The evolution of parasitism in phytoplankton-infecting Chytridiomycota

• Applicant: Dr. Jürgen Strassert
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 432453260

Chytrids (Chytridiomycota) are a group of early-diverging, zoosporic fungi that can be found in a broad range of aquatic and terrestric environments. They live either as saprophytes, obligate parasites, or as intermediate forms. In all chytrids, free-swimming zoospores settle on detritus or a host and extract nutrients while developing into sporangia that release new zoospores. Due to their small size and inconspicuous morphological features, the zoospores remained almost unrecognized for many decades in studies of marine and freshwater plankton communities. More recently, molecular-based techniques have revealed a high abundance and diversity of chytrids in aquatic environments. Some species have been found to infect phytoplankton belonging to different taxa including cyanobacteria, diatoms, and dinoflagellates. Chytrids are therefore assumed to play an ecologically significant role in controlling phytoplankton blooms. Surprisingly, information about the trophic lifestyle is available for only a few species that exist in culture and the genomic innovations that are linked to infection strategies in phytoplankton parasites are completely unknown, hampering the interpretation of chytrid trophic lifestyles in field studies surveying (meta)genomic eDNA. The phylogenetic relationships within chytrids are far from being resolved, making it impossible to understand the origins and diversification of parasitism across ecologically diverse lineages. In this project, I aim to reveal the molecular prerequisites for a parasitic lifestyle in phytoplankton-infecting chytrids. Comparative genome analyses of four phytoplankton-infecting chytrid species with closely related saprophytic species will provide novel insight into the genetic ‘toolkit’ for parasitism (e.g., parasite specific virulence genes). Further, I plan to infer a robust phylogenetic tree including circa 40 chytrid species for which information about the trophic lifestyle is available. Phylogenomic analyses using almost 400 protein-coding genes, retrieved from genomes/transcriptomes that are publicly available or newly sequenced in this project, will allow to untangle the early diversification of chytrids. The newly generated sequence data will also be screened for virulence genes identified in the first part of the project. The phylogenetic placement of chytrid lifestyles will shed light on the ancestral state of diverse groups and increase our understanding of the evolution of parasitism in phytoplankton-infecting chytrids.

• DFG Programme: Research Grants