(2955) Proposal to conserve the name Didymium against Mucilago and Spumaria (Physarales, Myxomycetes)

Abstract

(2955) Didymium Schrad., Nov. Gen. Pl.: 20. Mai 1797, nom. cons. prop. Typus: D. melanospermum (Pers.) T. Macbr. (N. Amer. Slime-moulds: 88. Sep 1899) (Physarum melanospermum Pers.), typ. cons. prop. (=) Mucilago P. Micheli ex F.H. Wigg., Prim. Fl. Holsat.: 112. 29 Mar 1780, nom. rej. prop. Typus (vide García-Martín & al. in Persoonia 50, in press): M. crustacea F.H. Wigg. (Prim. Fl. Holsat.: 112. 29 Mar 1780). (=) Spumaria Pers. ex J.F. Gmel., Syst. Nat. 2: 1466. Apr (sero)–Oct 1792, nom. rej. prop. Typus: S. mucilago J.F. Gmel., nom. illeg. (Mucor spongiosus Leyss.). The generic name Didymium Schrad. (Nov. Gen. Pl.: 20. 1797) has been traditionally and consistently used for a morphologically well-characterized genus of the family Didymiaceae Rostaf. ex Cooke, order Physarales (Myxomycetes). This genus accommodates sporocarpic and plasmodiocarpic species frosted with stellate calcareous crystals on the peridium, sometimes agglomerated in a crust-like coating (Martin in N. Amer. Fl. 1(1): 140. 1949; Poulain & al., Myxomycètes: 195, 454. 2011). Didymium was erected by Schrader (l.c.) based on eight sporocarpic species. Subsequently, D. farinaceum Schrad. (l.c.: 22) was designated as type by Hagelstein (Mycetozoa N. America: 116. 1944), being, in fact, the only species name out of the eight original ones that remains in Didymium (Martin in State Univ. Iowa Stud. Nat. Hist. 20: 15. 1966; Martin & Alexopoulos, Myxomycetes. 1969). However, D. farinaceum is an illegitimate name (Art. 52.1 of the ICN, Turland & al. in Regnum Veg. 159. 2018) because Physarum melanospermum Pers. (in Neues Mag. Bot. 1: 88. 1794), basionym of D. melanospermum (Pers.) T. Macbr. (N. Amer. Slime-moulds: 88. 1899), was cited in synonymy. Many species have since been added to Didymium, which currently totals more than 90 recognized species and more than 320 validly published species names (www.eumycetozoa.com), being the second-largest genus of Myxomycetes after Physarum, also in the order Physarales. Moreover, Didymium includes the species D. iridis (Ditmar) Fr. (Syst. Mycol. 3: 120. 1829), which is a well-known research model organism, object of many investigations on reproductive systems, gene inheritance, intron content of the nuclear ribosomal small subunit gene (nrSSU), mitochondrial editing, and senescence, among other topics (Aldrich & Daniels, Cell Biol. Physarum Didymium 1. 1982; Silliker & Collins in Molec. Gen. Genet. 213: 370–378. 1988; Traphagen & al. in RNA 16: 828–838. 2010). Thus, after decades of research, Didymium has become a generic name familiar to many people, not only taxonomists and experts on Myxomycetes, but also researchers in the mentioned disciplines, and amateurs and enthusiasts of these organisms. However, Didymium is not the only genus in the family Didymiaceae with crystalline calcareous deposits on the peridium, since Mucilago F.H. Wigg. (Prim. Fl. Holsat.: 112. 1780) also has this character. Traditionally, the only significant difference between the two genera has been the shape of their sporophores: sporocarpic or plasmodiocarpic in Didymium, but always aethalic in Mucilago. The intended name “Mucilago” was first coined by Micheli (Nov. Pl. Gen.: 216, t. 96. 1729; not validly published, Art. F.1.1), and then used by Battarra (Fungi Arimin.: 76. 1755), Adanson (Fam. Pl. 2: 7. 1763), and Haller (Hist. Stirp. Helv. 3: 110. 1768), none of whom validly published the name because the word “mucilago” is a technical term that was not accompanied by a species name in accordance with binomial nomenclature (Art. 20.2). This generic name was first validly published by Wiggers (l.c.), who included four species names. Of these, only Mucilago crustacea F.H. Wigg. (l.c.) has subsequently been used, and therefore, Mucilago is currently treated as if it were a monospecific genus. Spumaria is another generic name, ascribed to Persoon (in Gmelin, Syst. Nat. 2: 1466. 1792), of which the original type, S. mucilago J.F. Gmel. (l.c.), is an accepted synonym of Mucilago crustacea. Spumaria mucilago is an illegitimate name because Mucor spongiosus Leyss. (Fl. Halens., ed. 2: 305. 1783) was cited as a synonym (Art. 52.1), so the type of Spumaria mucilago is the type of Mucor spongiosus. The latter is unanimously considered conspecific with Mucilago crustacea, a view adopted by García-Martín & al. (in Persoonia 50, in press), who designated types for both Mucor spongiosus and Mucilago crustacea. Molecular studies based on nrSSU (Fiore-Donno & al. in Molec. Phylogen. Evol. 46: 878–889. 