Orbilia fimicola, a nematophagous discomycete and its Arthrobotrys anamorph

Morphological and molecular characterization of Orbilia pseudopolybrocha and O. tonghaiensis, two new species of Orbiliaceae from China

This study examined over 20 fungal specimens of the Orbiliaceae (Orbiliomycetes) from different regions in China. Our analyses based on morphological traits and the ITS rDNA sequences revealed two new Orbilia species with drechslerella-like asexual morphs. These new species are able to trap nematodes with constricting rings. In addition, Orbilia cf. orientalis is reported as a new cryptic Chinese variant of European collections of O. orientalis. All three species are described and illustrated in detail in this paper. Their phylogenetic relationships with other orbiliaceous species were identified based on their ITS sequences.

Nematode-Trapping Fungi

Nematode-trapping fungi are a unique and intriguing group of carnivorous microorganisms that can trap and digest nematodes by means of specialized trapping structures. They can develop diverse trapping devices, such as adhesive hyphae, adhesive knobs, adhesive networks, constricting rings, and nonconstricting rings. Nematode-trapping fungi have been found in all regions of the world, from the tropics to Antarctica, from terrestrial to aquatic ecosystems. They play an important ecological role in regulating nematode dynamics in soil. Molecular phylogenetic studies have shown that the majority of nematode-trapping fungi belong to a monophyletic group in the order Orbiliales (Ascomycota). Nematode-trapping fungi serve as an excellent model system for understanding fungal evolution and interaction between fungi and nematodes. With the development of molecular techniques and genome sequencing, their evolutionary origins and divergence, and the mechanisms underlying fungus-nematode interactions have been well studied. In recent decades, an increasing concern about the environmental hazards of using chemical nematicides has led to the application of these biological control agents as a rapidly developing component of crop protection.

Phylogenic analysis of adhesion related genes Mad1 revealed a positive selection for the evolution of trapping devices of nematode-trapping fungi

Adhesions, the major components of the extracellular fibrillar polymers which accumulate on the outer surface of adhesive traps of nematode-trapping fungi, are thought to have played important roles during the evolution of trapping devices. Phylogenetic analyses based on the genes related to adhesive materials can be of great importance for understanding the evolution of trapping devices. Recently, AoMad1, one homologous gene of the entomopathogenic fungus Metarhizium anisopliae cell wall protein MAD1, has been functionally characterized as involved in the production of adhesions in the nematode-trapping fungus Arthrobotrys oligospora. In this study, we cloned Mad1 homologous genes from nematode-trapping fungi with various trapping devices. Phylogenetic analyses suggested that species which formed nonadhesive constricting ring (CR) traps more basally placed and species with adhesive traps evolved along two lineages. Likelihood ratio tests (LRT) revealed that significant positive selective pressure likely acted on the ancestral trapping devices including both adhesive and mechanical traps, indicating that the Mad1 genes likely played important roles during the evolution of nematode-trapping fungi. Our study provides new insights into the evolution of trapping devices of nematode-trapping fungi and also contributes to understanding the importance of adhesions during the evolution of nematode-trapping fungi.

PCR primers with enhanced specificity for nematode-trapping fungi (Orbiliales)

Nematode-trapping fungi, a monophyletic lineage within the Orbiliales (Ascomycota), use specialized structures to capture and consume nematodes in soil, leaf litter, and other substrates. These fungi have been studied both because of their unique predatory life history and because they are potential control agents of important plant- and animal-parasitic nematodes. Ecological studies of nematode-trapping fungi have primarily used culture-based methods, but molecular detection techniques are now available and should be useful for studying this group. We developed Orbiliales-specific PCR primers for the ITS and 28s rDNA to directly detect nematode-trapping fungi without culturing and also to screen fungal isolates for phylogenetic placement in the Orbiliales. We used these primers to selectively amplify, clone, and sequence Orbiliales DNA extracted from soil, litter, and wood, and we compared the results of molecular detection with those obtained using a culture-based method. Of the eight species of nematode-trapping Orbiliales detected with the culture-based assay, only three were detected with PCR. The molecular assay, however, detected 18 species of uncultured Orbiliales, many of which are closely related to nematode-trapping fungi and fungal parasites of nematode eggs. Our results suggest that the combined use of Orbiliales-specific primers and culture-based techniques may benefit future studies of nematophagous fungi.

