Cryptic speciation in Orbilia xanthostigma and O. leucostigma (Orbiliomycetes): an aggregate with worldwide distribution

Orbilia xanthostigma, with golden yellow to yellow-orange apothecia and O. leucostigma, with white to very pale rose-lilaceous apothecia, were described by E.M. Fries over 200 years ago. Each of the two taxa, which are not easy to interpret because type material is lacking, was proposed in the past as lectotype of the genus Orbilia. In the here presented circumscription, which follows the current usage, O. xanthostigma is among the most frequently recorded species of the genus, whereas O. leucostigma appears to be much less common. Both grow gregariously on decorticated hygric gymno- and angiosperm wood or rarely bark and show a worldwide distribution. They are characterised by minute, strongly curved, warted ascospores and a dicranidion-like anamorph. Except for apothecial colour, there are no other notable morphological differences between them, either in the teleomorph (asci, ascospores, paraphyses) or anamorph (conidiophores, conidia). Because of their strong similarities, the two taxa have often been treated as infraspecific variants (subspecies, varieties) of a single species or even as synonyms. In order to overcome the ambiguities associated with the two names, O. delicatula, a name proposed by P.A. Karsten for a collection with golden yellow apothecia, was suggested by B. Spooner as a replacement name for them. The present study reveals unexpectedly high ITS and LSU variation within a morphologically extremely homogeneous group, representing over 16 more or less invariable genotypes whenever more than one sample with a sequence was available. ITS and LSU rDNA data from European (Luxembourg, Germany, Ukraine) and Macaronesian (Tenerife) collections suggest that the two colour variants represent two distinct species with a 16.5% ITS and 3.5% LSU D1–D2 distance, but very low infraspecific variation (0.2% ITS, 0% LSU). A sample with scattered yellow apothecia from Luxembourg on a xeric branch deviates from typical European-Macaronesian O. xanthostigma by 4% (ITS) and 0.7% (LSU). Further available sequences from samples from Asia, New Zealand and North America with mainly yellow apothecia clustered in various other clades that represent further distinct genotypes. In the absence of morphological characteristics, none of these genotypes are given names pending further investigation. Only two of these genotypes are sufficiently distinct to be recognised morphologically: O. aureocrenulata from tropical, Middle and South America, with golden yellow apothecia with a crenulate margin and prominent stipe, and O. xanthoflexa from temperate, northeastern North America with yellow sessile apothecia with a smooth margin and larger, less curved, smooth-walled ascospores.