<i>Arcyria</i> and allied genera: taxonomic backbone and character evolution

<i>Arcyria</i>, one of the most diverse genera of bright-spored myxomycetes, has recently been identified as polyphyletic within the paraphyletic family <i>Arcyriaceae sensu lato</i>. However, due to the high variability and inconsistency of morphological traits in <i>Trichiales</i>, no taxonomic decision has yet been proposed to rectify this situation. A thorough revision of the genus is necessary. To address this, we present results of the extensive taxonomic and geographic sampling of the genus by incorporating sequences of two marker genes from 192 specimens of <i>Arcyria</i> spp. from 19 countries and six continents into the existing three gene phylogenetic backbone of <i>Trichiales</i>. Our phylogenies decisively confirm the polyphyletic nature of <i>Arcyria</i>, with a considerable part of the genus forming a sister clade to <i>Hemitrichia</i>. Consequently, we erect here the new family <i>Hemitrichiaceae</i> and resurrect the genus <i>Heterotrichia</i> to accommodate the second major branch of former <i>Arcyria</i> species. Additionally, we describe the new genus <i>Spiromyxa</i> with intermediate phylogenetic position and unique combination of morphological traits. Furthermore, we show that most of the <i>Arcyria</i> morphospecies included into this study are either species complexes or para- or polyphyletic taxa. Traits, previously thought to delimit the genus <i>Arcyria</i>, show little consistency with the phylogeny. In particular, the expanding, net-like capillitium, attached to the cup or stalk, appears to have been present in the ancestor of three families of <i>Trichiales</i>, afterwards mostly persisted in two of them and evolved into something else in the third. Such traits as ornamentation of capillitium, cup and spores retain their taxonomic value primarily at the species level. However, the colour of the spore mass in fresh condition partially aligns with our phylogenetic results: most of the yellow "<i>Arcyria</i>" species cluster within <i>Heterotrichia</i>, while red and grey species sort mostly into separate subclades within the clade incorporating the remaining species of the genus <i>Arcyria</i>. <b>Citation:</b> Yatsiuk I, Leontyev D, Schnittler M, Ehlers T, Mikryukov V, Kõljalg U (2025). <i>Arcyria</i> and allied genera: taxonomic backbone and character evolution. <i>Fungal Systematics and Evolution</i> <b>15</b>: 97-118. doi: 10.3114/fuse.2025.15.04.