The rust fungi (Pucciniales) that infect ferns, early diverging vascular plants, are neither “primitive” nor monophyletic, as once hypothesized. The neotropical fern pathogen, Puccinia lygodii (Pucciniaceae), specializes on species of Lygodium. Lygodium is believed to have evolved in a period ca. 211 mya, which is after the evolution of the temperate fern rust fungi that parasitize later diverged ferns. Puccinia lygodii is the only rust species in the genus Puccinia known to infect ferns, the majority of which infect flowering plants. In this study we examined multiple new and herbarium specimens of P. lygodii and reconstructed its phylogenetic history with data generated from the 28S nuclear rDNA repeat. Puccinia lygodii is the sister species to another neotropical fern rust, Desmella aneimiae (Pucciniaceae), which also infects early diverged leptosporangiate fern species, and the new combination D. lygodii is made. Interestingly, P. lygodii and D. aneimiae differ primarily in sorus structure, i.e., subepidermal in the former vs. suprastomatal in the latter fungus. Characters such as suprastomatal sori and probasidia that germinate without dormancy are now known to represent a suite of adaptations that have been derived multiple times within Pucciniales, most likely in response to tropical climates.
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