Molecular Phylogeny and Description of the Novel Katablepharid Roombia truncata gen. et sp. nov., and Establishment of the Hacrobia Taxon nov

Abstract

BackgroundPhotosynthetic eukaryotes with a secondary plastid of red algal origin (cryptophytes, haptophytes, stramenopiles, dinoflagellates, and apicomplexans) are hypothesized to share a single origin of plastid acquisition according to Chromalveolate hypothesis. Recent phylogenomic analyses suggest that photosynthetic “chromalveolates” form a large clade with inclusion of several non-photosynthetic protist lineages. Katablepharids are one such non-photosynthetic lineage closely related to cryptophytes. Despite their evolutionary and ecological importance, katablepharids are poorly investigated.Methodology/Principal FindingsHere, we report a newly discovered flagellate, Roombia truncata gen. et sp. nov., that is related to katablepharids, but is morphologically distinct from othermembers of the group in the following ways: (1) two flagella emerge from a papilla-like subapical protrusion, (2) conspicuous ejectisomes are aligned in multiple (5–11) rows, (3) each ejectisome increases in size towards the posterior end of the rows, and (4) upon feeding, a part of cytoplasm elastically stretch to engulf whole prey cell. Molecular phylogenies inferred from Hsp90, SSU rDNA, and LSU rDNA sequences consistently and strongly show R. truncata as the sister lineage to all other katablepharids, including lineages known only from environmental sequence surveys. A close association between katablepharids and cryptophytes was also recovered in most analyses. Katablepharids and cryptophytes are together part of a larger, more inclusive, group that also contains haptophytes, telonemids, centrohelids and perhaps biliphytes. The monophyly of this group is supported by several different molecular phylogenetic datasets and one shared lateral gene transfer; therefore, we formally establish this diverse clade as the “Hacrobia.”Conclusions/SignificanceOur discovery of R. truncata not only expands our knowledge in the less studied flagellate group, but provide a better understanding of phylogenetic relationship and evolutionary view of plastid acquisition/losses of Hacrobia. Being an ancestral to all katablepharids, and readily cultivable, R. truncata is a good candidate for multiple gene analyses that will contribute to future phylogenetic studies of Hacrobia.

The evolutionary history of haptophytes and cryptophytes: phylogenomic evidence for separate origins

“…Katablepharid phylogenomic data are important to aid in the detection of hidden multiple substitutions that could have occurred along the branch leading to cryptophytes, a common source of non-phylogenetic signal, as well as the detection of endosymbiont-derived genes, since no plastid has been reported in katablepharids. Accordingly, we developed the first genome-wide survey of the katablepharid lineage, a transcriptome of the deep-branching R. truncata [25]. Using a fractionation method to remove this predator from its diatom prey (Navicula sp.…”

Monophyly of Archaeplastida supergroup and relationships among its lineages in the light of phylogenetic and phylogenomic studies. Are we close to a consensus?

“…2. A basal Viridiplantae was proposed only from an Hsp90 tree, but without significant support [159]. What is more, the glaucophyte-first hypothesis was significantly supported by the larger number of studies, e.g.…”

Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life

“…SAR includes more protist species than all other eukaryotic groups combined and is extremely diverse in terms of cytology, body organization, and lifestyles. Haptophytes and cryptophytes plus several minor eukaryotic lineages were proposed to constitute a novel supergroup denoted Hacrobia or CCTH (118). However, monophyly of this grouping has not been supported in the latest analyses (117,119), so it seems more appropriate to acknowledge at least five lineages with unresolved affinities: Cryptista, Haptophyta, Telonemia, Centrohelida, and Picozoa (previously called picobiliphytes) (120).…”