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A comparative analysis of the chloroplast genome of Euglena mutabilis underlined a high diversity in the evolution of plastids in euglenids. Gene clusters in more derived Euglenales increased in complexity with only a few, but remarkable changes in the genus Euglena. Euglena mutabilis differed from other Euglena species in a mirror-inverted arrangement of 12 from 15 identified clusters, making it very likely that the emergence at the base of the genus Euglena, which has been considered a long branch artifact, is truly a probable position. This was corroborated by many similarities in gene arrangement and orientation with Strombomonas and Monomorphina, rendering the genome organization of E. mutabilis in certain clusters as plesiomorphic feature. By RNA analysis exact exon-intron boundaries and the type of the 77 introns identified were mostly determined unambiguously. A detailed intron study of psbC pointed at two important issues: First, the number of introns varied even between species, and no trend from few to many introns could be observed. Second, mat1 was localized in Eutreptiales exclusively in intron 1, and mat2 was not identified. With the emergence of Euglenaceae in most species, a new intron containing mat2 inserted in front of the previous intron 1 and thereby became intron 2 with mat1., (© 2016 The Author(s) Journal of Eukaryotic Microbiology © 2016 International Society of Protistologists.)

The gene coding for the small ribosomal subunit RNA of Ploeotia costata contains an actively splicing group I intron (Pco.S516) which is unique among euglenozoans. Secondary structure predictions indicate that paired segments P1-P10 as well as several conserved elements typical of group I introns and of subclass IC1 in particular are present. Phylogenetic analyses of SSU rDNA sequences demonstrate a well-supported placement of Ploeotia costata within the Euglenozoa; whereas, analyses of intron data sets uncover a close phylogenetic relation of Pco.S516 to S-516 introns from Acanthamoeba, Aureoumbra lagunensis (Stramenopila) and red algae of the order Bangiales. Discrepancies between SSU rDNA and intron phylogenies suggest horizontal spread of the group I intron. Monophyly of IC1 516 introns from Ploeotia costata, A. lagunensis and rhodophytes is supported by a unique secondary structure element: helix P5b possesses an insertion of 19 nt length with a highly conserved tetraloop which is supposed to take part in tertiary interactions. Neither functional nor degenerated ORFs coding for homing endonucleases can be identified in Pco.S516. Nevertheless, degenerated ORFs with His-Cys box motifs in closely related intron sequences indicate that homing may have occurred during evolution of the investigated intron group.

Abstract: The latest studies of chloroplast genomes of phototrophic euglenoids yielded different results according to intrageneric variability such as cluster arrangement or diversity of introns. Although the genera Euglena and Monomorphina in those studies show high syntenic arrangements at the intrageneric level, the two investigated Eutreptiella species comprise low synteny. Furthermore Trachelomonas volvocina show low synteny to the chloroplast genomes of the sister genera Monomorphina aenigmatica, M. parapyrum, Cryptoglena skujae, Euglenaria anabaena, Strombomonas acuminata, all of which were highly syntenic. Consequently, this study aims at the analysis of the cpGenome of Trachelomonas grandis and a comparative examination of T. volvocina to investigate whether the cpGenomes are of such resemblance as could be expected for a genus within the Euglenaceae. Although these analyses resulted in almost identical gene content to other Euglenaceae, the chloroplast genome showed significant novelties: In the rRNA operon, we detected group II introns, not yet found in any other cpGenome of Euglenaceae and a substantially heterogeneous cluster arrangement in the genus Trachelomonas. The phylogenomic analysis with 84 genes of 19 phototrophic euglenoids and 18 cpGenome sequences from Chlorophyta and Streptophyta resulted in a well‐supported cpGenome phylogeny, which is in accordance to former phylogenetic analyses. [ABSTRACT FROM AUTHOR]

[Display omitted] • We obtained four new complete plastid genomes of marine photosynthetic euglenids. • Plastid-based phylogeny indicates both Eutreptiella and order Eutreptiales to be paraphyletic. • Plastid genomes of Eutreptiales possess unique group II intron maturase genes. • A majority of euglenid plastid-encoded maturases is functionally deficient. • Plastid-encoded maturases have diverse origins in euglenids, as some were acquired by HGT. Organellar genomes often carry group II introns, which occasionally encode proteins called maturases that are important for splicing. The number of introns varies substantially among various organellar genomes, and bursts of introns have been observed in multiple eukaryotic lineages, including euglenophytes, with more than 100 introns in their plastid genomes. To examine the evolutionary diversity and history of maturases, an essential gene family among euglenophytes, we searched for their homologs in newly sequenced and published plastid genomes representing all major euglenophyte lineages. We found that maturase content in plastid genomes has a patchy distribution, with a maximum of eight of them present in Eutreptiella eupharyngea. The most basal lineages of euglenophytes, Eutreptiales, share the highest number of maturases, but the lowest number of introns. We also identified a peculiar convoluted structure of a gene located in an intron, in a gene within an intron, within yet another gene, present in some Eutreptiales. Further investigation of functional domains of identified maturases show that most of them lost at least one of the functional domains, which implies that the patchy maturase distribution is due to frequent inactivation and eventual loss over time. Finally, we identified the diversified evolutionary origin of analysed maturases, which were acquired along with the green algal plastid or horizontally transferred. These findings indicate that euglenophytes' plastid maturases have experienced a surprisingly dynamic history due to gains from diversified donors, their retention, and loss. [ABSTRACT FROM AUTHOR]