The Pleurotus Ostreatus Species Complex:Species Recognition,Biogeography,and Germplasm Resources

Pleurotus ostreatus species complex(the oyster mushrooms),belonging to the genus Pleurotus of the family Pleurotaceae,is saprotrophic and of significant economic and ecological importance in the Northern Hemisphere.However,previous studies of species delimitation,biogeography and germplasm resources have been limited.Although species of the P.ostreatus complex are one of the main groups of cultivated edible mushrooms in China,the inconsistent nomenclature is controversial and has led to much confusion due to phenotypic plasticity and morphological stasis.The previous molecular phylogenetic analyses based on limited sampling or gene fragments were insufficient to reveal its species diversity.How to recognize the species of the complex rapidly and accurately is an urgent task in the mushroom industry.In addition,Pleurotus ostreatus is more widely distributed in China than other oyster mushrooms,however,its genetic diversity is not well known.The cultivated strains of this species have been frequently exchanged domestically and internationally,but no detailed breeding history has been documented.With samples gathered from a broad survey in China,molecular analyses of the P.ostreatus species complex were conducted using nucleotide sequences of four-gene loci to uncover the taxonomic position of the oyster mushrooms strains.With samples collected and loaned from different herbaria of the world,nuclear single-copy orthologous gene sequencing was used to reconstruct molecular phylogenetic frame and conduct biogeographic analyses of the P.ostreatus species complex.Genetic diversity study of P.ostreatus strains based on SSR molecular marker was also conducted.The main results are summarized as follows.1.The species delimitation of the P.ostreatus species complex in ChinaMolecular identification of 284 samples gathered from different mushroom spawn preservation centers,companies,and field isolations was conducted based on 1001 newly generated and other available sequences of the internal transcribed spacer region(ITS),translation elongation factor 1-?(TEF1)and two genes that encode subunits of RNA polymerase ??(RPB1 and RPB2).Our analyses elucidated the relationships of the oyster mushrooms of China and indicated that all of the strains could be divided into seven lineages,representing one morphological species each,namely P.abieticola,P.cf.floridanus,P.eryngii var.eryngii,P.ostreatus,P.placentodes,P.pulmonarius,and P.tuoliensis.The study revealed that 56 % of all the samples were labeled with improper scientific names.Meanwhile,the suitability of the four candidate segments as DNA barcodes were evaluated.It was indicated that RPB2 was the most promising candidate segment for the recognition of the species in the P.ostreatus complex,taking into consideration the polymorphisms and other aspects of the four markers.2.The phylogenetic reconstruction and species recognition of the P.ostreatus species complex in the worldWith samples of the P.ostreatus species complex from East Asia,Europe,North and South America,and Africa,the phylogenetic analyses were carried out based on 1762 newly generated sequences of 40 nuclear single-copy orthologous genes.Here,a robust phylogeny of the P.ostreatus species complex was presented,with a full resolution of the relationships of the major clades and the species.The P.ostreatus species complex was strongly supported as monophyletic,including three major clades and seven subclades.Clade ? was the most basal lineage in the P.ostreatus species complex.Clade ? was the largest clade among three clades,and subclade ?a was firstly discovered as a unique clade,sister to other three subclades in the Northern Hemisphere.Twenty phylogenetic species were recognized based on Genealogical Concordance Phylogical Species Recognition(GCPSR),with seven putatively new species.3.The biogeography of the P.ostreatus species complexThe biogeographic analyses of the P.ostreatus species complex were conducted based on molecular clock estimation and ancestral area reconstruction for the first time.The data suggested that the most recent common ancestor of the P.ostreatus species complex diversified in the late Eocene(about 38 Ma)in East Asia,and diversification of the major clades may have been triggered by the uplifts of the Qinghai-Tibet Plateau(QTP)and global cooling of the late Eocene.Species of the P.ostreatus complex migrated from the East Asia into North America across the Bering Land Bridge or the North Atlantic Land Bridge at different times during the late Oligocene,late Miocene and late Pliocene,and then diversified in the Old and New Worlds simultaneously through multiple dispersal and vicariance events.The dispersal from East Asia to South America in the middle Oligocene was probably achieved by a long-distance dispersal event.Intensification of aridity and climate cooling events in the late Miocene and Quaternary glacial probably had a significant influence on diversification patterns within the P.ostreatus species complex.The disjunctions among East Asia,Europe,North America and Africa within Clade ?c are hypothesized to be a result of allopatric speciation.Plant debris from coniferious and deciduous trees,such as families Pinaceae and Fagaceae,was the earlier substrate for the P.ostreatus species complex.Substrate transitions to Apiaceae probably occurred no earlier than 6 Ma.The diversification rate is variable in the evolutionary history of the P.ostreatus species complex and one radiation occurred in the late Miocene.4.Genetic diversity of P.ostreatus in China91 strains of P.ostreatus were analysed using 21 simple sequence repeats(SSR)developed from an enriched genomic library of P.cf.floridanus genome.Among the cultivated strains,10.5 % were previously labelled with improper names,suggesting that the cultivated strains of P.ostreatus in China exist the strain name confusion.It's indicated that wild strains harbored greater genetic diversity than the cultivated strains and speculated that wild stains may harbor a large gene pool in nature available for strain breeding.With regard to the cultivation history of P.ostreatus,the cultivated strains in China have three sources: introduction from Europe,domestication from wild strains from China,or hybridisation of the European and Chinese strains.Furthermore,a core collection of P.ostreatus with 34 strains was proposed.