Comparative Ultrastructure of Fornicate Excavates, Including a Novel Free-living Relative of Diplomonads: Aduncisulcus paluster gen. et sp. nov.

The Fornicata (Excavata) is a group of microbial eukaryotes consisting of both free-living lineages (e.g., Carpediemonas) and parasitic lineages (e.g. Giardia and Retortamonas) that share several molecular and ultrastructural traits. Carpediemonas-like organisms (CLOs) are free-living lineages that diverged early within the Fornicata, making them important for inferring the early evolutionary history of the group. Molecular phylogenetic analyses of free-living fornicates, including sequences from environmental PCR surveys, have demonstrated that CLOs form six different lineages. Representatives from five of these lineages have been studied at the ultrastructural level. The sixth lineage has been labeled "CL2" but has yet to be described with ultrastructural data. Improved understanding of CL2 is expected to help elucidate character evolution within the Fornicata. Therefore, we comprehensively characterized CL2 (NY0171) in order to understand the ultrastructural traits in this lineage, especially the organization of the microtubular root system (i.e., the flagellar apparatus). CL2 shared several morphological features with other fornicates, including reduced mitochondria and an arched B fiber bridging flagellar roots 1 and 2. The molecular phylogenetic position combined with some distinctive ultrastructural traits (e.g., a curved ventral groove) in CL2 required us to establish a new genus and species, Aduncisulcus paluster gen. et sp. nov.