Character evolution of modern fly-speck fungi and implications for interpreting thyriothecial fossils

摘要

Premise Fossils show that fly-speck fungi have been reproducing with small, black thyriothecia on leaf surfaces for ~250 million years. We analyzed morphological characters of extant thyriothecial fungi to develop a phylogenetic framework for interpreting fossil taxa. Methods We placed 59 extant fly-speck fungi in a phylogeny of 320 Ascomycota using nuclear ribosomal large and small subunit sequences, including newly determined sequences from nine taxa. We reconstructed ancestral character states using BayesTraits and maximum likelihood after coding 11 morphological characters based on original observations and literature. We analyzed the relationships of three previously published Mesozoic fossils using parsimony and our morphological character matrix, constrained by the molecular phylogeny. Results Thyriothecia evolved convergently in multiple lineages of superficial, leaf-inhabiting ascomycetes. The radiate and ostiolate scutellum organization is restricted to Dothideomycetes. Scutellum initiation by intercalary septation of a single hypha characterizes Asterinales and Asterotexiales, and initiation by coordinated growth of two or more adjacent hyphae characterizes Aulographaceae. Scutella in Microthyriales are initiated apically on a lateral hyphal branch. Patterns of hyphal branching in scutella contribute to distinguishing among orders. Parsimony resolves three fossil taxa as Dothideomycetes; one is further resolved as a member of a Microthyriales-Zeloasperisporiales clade within Dothideomycetes. Conclusions This is the most comprehensive systematic study of thyriothecial fungi and their relatives to date. Parsimony analysis of the matrix of character states of modern taxa provides an objective basis for interpreting fossils, leading to insights into morphological evolution and geological ages of Dothideomycetes clades.