The Transcriptome of Paraphelidium Tribonemae Illuminates the Ancestry of Fungi and Opisthosporidia

Aphelids constitute a group of diverse, yet poorly known, parasites of algae. Their life cycle and morphology resemble those of zoosporic fungi (chytrids) and rozellids (Cryptomycota/Rozellosporidia), another specious group of parasites of fungi and oomycetes. Unlike fungi, which are osmotrophs, aphelids and rozellids are phagotrophs, feeding on the host’s cytoplasm. Combined RNA polymerase and rRNA gene trees suggested that aphelids and rozellids relate to Microsporidia, extremely reduced parasites with remnant mitochondria. Accordingly, aphelids, rozellids and Microsporidia were proposed to form a monophyletic clade sister to Fungi, called Opisthosporidia. Microsporidia would have subsequently lost the ancestral opisthosporidian phagotrophy. However, the limited phylogenetic signal of those genes combined with microsporidian fast-evolving sequences have resulted in incongruent tree topologies, showing either rozellids or aphelids as the earliestbranching lineages of Opisthosporidia, and challenging their monophyly. We have generated the first transcriptome data for one aphelid species, Paraphelidium tribonemae. Multi-gene phylogenomic analyses clearly confirm the monophyly of Opisthosporidia, placing aphelids as the earliest branching opisthosporidian lineage. This is consistent with the rich proteome inferred for P. tribonemae, which includes cellulases likely involved in algal cell-wall penetration, enzymes involved in chitin biosynthesis and several metabolic pathways that were lost in the comparatively reduced Rozella allomycis genome. Contrary to recent claims suggesting a parasitic root for Fungi, our results suggest that Fungi and Opisthosporidia evolved from a free-living phagotrophic ancestor that became osmotrophic at the fungal root and evolved towards phagotrophic parasitism in the opisthosporidian line.