Evaluation of genome size and quantitative features of the dolipore septum as taxonomic predictors for the Serendipita 'williamsii' species complex

Despite multiple taxonomic revisions, several uncertainties at the genus and species level remain to be resolved within the Serendipitaceae family (Sebacinales). This volatile classification is attributed to the limited number of available axenic cultures and the scarcity of useful morphological traits. In the current study, we attempted to discover alternative taxonomic markers not relying on DNA sequences to differentiate among the closely related members of our Congolese Serendipita isolate collection and the reference strains S. indica (syn. Piriformospora indica) and S. williamsii (syn. P. williamsii). We demonstrated that nuclear distribution across hyphal cells and genome size (determined by flow cytometry) did not have enough resolving power, but quantitative and qualitative variations in the ultrastructure of the dolipore septa investigated by transmission electron microscopy did provide useful markers. Multivariate analysis revealed that subtle differences in ultrastructural characteristics of the parenthesome and the attached endoplasmic reticulum are most relevant when studying this fungal group. Moreover, the observed clustering pattern showed that there might be more diversity amongst the Congolese isolates within the S. 'williamsii' species complex than previously anticipated based on molecular data. Altogether, our results provide novel perspectives on the use of integrative approaches to support sebacinoid and Serendipitaceae taxonomy.

When the ribosomal DNA does not tell the truth: The case of the taxonomic position of Kurtia argillacea, an ericoid mycorrhizal fungus residing among Hymenochaetales

The nuclear ribosomal DNA (nuc-rDNA) is widely used for the identification and phylogenetic reconstruction of Agaricomycetes. However, nuc-rDNA-based phylogenies may sometimes be in conflict with phylogenetic relationships derived from protein coding genes. In this study, the taxonomic position of the basidiomycetous mycobiont that forms the recently discovered sheathed ericoid mycorrhiza was investigated, because its nuc-rDNA is highly dissimilar to any other available fungal sequences in terms of nucleotide composition and length, and its nuc-rDNA-based phylogeny is inconclusive and significantly disagrees with protein coding sequences and morphological data. In the present work, this mycobiont was identified as Kurtia argillacea (= Hyphoderma argillaceum) residing in the order Hymenochaetales (Basidiomycota). Bioinformatic screening of the Kurtia ribosomal DNA sequence indicates that it represents a gene with a non-standard substitution rate or nucleotide composition heterogeneity rather than a deep paralogue or a pseudogene. Such a phenomenon probably also occurs in other lineages of the Fungi and should be taken into consideration when nuc-rDNA (especially that with unusual nucleotide composition) is used as a sole marker for phylogenetic reconstructions. Kurtia argillacea so far represents the only confirmed non-sebacinoid ericoid mycorrhizal fungus in the Basidiomycota and its intriguing placement among mostly saprobic and parasitic Hymenochaetales begs further investigation of its eco-physiology.

Conservation of cytoplasmic organization in the cystidia of Suillus species

Cystidia of Suillus americanus and S. granulatus (Boletales) were examined cytochemically and ultrastructurally with cells prepared by freeze substitution. We present the first study showing ultrastructural details and cytological functions of the cystidium to be conserved in two closely related species. The results are presented for inclusion in the AFTOL Structural and Biochemical Database <http://aftol.umn.edu> to aid in the application of morphological characters to phylogenetic studies. The cystidia of these Suillus species appear to be united by a series of conserved characters, including specialized secretion mechanisms, smooth tubular endoplasmic reticulum and abundant free ribosomes. The conservation of these subcellular traits among members of this genus suggests that ultrastructural details of cystidia may provide a suite of phylogenetically informative characters. Inclusion of such characters in phylogenetic analyses might resolve or provide support for monophyletic groups at the level of family or genus.

Automated electron tomography of the septal pore cap in Rhizoctonia solani

Dolipore septa and septal pore caps (SPCs) in filamentous basidiomycetes may play an important role in maintaining the integrity of hyphal cells. We have investigated the ultrastructure of the dolipore septum and the SPC in Rhizoctonia solani hyphal cells after high-pressure freezing, freeze substitution, and Spurr embedding. We visualized the SPC with associated cell ultrastructures in three dimensions by automated electron tomography of thick-sectioned cells, followed by 3D tomographic reconstructions. Using these methods we were able to document the passage of mitochondria through the SPC, small tubular membranous structures at the entrance of the septal pore channel, filamentous structures connecting the inner side of the SPC with pore-plugging material, thin filaments anchoring the pore-plugging material with the plasma membrane, small vesicles attached to the plugging material, and tubular endoplasmic reticulum continuous with the base of the SPC. We hypothesize that the SPC, the filamentous structures, the plugging material, and the endoplasmic reticulum act in a coordinated fashion to maintain cellular integrity, intercellular communication, and the transport of solutes and cell organelles in the filamentous fungus R. solani.

Ultrastructure of the dikaryotic form of Cyathus bulleri Brodie

The fine structure of the dikaryotic form of Cyathus bulleri Brodie was generally found to be similar to that of other hyphal forms of the Basidiomycetes. The nuclear walls were doubled, porous and in some cases connected to the endoplasmic reticulum. The presence of typical as well as filamentous and U-shaped mitochondria was confirmed. Other cellular structures and organelles, among them vacuoles, vesicular and myelinoid-like bodies, often associated with the cell membranes, glycogen and ribosomes were also observed in the cytoplasm. The presence of the dolipore/parenthesome apparatus and clamp connections typical of the Basidiomycetes was established.