Taxonomic and phylogenetic studies reveal three new species of corticioid fungi (Russulales, Basidiomycota) from Southwestern China

Technical Summary: Taxonomic and Phylogenetic Studies of Three New Corticioid Fungi from Southwestern China

Problem Statement
The southwestern region of China is recognized as a global biodiversity hotspot; however, the macrofungal diversity of the order Russulales (Basidiomycota) remains poorly documented, with numerous species expected to be undescribed. Specifically, the taxonomy and phylogeny of wood-inhabiting fungi (WIF) within the families Gloeocystidiellaceae and Peniophoraceae are understudied. Genera such as Gloeocystidiellum and Vararia are known to be highly polyphyletic, and taxonomic gaps persist due to limited molecular sequence data and the absence of stable morphological or molecular characteristics for clear classification. Consequently, the relationships among these taxa remain unresolved, necessitating further collection and analysis to define their evolutionary positions.

Methodology
The study employed an integrative approach combining detailed morphological observations with molecular phylogenetic analyses.

• Sample Collection: Fresh basidiomata were collected from Dehong and Puer in Yunnan Province, China. Specimens were photographed in situ, dried, and deposited in the herbarium of Southwest Forestry University (SWFC).
• Morphological Analysis: Macroscopic features were recorded using standard color terminology. Microscopic structures (hyphal systems, cystidia, basidia, and basidiospores) were examined from dried specimens using light microscopy, with measurements and chemical reactions (KOH, IKI, CB) documented.
• Molecular Analysis: Genomic DNA was extracted from dried specimens. Two genetic markers were amplified and sequenced: the nuclear ribosomal internal transcribed spacer (ITS) region and the large subunit (nrLSU) gene.
• Phylogenetic Inference: Newly generated sequences were aligned with related taxa retrieved from GenBank using MAFFT. Phylogenetic trees were constructed using Maximum Likelihood (ML) via RAxML and Bayesian Inference (BI) via MrBayes. Support values were assessed using nonparametric bootstrapping (1000 replicates) and Bayesian posterior probabilities.
• Species Delimitation: To confirm the distinctness of the new taxa from closely related species, Pairwise Homoplasy Index (PHI) tests were conducted on ITS datasets to detect significant recombination events.

Key Contributions and Results
The study successfully identified and described three new species of corticioid fungi, assigning them to the genera Gloeocystidiellum and Vararia based on both morphological and molecular evidence:

1. Gloeocystidiellum tenuissimum sp. nov. (Family: Gloeocystidiellaceae):

   • Morphology: Characterized by a thin, white to cream, tuberculata hymenial surface; a monomitic hyphal system with clamped generative hyphae; and subglobose, verrucose basidiospores (3.5–4.5 × 2.5–3 µm).
   • Phylogeny: The species forms a distinct clade within Gloeocystidiellum, closely related to G. sinense.
   • Distinction: It differs from G. sinense by its tuberculata (vs. smooth) hymenial surface and smaller gloeocystidia. It differs from the type species G. porosum by having smaller gloeocystidia and basidiospores.

2. Vararia farinacea sp. nov. (Family: Peniophoraceae):

   • Morphology: Features a farinaceous (mealy), white to pale yellow hymenial surface; a dimitic hyphal system; and ellipsoid, smooth basidiospores (4.5–6.5 × 3–4.5 µm).
   • Phylogeny: Forms a strongly supported monophyletic clade within Vararia, closely related to V. cremea and V. gillesii.
   • Distinction: It is distinguished from V. cremea and V. gillesii by significantly smaller gloeocystidia, basidia, and basidiospores.

3. Vararia ruiliensis sp. nov. (Family: Peniophoraceae):

   • Morphology: Distinguished by a cream to slightly yellowish hymenial surface; a dimitic hyphal system with simple-septate generative hyphae; and oblong ellipsoid basidiospores (5–7.5 × 2–3.5 µm).
   • Phylogeny: Groups within Vararia, closely related to V. trinidadensis, V. gomezii, and V. sigmatospora.
   • Distinction: Morphologically distinct from its close relatives by having smaller gloeocystidia and basidiospores, as well as thinner dichohyphae compared to V. gomezii.

PHI Test Results:
The PHI tests confirmed the absence of significant recombination between the new species and their closest phylogenetic relatives (Φw values > 0.05), supporting their status as distinct biological species.

Significance
The paper claims that these findings augment the existing knowledge of species diversity and taxonomy of fungi in southwestern China. By providing detailed descriptions, illustrations, and phylogenetic placements, the study addresses taxonomic gaps within the polyphyletic genera Gloeocystidiellum and Vararia. The results contribute to the global understanding of wood-inhabiting fungi in the order Russulales, highlighting the region's rich, yet under-documented, fungal biodiversity. The study does not propose new applications or future experimental directions beyond the immediate taxonomic clarification of these taxa.