Morphology, Ultrastructure, and Classification of Barthelonids (Metamonada Incertae Sedis): With Descriptions of <i>Microbarthelona</i> Gen. Nov., and <i>Parabarthelona</i> Gen. Nov.

Barthelona is a genus of anaerobic flagellates that forms a sister lineage to Fornicata along with Skoliomonas. Although "Barthelona spp." are known to separate into three distinct lineages, their detailed morphology has not been examined, and their taxonomic classification is incomplete. In this study, we investigated the morphology, especially ultrastructure, of three cultures of barthelonids, each representing one lineage. These flagellates possess an inconspicuous ventral groove and a highly elongated cytopharynx that originates near the posterior end of the cell and extends toward the anterior end. Their posterior flagellum lacks a vane. The flagellar apparatus shares some features with those of other excavate flagellates in possessing microtubular roots R1, split R2 and R3, but lacks a singlet root, B fiber and discernable C fiber. Notably, R2 splits near the posterior end of the cell. Cell size and the number of microtubules comprising R1 and R2 vary among strains. Based on these observations, we describe two new genera and species, Microbarthelona vorax gen. nov., sp. nov. and Parabarthelona vacuolata gen. nov., sp. nov. Additionally, we propose the following higher taxa for Barthelona, Microbarthelona, and Parabarthelona: Notopharyngea cl. nov., Barthelonida ord. nov., and Barthelonidae fam. nov.