Hybrids of RNA viruses and viroid-like elements replicate in fungi.

Earth's life may have originated as self-replicating RNA, and it has been argued that RNA viruses and viroid-like elements are remnants of such pre-cellular RNA world. RNA viruses are defined by linear RNA genomes encoding an RNA-dependent RNA polymerase (RdRp), whereas viroid-like elements consist of small, single-stranded, circular RNA genomes that, in some cases, encode paired self-cleaving ribozymes. Here we show that the number of candidate viroid-like elements occurring in geographically and ecologically diverse niches is much higher than previously thought. We report that, amongst these circular genomes, fungal ambiviruses are viroid-like elements that undergo rolling circle replication and encode their own viral RdRp. Thus, ambiviruses are distinct infectious RNAs showing hybrid features of viroid-like RNAs and viruses. We also detected similar circular RNAs, containing active ribozymes and encoding RdRps, related to mitochondrial-like fungal viruses, highlighting fungi as an evolutionary hub for RNA viruses and viroid-like elements. Our findings point to a deep co-evolutionary history between RNA viruses and subviral elements and offer new perspectives in the origin and evolution of primordial infectious agents, and RNA life.

Transmission electron microscopy of Tuberculina species (Helicobasidiales) reveals an unique mode of conidiogenesis within Basidiomycota.

Tuberculina species represent the asexual life stage of the plant-parasitic sexual genus Helicobasidium. Tuberculina species are distributed all over the world, living in antagonistic symbiosis with over 150 rust species from at least 15 genera. Within the Basidiomycota, besides the spermogonia of rust fungi, only Tuberculina species develop distinct fructifications in the haplophase. However, the knowledge of conidiogenesis in Tuberculina is meagre. Therefore, conidial development in Tuberculina maxima, Tuberculina persicina, and Tuberculina sbrozzii was studied using transmission electron microscopy, and compared to each other as well as to spermatia formation in rust fungi. Significant ultrastructural characteristics such as the movement of nuclei in the process of conidium formation, and formation of the initial and late stages of conidiogenesis are documented. The mode of conidiogenesis of Tuberculina species is unique within the Basidiomycota in that (1) it is realized by haploid fructifications, (2) it is holoblastic, without annellidic proliferation, (3) the nucleus of the conidiogenous cell moves towards the forming conidium, divides, and no daughter nucleus remains inside the conidiogenous cell, and (4) the conidiogenous cell retains only cytoplasmic residues after the development of a single conidium, and a successive conidium is not produced.

Isolations from the redbay ambrosia beetle, Xyleborus glabratus, confirm that the laurel wilt pathogen, Raffaelea lauricola, originated in Asia.

The laurel wilt pathogen Raffaelea lauricola was hypothesized to have been introduced to the southeastern USA in the mycangium of the redbay ambrosia beetle, Xyleborus glabratus, which is native to Asia. To test this hypothesis adult X. glabratus were trapped in Taiwan and on Kyushu Island, Japan, in 2009, and dead beetles were sent to USA for isolation of fungal symbionts. Individual X. glabratus were macerated in glass tissue grinders, and the slurry was serially diluted and plated onto malt agar medium amended with cycloheximide, a medium semiselective for Ophiostoma species and their anamorphs, including members of Raffaelea. R. lauricola was isolated from 56 of 85 beetles in Taiwan and 10 of 16 beetles in Japan at up to an estimated 10 000 CFUs per beetle. The next most commonly isolated species was R. ellipticospora, which also has been recovered from X. glabratus trapped in the USA, as were two other fungi isolated from beetles in Taiwan, R. fusca and R. subfusca. Three unidentified Raffaelea spp. and three unidentified Ophiostoma spp. were isolated rarely from X. glabratus collected in Taiwan. Isolations from beetles similarly trapped in Georgia, USA, yielded R. lauricola and R. ellipticospora in numbers similar to those from beetles trapped in Taiwan and Japan. The results support the hypothesis that R. lauricola was introduced into the USA in mycangia of X. glabratus shipped to USA in solid wood packing material from Asia. However differences in the mycangial mycoflora of X. glabratus in Taiwan, Japan and USA suggest that the X. glabratus population established in USA originated in another part of Asia.

Ophiostoma dentifundum sp. nov. from oak in Europe, characterized using molecular phylogenetic data and morphology.

Previous phylogenetic studies based on ITS sequence data have shown that Ophiostoma species with Sporothrix anamorphs include several species complexes. Isolates from oak in Poland and Hungary, which have previously been referred to as O. stenoceras, as well as isolates morphologically similar to S. inflata formed the basis of this study. Identification was based on sequences for the ITS region of rDNA operon and partial beta-tubulin gene. Analyses showed that isolates from Poland and Hungary reside in a well resolved clade, separate from those in the O. stenoceras-complex. The morphology of these isolates was compared with those of strains in the O. stenoceras complex and S. inflata. Morphological differences in teleomorph and anamorph structures were found between the isolates from Poland and Hungary and those in the O. stenoceras-complex. Growth characteristics and the presence of the teleomorph in culture could be used to separate this fungus from isolates in the S. inflata-complex. The fungus from Poland and Hungary is described here as O. dentifundum sp. nov. It is phylogenetically most closely related to isolates of S. inflata, which represent four well defined groups based on morphology and DNA sequence phylogeny.

Two new Ophiostoma species with Sporothrix anamorphs from Austria and Azerbaijan.

The genus Ophiostoma includes numerous species of primarily insect-vectored, wood-staining fungi. Several anamorph genera that differ in their micronematous or macronematous conidiogenous cells have been associated with Ophiostoma species. Among the former group, Sporothrix is associated with many species and is characterized by conidiogenous cells that arise laterally or terminally from any place on the hyphae and produce nonseptate conidia on sympodially developing denticles. The purpose of this study was to characterize ophiostomatoid isolates with Sporothrix anamorphs recently collected in Austria and Azerbaijan. The isolates were characterized based on comparisons of rDNA and ß-tubulin sequence data. Morphology, growth in culture, and sexual reproductive mode were also considered. Phylogenetic analyses of the combined sequence data showed that the isolates formed two distinct groups, one including isolates from Austria and the other isolates from Austria and Azerbaijan. Growth at 25 C and morphology revealed some differences between the two groups, and supported the view that they represent two new species, which we describe here as Ophiostoma fusiforme sp. nov. and Ophiostoma lunatum sp. nov. Both these groups phylogenetically were related to, but distinct from, Ophiostoma stenoceras.