Ontogeny and phylogeny of a Scutellospora heterogama mutant, with implications for morphological recognition of species in Glomeromycota

Celebrating INVAM: 35 years of the largest living culture collection of arbuscular mycorrhizal fungi

The International Culture Collection of (Vesicular-) Arbuscular Mycorrhizal Fungi-INVAM-the largest living culture collection of arbuscular mycorrhizal fungi (AMF) celebrated its 35th year in 2020. The authors record here the mission and goals of INVAM, its contribution as a living culture collection, some historical aspects of INVAM, and describe the advances in mycorrhizology and AMF systematics after INVAM moved to West Virginia University. This commentary emphasizes the importance of a living culture collection to preserve germplasm and to educate and assist researchers in mycorrhizal science.

A combination of morphology and 28S rRNA gene sequences provide grouping and ranking criteria to merge eight into three Ambispora species (Ambisporaceae, Glomeromycota)

Ambispora, the only genus in Ambisporaceae and one of three deeply rooted families in Archaeosporales, Glomeromycetes, is amended. Analysis of the morphology of specimens from types and living cultures and 28S ribosomal DNA (rDNA; LSU) sequences resulted in two major changes that redefined Ambispora to include only species with the potential for spore dimorphism (acaulosporoid and glomoid). First, species described as producing only glomoid spores (Ambispora leptoticha, Ambispora fecundispora, and Ambispora callosa), only acaulosporoid spores (Ambispora jimgerdemannii), or both spore morphotypes (Ambispora appendicula) were synonymized with a redefined dimorphic species, A. leptoticha. LSU sequences and more conserved SSU gene data indicated little divergence between genotypes formerly classified as separate species. Second, Ambispora fennica was synonymized with Ambispora gerdemannii based on morphological and LSU sequence variation equivalent to that measured in the sister clade A. leptoticha. With this analysis, Ambispora was reduced to three species: A. leptoticha, A. gerdemannii, and Ambispora granatensis. Morphological and molecular characters were given equal treatment in this study, as each data set informed and clarified grouping and ranking decisions. The two inner layers of the acaulosporoid spore wall were the only structural characters uniquely defining each of these three species; all other characters were shared. Phenotypes of glomoid spores were indistinguishable between species, and thus were informative only at the genus level. Distinct subclade structure of the LSU gene tree suggests fixation of discrete variants typical of clonal reproduction and possible retention of polymorphisms in rDNA repeats, so that not all discrete genetic variants are indicative of speciation.

An evidence-based consensus for the classification of arbuscular mycorrhizal fungi (Glomeromycota)

The publication of a large number of taxon names at all levels within the arbuscular mycorrhizal fungi (Glomeromycota) has resulted in conflicting systematic schemes and generated considerable confusion among biologists working with these important plant symbionts. A group of biologists with more than a century of collective experience in the systematics of Glomeromycota examined all available molecular-phylogenetic evidence within the framework of phylogenetic hypotheses, incorporating morphological characters when they were congruent. This study is the outcome, wherein the classification of Glomeromycota is revised by rejecting some new names on the grounds that they are founded in error and by synonymizing others that, while validly published, are not evidence-based. The proposed "consensus" will provide a framework for additional original research aimed at clarifying the evolutionary history of this important group of symbiotic fungi.

Phylogenetic reference data for systematics and phylotaxonomy of arbuscular mycorrhizal fungi from phylum to species level

Although the molecular phylogeny, evolution and biodiversity of arbuscular mycorrhizal fungi (AMF) are becoming clearer, phylotaxonomically reliable sequence data are still limited. To fill this gap, a data set allowing resolution and environmental tracing across all taxonomic levels is provided. Two overlapping nuclear DNA regions, totalling c. 3 kb, were analysed: the small subunit (SSU) rRNA gene (up to 1800 bp) and a fragment spanning c. 250 bp of the SSU rDNA, the internal transcribed spacer (ITS) region (c. 475-520 bp) and c. 800 bp of the large subunit (LSU) rRNA gene. Both DNA regions together could be analysed for 35 described species, the SSU rDNA for c. 76 named and 18 as yet undefined species, and the ITS region or LSU rDNA, or a combination of both, for c. 91 named and 16 as yet undefined species. Present phylogenetic analyses, based on the three rDNA markers, provide reliable and robust resolution from phylum to species level. Altogether, 109 named species and 27 cultures representing as yet undefined species were analysed. This study provides a reference data set for molecular systematics and environmental community analyses of AMF, including analyses based on deep sequencing.