Molecular phylogenetic analyses and micromorphology reveal placement of the enigmatic tropical discomycete Polydiscidium in Sclerococcum (Sclerococcales, Eurotiomycetes)

ABSTRACT

Polydiscidium is an enigmatic, monotypic, and rarely reported genus of Ascomycota of uncertain placement. The morphologically unique Polydiscidium martynii grows on dead wood and forms compound ascomata composed of thick, black, gelatinous somatic tissue that branches out from a common base. Multiple apothecia are located on the branches, mostly toward the tips, and are composed of 8-spored asci and paraphyses embedded in a gelatinous matrix that turns blue in Melzer’s reagent. The species was previously known from only three collections from Guyana (holotype), Trinidad, and the Democratic Republic of the Congo and no sequences exist. Due to its peculiar morphology, taxonomic affinities of Polydiscidium have been debated, with different authors having placed it in Helotiaceae, Leotiaceae, or Leotiomycetes incertae sedis. Recent collections of this species resulting from long-term field work in Guyana and Cameroon led us to revisit the morphology and phylogenetic position of this fungus. Newly generated sequences of P. martynii were added to an Ascomycota-wide six-locus data set. The resulting phylogeny showed Polydiscidium to be a member of order Sclerococcales (Eurotiomycetes). Next, a four-locus (18S, ITS, 28S, mtSSU) phylogenetic reconstruction revealed that Polydiscidium is congeneric with Sclerococcum. A new combination is proposed for this species, Sclerococcum martynii. Micromorphological features, including the gelatinous hymenium composed of asci with amyloid gel cap and septate brown ascospores, are in agreement with Sclerococcum. New combinations are proposed for two additional species: Sclerococcum chiangraiensis and S. fusiformis. Finally, Dactylosporales is considered a later synonym of Sclerococcales.

KEYWORDS:

• Cameroon
• Guiana Shield
• multilocus phylogeny
• rare fungi
• Sclerococcaceae
• systematics
• taxonomy
• 3 new taxa

ACKNOWLEDGMENTS

Field assistance in Guyana was provided by Christopher Andrew, Mei Lin Chin, Francino Edmund, Luciano Edmund, Pedro Joseph, Valentino Joseph, and Rachel A. Koch. Research permits were granted by the Guyana Environmental Protection Agency. Field assistance in Cameroon was provided by Constantin Engoulou, Alamane Gabriel (a.k.a. Sikiro), Essambe Jean-Pierre (a.k.a. Papa Chef), and Blaise Jumbam. Cameroon’s Ministry of Research and Scientific Innovation issued research permits. We thank Pieter Asselman (Ghent University) for molecular work, Lee Davies (Royal Botanic Gardens, Kew) for locating and databasing the holotype collection of P. martynii at K, Dillon R. Husbands (Purdue University) for sharing her Guyanese collection of this species, Jingyu (Lucy) Liu (Purdue University) for technical laboratory assistance, D. Jean Lodge (University of Georgia) for sharing her observation of S. martynii in Suriname, Noah Siegel for providing a field photo of collection TH 11122, and Bryce Westman (Purdue University) for kind assistance with PUL collections.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the U.S. National Science Foundation (DEB-2018098 to D.H., DEB-1556338 to T.W.H., DEB-0732968 to M.C.A.), the National Geographic Society Committee for Research and Exploration (grant 6679-99 to T.W.H.), and the U.S. Department of Agriculture National Institute of Food and Agriculture (Hatch project 1010662 to M.C.A.).