Abstract
Photosynthetic euglenids (Euglenophyta) are a monophyletic group of unicellular eukaryotes characterized by the presence of plastids, which arose as the result of the secondary endosymbiosis. Many Euglenophyta plastid (pt) genomes have been characterized recently, but they represented mainly one family – Euglenaceae. Here, we report a comparative analysis of plastid genomes from eight representatives of the family Phacaceae. Newly sequenced plastid genomes share a number of features including synteny and gene content, except for genes mat2 and mat5 encoding maturases. The observed diversity of intron number and presence/absence of maturases corroborated previously suggested correlation between the number of maturases in the pt genome and intron proliferation. Surprisingly, pt genomes of taxa belonging to Discoplastis and Lepocinclis encode two inverted repeat (IR) regions containing the rDNA operon, which are absent from the Euglenaceae. By mapping the presence/absence of IR region on the obtained phylogenomic tree, we reconstructed the most probable events in the evolution of IRs in the Euglenophyta. Our study highlights the dynamic nature of the Euglenophyta plastid genome, in particular with regards to the IR regions that underwent losses repeatedly.
Highlights
Plastids derived from a single endosymbiotic event between a cyanobacterium and the common ancestor of the green algae, red algae and glaucophytes - an event called primary endosymbiosis[1]
The pt genome of Eutreptia viridis contained the smallest genome among Euglenophyta at 65,513 bp[17], followed by that of the fellow member of Eutreptiales – Eutreptiella gymnastica (67, 622 bp)[12], leading to the assumption that there was an expansion in the genome size in the freshwater euglenids (Euglenales)
All the strains sequenced in this study possess relatively small pt genomes (Fig. 1, Table 1, Supplementary Table 1) and the P. inflexus pt genome (~58 Kb) was identified as the smallest Euglenophyceae pt genome sequenced to date, even smaller than the genome of Eutreptiella gymnastica
Summary
Plastids derived from a single endosymbiotic event between a cyanobacterium and the common ancestor of the green algae (including land plants), red algae and glaucophytes - an event called primary endosymbiosis[1] The plastids of both green algae and red algae were subsequently transferred to other eukaryotic lineages, which gave rise to secondary plastids. With the advent of molecular sequencing, the division of taxa between Phacus and Lepocinclis have been validated and a new genus Discoplastis was erected to accommodate several species formerly belonging to the genus Euglena and characterized by numerous small plastids without pyrenoids and strong metaboly of the cell[26]. We report the structural features of the eight newly sequenced plastid genomes of the representatives of the family Phacaceae, including five taxa from Lepocinclis, two taxa from Phacus, and one Discoplastis taxon. Our results highlight the repeated losses of IR region during the evolution of Euglenophyta
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