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Gazis & Chaverri (2012)
Gazis & Chaverri (2012)
Xylonomycetes is a monotypic class, belonging to the Phylum Ascomycota (Fungi), recently described based on the species Xylona heveae. Strains of this species were isolated mainly as sapwood endophytes of Hevea brasiliensis (rubber tree) distributed in remote areas of the Peruvian Amazon. Six molecular markers were sequenced to place this new lineage within the superclass 'Leotiomyceta'. However, its evolutionary relationships with the other Ascomycota classes remained unresolved at the time this novel taxon was introduced. Only the morphology of its anamorphic state is known because in vitro crosses among different strains were not successful and no teleomorph could be obtained. Based on its restricted distribution and specialized substrate, it has been suggested that Xylona may represent a true endophytic lineage since no close relatives have been found as saprotrophs or plant pathogens, not even from environmental surveys. In marked contrast to many foliar endophytes, all strains from Xylona heveae failed to degrade the plant polymers cellulose and lignin in vitro, suggesting that this species might use different pathways to obtain nutrients from their hosts. In 2013 the genome and transcriptome of Xylona heveae were sequenced as part of the 1,000 fungal genome project sponsored by the DOE Joint Genome Institute and it can be explored through the MycoCosm portal (http://genome.jgi.doe.gov/Xylhe1/Xylhe1.home.html). Preliminary results are already shedding light into Xylona’s evolutionary history and its potential ecological role within the ecosystem.
Etymology Xylonomycetes comes from the Greek “xylon” =wood, referred to the most common substrate from which the type species was isolated from, and the Greek ‘mykes” = fungus. The genus Xylona comes from the Greek “Xylona” = from the forest and the epithet heveae refers to the name of the host (Hevea) from which the type species was isolated.
Colonies growing in Potato Dextrose Agar (PDA) or Malta Extract Agar (MEA) at first are white but change to grayish sepia to fuscous black with time (A-D). Mycelium grows mostly superficial and is composed by smooth branching hyphae that sometimes become anastomose. Pycnidia are produced along the entire mycelial mat but they are produced in greater number at the edges of the colony. After ca. 3 weeks cultures acquire a vinaceous color due to a diffusing pigment produced by the strains, especially at the growing margins of the colony.
Conidiomata are pycnidial, immersed when young, fuscous black, subspherical, and entirely closed (ostiole absent). Pycnidial wall composed of thin-walled cells forming tissue textura angularis (E). Conidial masses are liberated through the apical rupture of the pycnidia (F). Chlamydospores are rarely produced (G). Conidiophores are absent. Conidiogenous cells are enteroblastic, phialidic, discrete, hyaline and presented one collarette (H). Conidia are apically rounded with two lateral obtuse projections appearing heart-shaped (narrower and truncated at base), hyaline when young, turning dark brick when mature, aseptate, thick-walled, smooth and guttulated (I-J).
Ecology and Distribution
The majority of the strains reported in the original description of Xylona heveae were isolated from living sapwood of Hevea brasiliensis (K-L), but few strains were also isolated from healthy leaf tissue. The ecological role of Xylona heveae within its habitat and the relationship with its host remain unknown. By performing cellulase and ligninase activity assays it was determined that this species was not capable of breaking down cellulose or lignin, suggesting that Xylona might not able to switch from an endophytic to a saprotrophic habit. It also remains unknown how strains of Xylona heveae infect their hosts. Typically, horizontally transmitted endophytes can infect their hosts by entering the leaf tissue through the stomata or directly by piercing the leaf's cuticle. However, the mode of transmission of Xylona heveae is still unknown. The fact that it was isolated from both, leaves and sapwood of Hevea, suggests that this fungus can enter their host through the leaves and then migrate to the trunk through the vascular system, or enter through the root system and then migrate to the crown of the tree. Isolates of Xylona heveae were mainly collected from rubber plantations in central Peru (San Martin), but also from wild populations of Hevea brasiliensis and Hevea guianensis in North East(Loreto) and South East (Madre de Dios) Peru. These sites differed in management regime, elevation, land use history among other environmental factors, suggesting that Xylona may occur over a wide range of conditions.
Based on six markers, commonly used in fungal systematics (nucLSU, nucSSU, 5.8S, mitSSU, RPB1 and RPB2), Xylona heveae was inferred to belong within the super class ‘Leotiomyceta’ (Pezizomycotina: Ascomycota). Nevertheless, the information obtained from these markers was not enough to resolve its exact position (M). Morphology was not useful for determining to which group Xylonomycetes is more closely related since few morphological characters were available and these were shared by several other classes within the 'Leotiomyceta' (e.g., conidiomata producing pycnidial anamorph, non-ostiolate pycnidium, enteroblastic phialidic conidiogenesis).
Xylona’s predicted unique genomic content and expression data could shed light into our understanding of the mechanisms behind endophytism. In addition, Xylona heveae as the only representative of a highly distinct group of Ascomycota, might have novel enzymes and secondary metabolites of interest in bioprospection. The genome of Xylona heveae is small (24 Mbp)in comparison to other fungal species representing closely related groups. Genomic data has contributed with additional molecular characters, helping resolve its position and relationship with other Ascomycota classes (N).
- Gazis et al. 2012
- Grigoriev et al. 2014
1. Gazis, R., Miadlikowska, J., Lutzoni, F., Arnold, A. E., & Chaverri, P. (2012). Culture-based study of endophytes associated with rubber trees in Peru reveals a new class of Pezizomycotina: Xylonomycetes. Molecular Phylogenetics and Evolution 65: 294-304.
2. Grigoriev, Igor V., Roman Nikitin, Sajeet Haridas, Alan Kuo, Robin Ohm, Robert Otillar, Robert Riley et al. (2014). MycoCosm portal: gearing up for 1000 fungal genomes. Nucleic acids research 42, no. D1: D699-D704.