Extending the fossil record for foliicolous Dothideomycetes: Bleximothyrium ostiolatum gen. et sp. nov., a unique fly‐speck fungus from the Lower Cretaceous of Virginia, USA

Abstract

Fossils can reveal long-vanished characters that inform inferences about the timing and patterns of diversification of living fungi. Through analyzing well-preserved fossil scutella, shield-like covers of fungal sporocarps, we describe a new taxon of early Dothideomycetes with a combination of characters unknown among extant taxa. METHODS: Macerated clays from the Potomac Group, lower Zone 1, from the Lower Cretaceous (Aptian, 125-113 Ma) of Virginia USA yielded one gymnospermous leaf cuticle colonized by 21 sporocarps of a single fungal morphotype. We inferred a tree from nuclear ribosomal DNA of extant species, and coded morphological characters to evaluate alternative, equally parsimonious placements of the fossil in a molecular constraint tree of extant species. RESULTS: Bleximothyrium ostiolatum gen. et sp. nov. has an ostiolate scutellum of radiate, dichotomizing hyphae. Unlike otherwise similar extant and fossil taxa, B. ostiolatum has tangled hyphae at its scutellum margin. Scutella of B. ostiolatum are connected to superficial mycelium, to intercalary and lateral appressoria, and to extensive subcuticular "mycélium en palmettes". The gymnospermous host has characters consistent with identity as a non-papillate ginkgophyte or cycad. CONCLUSIONS: Bleximothyrium ostiolatum is the oldest known fossil fly-speck fungus that occurs on plant cuticles and has the radiate, ostiolate scutellum known only from Dothideomycetes. Its combination of characters, its scutellum margin, and mycélium en palmettes are unknown in other extant and fossil species, and Bleximothyrium ostiolatum likely represents a new group of fly-speck fungi that may now be extinct.

Novel epiphytic root-fungus symbiosis in the Indo-Pacific seagrass Thalassodendron ciliatum

“…On the other hand, the mycobionts of both seagrasses form extensive hyphal mantles on the root surface (Vohník, 2022;Vohník et al, 2017), this study) that are morphologically identical to those formed by DSE and certain ectomycorrhizal (EcM) fungi on the roots of compatible terrestrial plants (e.g., Kaldorf et al, 2004). Intriguingly, similar structures called mycélium en palmettes (Ducomet, 1907) are formed by some foliicolous Dothideomycetes on the leaf and twig cuticle where the respective mycobionts eventually form thyriothecia (Le Renard et al, 2021). However, these have not been detected on the roots investigated here.…”

“…3A). The root surface of darker/older roots was often densely colonized by DS fungal hyphae that formed discontinuous hyphal mantles resembling the pseudoparenchymatous nets formed by the terrestrial DSE B) or "mycélium en palmettes" (Ducomet, 1907;Le Renard et al, 2021). These typically originated from individual hyphae growing more or less linearly on the roots surface but later starting to produce shorter isodiametric cells that radially spread around (Figs 4A, B).…”