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Evolution, Phylogeny and Systematics of Euglenozoa

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Euglenophyceae are an interesting group of mostly photosynthesizing Protista. They belong to Euglenida, which together with Diplonemea, Symbiontida and Kinetoplastea form a larger taxon Euglenozoa. Euglenozoa, considered by some researchers to be one of the earliest diverging branches of the eukaryotic tree ...

Euglenophyceae are an interesting group of mostly photosynthesizing Protista. They belong to Euglenida, which together with Diplonemea, Symbiontida and Kinetoplastea form a larger taxon Euglenozoa. Euglenozoa, considered by some researchers to be one of the earliest diverging branches of the eukaryotic tree of life, have unusual genomic structures and an enormous diversity of gene expression. Green and secondarily colorless Euglenophyceae form a monophyletic group, suggesting that the acquisition of a chloroplast happened only once during the evolutionary history of this group. The Euglenophyceae chloroplast possesses chlorophyll a and b and is surrounded by a triple membrane, the consequence of secondary endosymbiosis. The closest living relative of this donor algae has been identified as Pyramimonas, a Prasinophytae green algae.

The structure of the Euglenophyceae genome is quite complicated and has not yet been fully elucidated. Since the Euglenophyceae cell has been created by secondary endosymbiosis, it potentially contains the vestiges of different genomes, coming from both the donor and recipient cells. This includes: nuclear, chloroplast and mitochondrial genomes of the donor; nuclear and mitochondrial genomes of the host cell. While Euglenophyceae have nuclear and extra-nuclear genomes in the form of mitochondrial, chloroplast and cytoplasmic plasmid-like molecules, no nucleomorph structure has been identified. The chloroplast genome sequenced from several species proved to be substantially diversified in terms of size - mostly due to the highly disparate number of introns – but not due to gene content, which is almost identical in all species surveyed. Introns in Euglenophyceae also display unusual diversity. In addition to spliceosomal and group I and II self-splicing introns, there are introns unique to Euglenophyceae, group III introns and twintrons – introns within introns – later identified in other organisms.

The structure of the Euglenophyceae mitochondrion genome has not yet been fully elucidated, but seems to consist of many small molecules, each coding for parts of particular genes. However, at least one gene, a cox1 in the Euglena gracilis gene seems to be coded for in full by a continuous molecule. In Euglena gracilis, in addition to the nuclear and organellar genomes, there are circular, plasmid-like DNA molecules coding for cytoplasmic ribosomal RNA.

Phylogenetic analyses of Euglenophyceae currently performed on increasingly more genes have helped to elucidate relationships between most of the popular genera and between many species. Several new monophyletic genera have recently been created and numerous species have been reassigned to other genera. However, less commonly found genera have not been sampled in recent molecular phylogenies and their phylogenetic affiliation remains unclear.

Given these unusual features and unresolved questions concerning the biology and evolution of Euglenophyceae, they constitute a good Research Topic at Frontiers. We welcome the submission of all article types accepted at Frontiers. While we wish to propose several special themes, listed below, any paper on the Evolution and Systematics of Euglenophyceae and their relatives is welcome.

Acquisition and loss of photosynthesis in Euglenophyceae, Structure and evolution of nuclear, mitochondrial and chloroplast genomes, Origin and evolution of introns in Euglenophyceae, Character evolution in Euglenophyceae, Phylogeny and Systematics of Euglenophyceae.


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