geographic hierarchy. A comparison of gene frequencies was consistent with the occurrence of hybridization between the cuitivar-like and the other strains in the coastal California population. Analyses of genetic diversity and genetic distance were all consistent with the historical record that cultivars and cultivar-like strains in the wild originated from Western Europe.

Specimens of Agaricus sections Xanthodermatei and Hondenses were collected in Tibet and the surrounding areas. All specimens were morphologically described and subjected to molecular phylogenetic analysis of the ITS and combined ITS, LSU and tef-1α sequence data using Bayesian and Maximum Likelihood methods. Twelve species were identified in this study, eleven of which were new species and one of which was identified in China for the first time. All of the identified species are described and illustrated with photo plates.