This is the opening paper in the special issue of Fungal Diversity, which collates the data on defining species. Defining and recognizing species has long been a controversial issue. Since Darwin's proposed origin of species, over 30 species criteria have been brought forth and used to define species boundaries. In recent times, phylogenetic analyses based on multiple loci have been extensively used as a method to define species boundaries. However, only a few mycologists are aware that phylogenetic species criteria can mask discordances among fungal groups, leading to inaccurately defined species boundaries. In the current review, we discuss species recognition criteria, how and where these criteria can be applied along with their limitations and derived alternatives. In order to delimit fungal species, authors need to take into account not only the phylogenetic and phenotypic coherence, but also the timing of events that lead to fungal speciation and subsequent diversifications. Variations in the rate of phenotypic diversifications and convergent fungal evolution make it difficult to establish a universal species recognition criterion. The best practice can only be defined in the context of each fungal group. In this review, we provide a set of guidelines, encouraging an integrative taxonomic approach for species delimitation that can be used to define fungal species boundaries in the future. The other papers in this special issue deal with fungal speciation in Ascomycota, Dothideomycetes, Basidiomycota, basal fungi, lichen-forming fungi, plant pathogenic fungi, and yeasts.

A comprehensive account of fungal classification from freshwater habitats is outlined and discussed in the present review based on literature of biodiversity studies and recent morpho-phylogenetic analyses. A total of 3,870 freshwater fungal species are listed with additional details on the isolation source, habitat, geographical distribution, and molecular data. The Ascomycota (2,968 species, 1,018 genera) dominated the freshwater fungal taxa wherein Sordariomycetes (823 species, 298 genera) had the largest number, followed by Dothideomycetes (677 species, 229 genera), Eurotiomycetes (276 species, 49 genera), and Leotiomycetes (260 species, 83 genera). Other phyla included in the updated classification of freshwater fungi are: Chytridiomycota (333 species, 97 genera), Rozellomycota (221 species, 105 genera), Basidiomycota (218 species, 100 genera), Blastocladiomycota (47 species, 10 genera), Monoblepharomycota (29 species, 6 genera), Mucoromycota (19 species, 10 genera), Aphelidiomycota (15 species, 3 genera), Entomophthoromycota (6 species, 4 genera), Mortierellomycota (5 species, 3 genera), Olpidiomycota (4 species, 1 genus), Zoopagomycota (3 species, 2 genera), and Sanchytriomycota (2 species, 2 genera). The freshwater fungi belong to 1,361 genera, 386 families and 145 orders. The Pleosporales and Laboulbeniaceae are the largest freshwater fungal order and family comprised of 391 and 185 species, respectively. The most speciose genera are Chitonomyces (87, Laboulbeniomycetes), Verrucaria (50, Eurotiomycetes), Rhizophydium (52, Rhizophydiomycetes), Penicillium (47, Eurotiomycetes), and Candida (42, Saccharomycetes).

Blastocystis is the most commonly found eukaryote in the gut of humans and other animals. This protist is extremely heterogeneous genetically and is classified into 28 subtypes (STs) based on the small subunit ribosomal RNA (SSU rRNA) gene. Numerous studies exist on prevalence of the organism, which usually focus on either humans or animals or the environment, while only a handful investigates all three sources simultaneously. Consequently, understanding of Blastocystis transmission dynamics remains inadequate. Our aim was to explore Blastocystis under the One Health perspective using a rural community in northern Thailand as our study area. We surveyed human, other animal and environmental samples using both morphological and molecular approaches. Prevalence rates of Blastocystis were 73% in human hosts (n = 45), 100% in non-human hosts (n = 44) and 91% in environmental samples (n = 35). Overall, ten subtypes were identified (ST1, ST2, ST3, ST4 ST5, ST6, ST7, ST10, ST23, and ST26), eight of which were detected in humans (ST1, ST2, ST3, ST4, ST5, ST7, ST10, and ST23), three in other animals (ST6, ST7, and ST23), while seven (ST1, ST3, ST6, ST7, ST10, ST23, and ST26) were found in the environment. In our investigation of transmission dynamics, we assessed various groupings both at the household and community level. Given the overall high prevalence rate, transmission amongst humans and between animals and humans are not as frequent as expected with only two subtypes being shared. This raises questions on the role of the environment on transmission of Blastocystis. Water and soil comprise the main reservoirs of the various subtypes in this community. Five subtypes are shared between humans and the environment, while three overlap between the latter and animal hosts. We propose soil as a novel route of transmission, which should be considered in future investigations. This study provides a thorough One Health perspective on Blastocystis. Using this type of approach advances our understanding on occurrence, diversity, ecology and transmission dynamics of this poorly understood, yet frequent gut resident.

Three recent studies of Blastocystis epidemiology in mammalian hosts identified four novel sequences that appeared to share B. lapemi as the most similar sequence. However, full-length ssu rRNA gene sequences were not available to confirm the validity of these new subtypes. In the present study, Nanopore MinION sequencing was used to obtain full-length reference sequences for each of the new subtypes. Additionally, phylogenetic analyses and pairwise distance comparisons were performed to confirm the validity of each of these new subtypes. We propose that the novel sequences described in this study should be assigned the subtype designations ST35-ST38. The full-length reference sequences of ST35-ST38 will assist in accurate sequence descriptions in future studies of Blastocystis epidemiology and subtype diversity.

Blastocystis is the most common gastrointestinal protist found in humans and animals. Although the clinical significance of Blastocystis remains unclear, the organism is increasingly being viewed as a commensal member of the gut microbiome. However, its impact on the microbiome is still being debated. It is unclear whether Blastocystis promotes a healthy gut and microbiome directly or whether it is more likely to colonize and persist in a healthy gut environment. In healthy people, Blastocystis is frequently associated with increased bacterial diversity and significant differences in the gut microbiome. Based on current knowledge, it is not possible to determine whether differences in the gut microbiome are the cause or result of Blastocystis colonization. Although it is possible that some aspects of this eukaryote’s role in the intestinal microbiome remain unknown and that its effects vary, possibly due to subtype and intra-subtype variations and immune modulation, more research is needed to characterize these mechanisms in greater detail. This review covers recent findings on the effects of Blastocystis in the gut microbiome and immune modulation, its impact on the microbiome in autoimmune diseases, whether Blastocystis has a role like bacteria in the gut–brain axis, and its relationship with probiotics.