Most classes in the phylum Ascomycota are well studied regarding their biology and systematics. Perhaps, the most poorly studied ascomycete class is Laboulbeniomycetes, which has not received much attention from the broader mycological community and for which no stable large-scale phylogeny exists. The most extensive molecular phylogenetic studies to date are still limited in both locus and taxon sampling. Only one genome is available, and no consortium of researchers is currently established to push the field forward, although steps are being taken to intensify international collaboration.
Three orders are formally recognized within Laboulbeniomycetes, Herpomycetales, Laboulbeniales, and Pyxidiophorales. Both Herpomycetales and Laboulbeniales are unique among related fungi in that they never form mycelia; their ascospores do not form germ tubes but divide mitotically after attachment to an arthropod host to form thalli of up to thousands of cells by determinate growth, and at maturity, structures are produced that form spermatia (antheridia) and ascospores (perithecia).
The Pyxidiophorales order is comprised of fungi that are associated with arthropods in their dispersal phase. They are mycoparasites; they parasitize and feed on the hyphae and sporocarp tissues of other fungi—species of Ascobolus, Asterophora, Fusarium, Inonotus, Lasiobolus. Their single-septate ascospores directly develop a phoretic “Thaxteriola” asexual state. These asexual states produce yeast-like phialoconidia that are transported to new substrates via phoretic mites. Onto the new substrate, These conidia germinate by germ tubes to form mycelia that may produce conidia and eventually perithecia. Sexual reproduction is unknown for several members of the order. For example, the asexual fungus Gliocephalis hyalina was placed within Pyxidiophorales only based on phylogenetic analysis.
The Laboulbeniales order is the most species-rich order, with 2,325 species in 145 genera. These fungi are obligate, microscopic ectoparasites of arthropods; as such they exemplify diverse and environmentally important groups of microfungi. Sometimes colloquially referred to as beetlehangers, Laboulbeniales fungi require an arthropod host for successful development. A bicellular ascospore adheres to a new host and either penetrates the cuticle making contact with the body cavity for nutrition and support or remains superficially attached without obvious penetration. Most species of Laboulbeniales are strictly host specific; whereas other species are habitat specific, having multiple hosts in phylogenetically unrelated groups that occur in the same micro-habitat, such as ant nests and subterranean caves. Laboulbeniales have often been suggested to have no severe effects on their hosts but recent experiments show that Hesperomyces virescens, a common parasite of a globally invasive ladybird, causes up to 50% host mortality. In other words, there may be potential for their use in biological control. No species of Laboulbeniales have been successfully grown in axenic culture. Some growth was obtained with one species of Stigmatomyces on brain-heart infusion agar, although thalli did not reach maturity. Recent attempts using multiple media including insect cuticle have failed.
The third order, Herpomycetales, was only recently erected. Only a single genus is described in the order, Herpomyces, with currently 27 described species. Hosts are exclusively cockroaches (order Blattodea). The genus Herpomyces was traditionally considered an early diverging group of the Laboulbeniales because of morphological features. However, molecular phylogenetic data in combination with morphological, developmental, and host usage traits strongly support its separation from Laboulbeniales. Our most recent molecular phylogenetic results point at an independent origin of the thallus in Herpomycetales and Laboulbeniales. This does make sense from a developmental and morphological point of view, because thalli of Herpomyces and Laboulbeniales are so structurally different.
Laboulbeniomycetes fungi have been overlooked in many aspects with emphasis primarily on Laboulbeniales. The biology, ecology, and within-class relationships are unresolved, and the morphological disparities have not been tracked. Molecular phylogenetic studies have not provided satisfactory support at any taxonomic level. Haelewaters et al. (2019), when describing Herpomycetales, the third order of the class in addition to Laboulbeniales and Pyxidiophorales, based their phylogenetic work on three ribosomal loci because no sequences of protein-coding genes were available. Since that time, Liu et al. (2020) published the first Laboulbeniales sequences of the translation elongation factor 1-α (tef1) gene but, to date, no efforts have been made to generate other protein-coding sequences.
In addition, many representatives of the class have not been sequenced; the vast majority of the class is known from morphology alone, and in many cases only from the type locality. Only 12 of 174 species of Laboulbeniomycetes described between 2010 and 2020 were accompanied by sequence data. The order Laboulbeniales has undergone a successful radiation, and species in the order use a wide variety of arthropod hosts. It is no surprise that many higher-level taxonomic clades are described in the order, although all have been based on developmental and morphological features alone; the few available molecular phylogenetic studies have pointed out that the majority of these higher clades are polyphyletic, pointing out a desperate need of revision.
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