Phylogenomics of novel ploeotid taxa contribute to the backbone of the euglenid tree

<b>Abstract</b><br/><p>Euglenids are a diverse group of flagellates that inhabit most environments and exhibit many different nutritional modes. The most prominent euglenids are phototrophs, but phagotrophs constitute the majority of phylogenetic diversity of euglenids. They are pivotal to our understanding of euglenid evolution, yet we are only starting to understand relationships amongst phagotrophs, with the backbone of the tree being the most elusive. Ploeotids make up most of this backbone diversity—yet despite their morphological similarities, SSU rDNA analyses and multigene analyses show they are non-monophyletic. As more ploeotid diversity is sampled, known taxa have coalesced into some subgroups (e.g. Alistosa), but the relationships between these are not always supported and some taxa remain unsampled for multigene phylogenetics. Here, we used light microscopy and single-cell transcriptomics to characterize five ploeotid euglenids and place them into a multigene phylogenetic framework. Our analyses place <em>Decastava</em> in Alistosa; while <em>Hemiolia</em> branches with <em>Liburna</em>, establishing the novel clade Karavia. We describe <em>Hemiolia limna</em>, a freshwater-dwelling species in an otherwise marine clade. Intriguingly, two undescribed ploeotids are found to occupy pivotal positions in the tree: <em>Chelandium granulatum</em> nov. gen. nov. sp. branches as sister to <em>Olkasia</em>, and <em>Gaulosia striata</em> nov. gen. nov. sp. remains an orphan taxon.</p>