2008; Fiore-Donno & al. in J. Eukaryot. Microbiol. 57: 189–196. 2010; Fiore-Donno & al. in PLoS One 7: e35359. 2012; Wrigley de Basanta & al. in Mycologia 107: 157–168. 2015) have repeatedly shown that Mucilago crustacea is placed within the so-called Didymium clade, questioning their separation. This phylogenetic position is confirmed based on extended gene and taxon sampling, and phylogenies based on three (Prikhodko & al. in Mycol. Progr. 22: 11. 2023) and four independent DNA regions (García-Martín & al., l.c.) show M. crustacea firmly and deeply nested within Didymium. Furthermore, the sporophore morphology also does not justify the segregation of Mucilago, first because aethalic morphologies have repeatedly arisen through the evolution of Physarales (García-Martín & al., l.c.), and second because there is already one accepted species of Didymium (D. yulii S.Y. Liu & F.Y. Zhao), not sister-related to M. crustacea, with aethalic sporophores (Zhao & al. in Mycologia 113: 930. 2021). As explained in García-Martín & al. (l.c.), the most appropriate option is to consider Didymium and Mucilago as synonyms to achieve monophyly for the resulting genus. To retain both names as different genera (i.e., to keep the types of each genus in independent and well-supported clades), the genus Didymium should be divided into at least 4–8 independent genera, and even with such splitting the clade containing M. crustacea would still include a mixture of sporocarpic and aethalic species. Besides, the resulting genera would be difficult to diagnose and separate from each other based on phenotypic characters. Therefore, this option is discouraged because it would complicate the taxonomy of Didymium for little or no gain. When treating Didymium, Mucilago, and Spumaria as synonyms, both Mucilago and Spumaria have priority over Didymium (Art. 11.3). Consequently, to preserve the current usage and provide nomenclatural stability by avoiding a plethora of combinations (almost a hundred), the conservation of the generic name Didymium against Mucilago and Spumaria is here proposed as the most straightforward nomenclatural solution, requiring the single combination D. spongiosum (Leyss.) J.M. García-Martín & al., made in García-Martín & al. (l.c.). The name M. crustacea cannot be combined in Didymium due to the existence of the name D. crustaceum Fr. (Syst. Mycol. 3: 124. 1829), whereas the name D. mucilago Prikhodko & al. (l.c.) is not tenable because the final epithet of the next earliest legitimate name at the same rank, Mucor spongiosus, is available, precluding publication of a replacement name (Art. 11.4 final sentence). In addition, the acceptance of this proposal would be most desirable since Didymium has been in use for an exceedingly well-known group of Myxomycetes, for a very long time, in the vast majority of the taxonomic literature (Lister, Monogr. Mycetozoa. 1894; Martin & Alexopoulos, l.c.; Farr in Fl. Neotrop. Monogr. 16. 1976; Nannenga-Bremekamp, Guide Temp. Myxomycetes. 1991; Neubert & al., Myxomyceten 2. 1995; Poulain & al., Myxomycètes. 2011; Clark & Haskins in Asian J. Mycol. 1: 22–62. 2018; and many others). Additionally, it would not preclude the use of Mucilago as a subgenus or section of Didymium, if that option would be considered adequate for the clade containing the type of Mucilago but not the type of Didymium (Art. 14.4). It would also not preclude the use of Mucilago as a genus if subsequent researchers decide to split Didymium, since Mucilago could still be applied to a clade harboring its type but not the type of Didymium (Art. 14.6). Given the current conservation proposal, we also decided to propose D. melanospermum as the conserved type for Didymium for simplicity, since the illegitimate and homotypic name D. farinaceum cannot be used for that species. JCZ, https://orcid.org/0000-0002-9243-2999 JMG-M, https://orcid.org/0000-0002-9526-2673 CL, https://orcid.org/0000-0002-6135-2873 We would like to thank Luis A. Parra for his valuable revision of an earlier version of this proposal, as well as John McNeill and John Wiersema for their crucial nomenclatural and editorial corrections. This research was supported by the Spanish Government through a Ph.D. research grant (BES2012-061641, second author JMG-M), and the Myxotropic Project PID2021-128499NB-I00 (MCIU/AEI/FEDER, UE).