Abstract
BackgroundFornicata is a relatively recently established group of protists that includes the diplokaryotic diplomonads (which have two similar nuclei per cell), and the monokaryotic enteromonads, retortamonads and Carpediemonas, with the more typical one nucleus per cell. The monophyly of the group was confirmed by molecular phylogenetic studies, but neither the internal phylogeny nor its position on the eukaryotic tree has been clearly resolved.ResultsHere we have introduced data for three genes (SSU rRNA, α-tubulin and HSP90) with a wide taxonomic sampling of Fornicata, including ten isolates of enteromonads, representing the genera Trimitus and Enteromonas, and a new undescribed enteromonad genus. The diplomonad sequences formed two main clades in individual gene and combined gene analyses, with Giardia (and Octomitus) on one side of the basal divergence and Spironucleus, Hexamita and Trepomonas on the other. Contrary to earlier evolutionary scenarios, none of the studied enteromonads appeared basal to diplokaryotic diplomonads. Instead, the enteromonad isolates were all robustly situated within the second of the two diplomonad clades. Furthermore, our analyses suggested that enteromonads do not constitute a monophyletic group, and enteromonad monophyly was statistically rejected in 'approximately unbiased' tests of the combined gene data.ConclusionWe suggest that all higher taxa intended to unite multiple enteromonad genera be abandoned, that Trimitus and Enteromonas be considered as part of Hexamitinae, and that the term 'enteromonads' be used in a strictly utilitarian sense. Our result suggests either that the diplokaryotic condition characteristic of diplomonads arose several times independently, or that the monokaryotic cell of enteromonads originated several times independently by secondary reduction from the diplokaryotic state. Both scenarios are evolutionarily complex. More comparative data on the similarity of the genomes of the two nuclei of diplomonads will be necessary to resolve which evolutionary scenario is more probable.
Highlights
Fornicata is a relatively recently established group of protists that includes the diplokaryotic diplomonads, and the monokaryotic enteromonads, retortamonads and Carpediemonas, with the more typical one nucleus per cell
SSU rDNA phylogeny Our analyses of SSU rRNA genes include the broadest taxonomic sampling of Fornicates examined so far
In addition to previously available data we have included five new isolates of the enteromonad genus Trimitus, three new isolates of the genus Enteromonas, two isolates of a new enteromonad genus, an isolate of Trepomonas steini, a new isolate corresponding to the morphospecies Trepomonas agilis (PPS-6), a novel Spironucleus isolate (GEPA2H) and one uncultured eukaryote (CHESI2)
Summary
Fornicata is a relatively recently established group of protists that includes the diplokaryotic diplomonads (which have two similar nuclei per cell), and the monokaryotic enteromonads, retortamonads and Carpediemonas, with the more typical one nucleus per cell. The monophyly of the group was confirmed by molecular phylogenetic studies, but neither the internal phylogeny nor its position on the eukaryotic tree has been clearly resolved Diplomonads and their close relatives – enteromonads, retortamonads and Carpediemonas membranifera – are small flagellates that tend to be found in low-oxygen habitats. They were classified within Fornicata (Metamonada, Excavata) [1]. All described enteromonads, and all described retortamonads except one are endobionts or parasites of animals, with several causing serious and highly prevalent diseases in fish, domestic animals and man [3] Diplomonads and their relatives have been interesting for students of the evolution of the eukaryotic cell for several reasons. The beststudied diplomonad – Giardia intestinalis (= G. lamblia) was looked to as a model for understanding early eukaryotic cells
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