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Xylariales

Order of fungi

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Taxonomy

Early classifications of the Xylariales varied considerably, with taxonomists recognising anywhere from three to eleven families within the order. A milestone in understanding the order's composition came from Smith, Liew, and Hyde's 2003 molecular phylogenetics study, which established the Xylariales as a monophyletic group containing seven families: Amphisphaeriaceae, Apiosporaceae, Clypeosphaeriaceae, Diatrypaceae, Graphostromataceae, Hyponectriaceae, and Xylariaceae.

During this period, fungi in the order were characterised by several common morphological features. They typically possessed well-developed stromata (compact fungal tissues) and perithecial ascomata (flask-shaped fruiting bodies) with thick walls. Their asci, which contained eight spores, featured a distinctive apical structure that stained blue in iodine (a J+ reaction), and their paraphyses (specialised sterile filaments) were free at the tips and originated from the hymenium. The ascospores were often pigmented, with germ pores or slits, and sometimes had transverse divisions or a jelly-like coating. Their asexual forms (anamorphs) were mainly hyphomycetous, producing spores through a simple, budding process called holoblastic conidiation.

Historical taxonomic debates centred around several key issues. The Myelospermataceae, which was tentatively included by some authors in the early 2000s, lacked sufficient molecular evidence for definitive placement. Earlier proposals to elevate certain groups to ordinal status (such as Amphisphaeriales and Diatrypales) were rejected based on molecular evidence. The Apiosporaceae's inclusion, formally proposed in 1998, represented one of the newer additions to the order at the time.

The early molecular studies of the 2000s, while groundbreaking in establishing the monophyly of the order, also revealed limitations in using ribosomal DNA sequences for resolving family-level relationships within the Xylariales. This suggested either a relatively recent evolution of the group or slower evolutionary rates compared to other fungal orders. The relatively low genetic variation observed in commonly used molecular markers presented challenges for understanding the precise relationships between families, leaving many questions about internal phylogenetic structure unresolved. More recent molecular studies have revised and enhanced the understanding of relationships within the order, such as the 2018 study that revised the families Graphostromataceae, Hypoxylaceae, Lopadostomataceae and Xylariaceae. DNA analysis in 2018 confirmed the placement of the order and subclass, as sister to the order Amphisphaeriales in the subclass Xylariomycetidae.

The Xylariales represents a morphologically diverse group of fungi characterised by their complex stromatic structures and ecological roles as endophytes, pathogens, or saprobes. Molecular phylogenetic analyses suggest that Xylariales diverged from the related order Amphisphaeriales approximately 154 million years ago (MYA), during the Jurassic period, with the crown group of Xylariales estimated to have appeared around 147 MYA. This evolutionary timeline aligns with important diversification events, including the development of novel stromatic forms and pigmentation in ascospores, traits that distinguish many Xylariales species.

Ancestral state reconstruction studies indicate that early members of Xylariales likely had inconspicuous, immersed ascomata with hyaline, septate ascospores lacking germ slits. Over time, these ancestral traits evolved into the more conspicuous and diverse stromatic structures observed in extant species, reflecting adaptations to varied ecological niches. Today, Xylariales is a highly polyphyletic group, encompassing families with considerable morphological and ecological diversity. Ongoing phylogenetic research continues to refine the taxonomy of the order, revealing previously unrecognised lineages and resolving uncertainties about its classification.

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