Agaricus is an important mushroom genus with some most common edible and medicinal species, but some members are also poisonous. Taxonomically, the genus is divided into seven subgenera including the subgenus Pseudochitonia. In this study, we analyzed some collections of Agaricus from southern Oman, belonging to A. subgenus Pseudochitonia. Agaricus subg. Pseudochitonia is the largest subgenus in terms of sections, with 13 sections by applying the modern taxonomic system in which supraspecific taxa are ranked according to their divergence times. We applied the same procedure using BEAST (Bayesian evolutionary analysis sampling tree) to evaluate the taxonomic position of our specimens. Our specimens formed a strong monophyletic clade with unclassified Agaricus species (A. lannaensis and A. sp. CA820), based on multigene sequence data (ITS, 28S and TEF1) and ITS alignment. The divergence time is estimated around 20–30 million years for this clade. We proposed a new section Catenulati following the basic principles of modern taxonomic system of Agaricus, which are (i) strong phylogenetic support, (ii) divergence times younger than 30 Ma, and (iii) specific morphological characters where possible. The new section Catenulati is represented by a new species Agaricus arabiensis. Moreover, the new section resolved the pending issue of A. lannaensis, previously as unclassified taxon in A. subg. Pseudochitonia and is now in A. sect. Catenulati. The new section is characterized by a phenolic odor of basidiomata and catenoid cheilocystidia.
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