Gigaspora Gerd. & Trappe Mycol. Mem. 5: 25 (1974)

Index Fungorum number: IF 20239; Mycobank number: MB 20239

Notes – Originally, Gigaspora was erected with five species (G. calospora (T.H. Nicolson & Gerd.) Gerd. & Trappe, G. coralloidea Trappe, Gerd. & I. Ho, G. gigantea (T.H. Nicolson & Gerd.) Gerd. & Trappe, G. gilmorei Trappe & Gerd., G. heterogama (T.H. Nicolson & Gerd.) Gerd. & Trappe) in Endogonaceae, which also comprised saprotrophic and ectomycorrhizal fungi, e.g., Endogone species (Gerdemann and Trappe 1974). The type species of Gigaspora is G. gigantea, which was originally described as Endogone gigantea T.H. Nicolson & Gerd. (Nicolson and Gerdemann 1968). The unique morphological characteristic linking the five species was the formation of spores at the top of a subtending hypha called sporogenous cell or bulbous suspensor-like cell. However, spores of G. gigantea had only one three–layered spore wall, while spores of the other four species had two or three spore walls. Moreover, G. gigantea spores germinated from germ warts, present on the inner surface of the third spore wall layer, and produced auxiliary cells ornamented with spines in the extraradical mycelium. Instead, the other species produced germination hyphae from a plate-like structure, called germination shield, formed on the upper surface of the innermost spore wall, and knobby auxiliary cells in the extraradical mycelium. Therefore, Walker and Sanders (1986) transferred the latter species to a new genus, Scutellospora, in Endogonaceae. Morton and Benny (1990) accommodated Gigaspora and Scutellospora in a new family, Gigasporaceae, established in a new order, Glomales, orthographically corrected into Glomerales (Schüßler et al. 2001), which included only fungi producing (or hypothetically producing) arbuscular mycorrhiza. Schüßler et al. (2001) transferred Gigasporaceae to a new order, Diversisporales, in a newly established phylum, Glomeromycota. Oehl et al. (2008) retained only Gigaspora in Gigasporaceae and distributed the known Scutellospora species in three new families with five new genera, which later were placed in Gigasporales, a new order introduced into Glomeromycota (Oehl et al. 2008, 2011a, 2011b; Goto et al. 2012). Differently from most Glomeromycota species, members of Gigasporales do not produce intraradical vesicles.

Currently, Gigaspora comprises eight species: G. albida N.C. Schenck & G.S. Sm., G. candida Bhattacharjee, Mukerji, J.P. Tewari & Skoropad, G. decipiens I.R. Hall & L.K. Abbott, G. gigantea, G. margarita W.N. Becker & I.R. Hall, G. polymorphira L.M. Yao, G.Y. Tao & L. Jiang, G. ramisporophora Spain, Sieverd. & N.C. Schenck, and G. rosea T.H. Nicolson & N.C. Schenck. Three other species have been described, but two do not possess the characteristics of Gigaspora (G. lazzarii Montecchi, Ruini & G. Gross— Montecchi et al. 1996 and G. tuberculata Neeraj, Mukerji, B.C. Sharma & A.K. Varma—Neeraj et al. 1993), and one, G. alboaurantiaca W.N. Chou, was synonymized with G. candida (Schüßler and Walker 2010). However, the latter synonymization is incongruent as G. alboaurantiaca description clearly states that the sporogenous cell is pale brown (Chou et al. 1991), while it is white in G. candida (Bhattacharjee et al. 1982) (Fig. 1).

Figure 1 –  Phylogram generated from Maximum Likelihood (ML) and Bayesian Inference (BI) analyses displaying the phylogenetic relationships between the Gigaspora siqueirae (in bold, blue) and members of the Gigasporaceae and Intraornatosporaceae. The family Dentiscutataceae, as sister to the clade Gigasporaceae–Intraornatosporaceae, was used to root the consensus tree. The trees were inferred using a dataset that includes concatenated sequences divided into five partitions (18S: 1–1786, ITS1: 1787–1883, 5.8S: 1884-2042, ITS2: 2043–2251, 28S: 2252–3047). For species in the Intraornatosporaceae only partial 28S sequences were available. Overall, the dataset included 15 species in six genera represented by 55 sequences. Sequence alignment was performed using MAFFT 7.243 (Katoh et al. 2019), strategy E-INS-i. In both ML and BI analyses, GTR+I+G was chosen as a nucleotide substitution model for each nucleotide partition (Abadi et al. 2019). The ML tree was estimated using RAxML-NG 1.0.1 (Kozlov et al. 2019), with a maximum likelihood/1000 bootstrapping run, and ML estimated proportion of invariable sites and base frequencies. In the BI analysis, four Markov chains were run over ten million generations in MrBayes 3.2 (Ronquist et al. 2012), sampling every 500 generations, with a burn-in at 30% of sampled trees. All parameters of the convergence diagnostic (Potential Scale Reduction Factor) indicated that convergence was obtained (Ronquist et al. 2012; Miller et al. 2015). The tree topology obtained from the ML analysis was identical to that generated in the BI analysis. Support values and posterior probabilities greater than 60% and 0.95, respectively, are indicated above or below the nodes. The bar indicates 0.005 expected change per site per branch

Species