Orbilia quercisp. nov. and its knob-forming nematophagous anamorph

Orbilia querci, a new nematode-trapping fungus, was found on rotten wood of Quercus sp. in Huai-rou County, Beijing, China. It is characterized by having a tear-shaped spore body in the cylindrical ascospore. Pure culture was obtained from the ascospores. Conidiophores were simple or occasionally branched, bearing a single conidium on the tip. Conidia were spindle-shaped, mostly with 3-septa. Nematodes were captured by means of adhesive stalked knobs. The adhesive knobs were produced frequently on nutritional agar plates even in the absence of challenging nematodes. Its anamorph is placed in Dactylellina and named as D. querci. The sequence divergence of the ITS1 region between the fungus and the other knob-forming species tested was 23.8-33.4%, supporting O. querci as a distinct species. This is the first report of the connection between a knob-forming nematophagous hyphomycete and an Orbilia teleomorph.

Growth of Arthrobotrys superba from a birch wood resource base into soil determined by radioactive tracing

The ability of a nematode-trapping fungus to establish in field soil is an important characteristic when considering its use as a biological control agent. The outgrowth of the nematode-trapping fungus Arthrobotrys superba from wood was recorded by labelling the fungus with [(14)C]3-O-methylglucose and [(32)P]orthophosphoric acid and by using the soil sprinkling method. The fungus reached a distance of 7-8 cm during 25 days in heat-treated (60 degrees C) soil, detected by either radioactive tracing or the soil sprinkling technique. The two labelled compounds were co-distributed at all sampling times (r(2)=0.946) which indicates that the glucose pool (as methylglucose) and phosphorus content were correlated throughout the mycelium. In natural, non-heat-treated soil the fungus reached a distance of 1.5 cm from one disc of birch wood after 30 days, while it reached 3.2 cm during the same period when the food base was a pile of five inoculated discs. The experiments showed, for the first time, that a nematophagous fungus, A. superba, can grow out into soil from a piece of wood and supported by nutrients translocated from the resource base to the edge of the mycelium.

How natural should anamorph genera be?

The taxonomy of anamorphic fungi has always been artificial, as a concession to the practical needs of identifying and naming important organisms, because of insufficient characters indicative of a more natural classification. Integration of anamorph taxa into a teleomorph classification is best served by retaining anamorph names for fungi that have no teleomorph fructification. Proposals to split more or less heterogeneous anamorph genera such as Aspergillus, Fusarium, Acremonium, Verticillium, and Gliocladium into more natural units have prompted this assay. While subdivision of these genera can be defended only partly, some nematophagous genera with affinities to Arthrobotrys or Monacrosporium are considered for lumping, to achieve a more natural classification. This raises the question of to what extent anamorph genera can be delimited naturally and how far they must reflect teleomorph associations. A consistent application of natural genus concepts for anamorphic fungi, even if that were possible, would completely upset nomenclature. While the segregation of morphologically divergent, obviously unrelated taxa from an anamorph genus is defendable, some admittedly deviating taxa (different teleomorphs) should be tolerated in somewhat artificial genera for the sake of identification. Conversely, lumping all anamorphs associated with one teleomorph genus into one genus is not supported when the criteria used for identification clearly favour splitting; otherwise it would render identification according to morphological criteria impossible. Therefore classification of anamorph genera cannot aim at genera that adequately reflect natural relationships and the most convenient generic delimitation must be considered for individual cases. Key words: anamorph, teleomorph, classification, nomenclature, Deuteromycetes, pleomorphism.