:Euglena proxima is a common, globally dispersed, and easily identified photosynthetic euglenoid. Previous phylogenetic analyses using nuclear-encoded small subunit (SSU) and large subunit (LSU) rRNA genes revealed that this taxon was paraphyletic with other Euglena species and was positioned as sister to all Euglenaceae. Despite this, authors were reluctant to remove this taxon from Euglena until additional data, or taxa pairing with E. proxima, were obtained because it created a monotypic genus. To clarify the taxonomy, the chloroplast genome of E. proxima was sequenced and compared with those of other photosynthetic euglenoids. Phylogenomic analyses were performed comparing 79 chloroplast-encoded genes from E. proxima with those found in seven photosynthetic euglenoids and three prasinophytes, the group from which the euglenoid chloroplast was probably derived. These analyses resulted in highly supported phylogenomic trees with topologies that were consistent with all previous phylogenetic analyses, i.e. they positioned E. proxima as sister to all of the Euglenaceae. In addition, a syntenic comparison was conducted between E. proxima and the chloroplast genomes of two Euglena taxa in order to determine their similarities. This analysis showed that the construction of the E. proxima chloroplast genome was very different from that of the two Euglena chloroplast genomes, which were extremely similar to each other. Based on these data, E. proxima was removed from Euglena, and a new genus, Euglenaformis, was erected to clarify the true relationship of this taxon to the rest of the photosynthetic euglenoids.

The chloroplast genomes of two photosynthetic euglenoids, Colacium vesiculosum Ehrenberg (128,889 bp), and Strombomonas acuminata (Schmarda) Deflandre (144,167 bp) have been sequenced. These chloroplast genomes in combination with those of Euglena gracilis, Eutreptia viridis, and Eutreptiella gymnastica provide a snapshot of euglenoid chloroplast evolution allowing comparisons of gene content, arrangement, and expansion. The gene content of the five chloroplast genomes is very similar varying only in the presence or absence of, rrn5, roaA, psaI, psaM, rpoA, and two tRNAs. Large gene rearrangements have occurred within the C. vesiculosum and S. acuminata chloroplast genomes. Most of these rearrangements represent repositioning of entire operons rather than single genes. When compared with previously sequenced genomes, C. vesiculosum and S. acuminata chloroplast genomes more closely resemble the E. gracilis chloroplast genome in size of the genome, number of introns, and gene order than they do those of the Eutreptiales. Overall, the chloroplast genomes of these five species show an evolutionary trend toward increased intron number, a decrease in gene density, and substantial rearrangement of gene clusters.

Functional gene clusters, containing two or more genes encoding different enzymes for the same pathway, are sometimes observed in plant genomes, most often when the genes specify the synthesis of specialized defensive metabolites. Here, we show that a cluster of genes in tomato (Solanum lycopersicum; Solanaceae) contains genes for terpene synthases (TPSs) that specify the synthesis of monoterpenes and diterpenes from cis-prenyl diphosphates, substrates that are synthesized by enzymes encoded by cis-prenyl transferase (CPT) genes also located within the same cluster. The monoterpene synthase genes in the cluster likely evolved from a diterpene synthase gene in the cluster by duplication and divergence. In the orthologous cluster in Solanum habrochaites, a new sesquiterpene synthase gene was created by a duplication event of a monoterpene synthase followed by a localized gene conversion event directed by a diterpene synthase gene. The TPS genes in the Solanum cluster encoding cis-prenyl diphosphate-utilizing enzymes are closely related to a tobacco (Nicotiana tabacum; Solanaceae) diterpene synthase encoding Z-abienol synthase (Nt-ABS). Nt-ABS uses the substrate copal-8-ol diphosphate, which is made from the all-trans geranylgeranyl diphosphate by copal-8-ol diphosphate synthase (Nt-CPS2). The Solanum gene cluster also contains an ortholog of Nt-CPS2, but it appears to encode a nonfunctional protein. Thus, the Solanum functional gene cluster evolved by duplication and divergence of TPS genes, together with alterations in substrate specificity to utilize cis-prenyl diphosphates and through the acquisition of CPT genes.

Bennett M.S., Wiegert K.E. and Triemer R.E. 2012. Comparative chloroplast genomics between Euglena viridis and Euglena gracilis (Euglenophyta). Phycologia 51: 711–718. DOI: 10.2216/12-017.1 The chloroplast genomes of Euglena gracilis, Eutreptia viridis, Eutreptiella gymnastica, Colacium vesiculosum, Strombomonas acuminata and the colourless Euglena (Astasia) longa, which had secondarily lost the ability to photosynthesize, were previously reported. These studies had shown that there was a great diversity in the size of euglenoid chloroplast genomes and in the arrangement of gene clusters. However, while these genomes provided important insights into the evolution of the chloroplast genome across genera, they did not address genomic variability within a genus. In an effort to continue with these investigations, we sequenced the chloroplast genome of Euglena viridis and compared this to the E. gracilis chloroplast genome in order to explore intrageneric chloroplast evolution. The chloroplast genome of E. viridis was also compared to those of the other previously published euglenoid chloroplast genomes. Our results showed that while the chloroplast genome of E. viridis most closely resembled that of E. gracilis, the chloroplast genomes did show significant differences. The chloroplast genome of E. viridis was far more compact, had a gene cluster that was reversed in both gene order and strand orientation, had a region that was comprised almost entirely of open reading frames and had substantially fewer introns. However, despite these differences, it was clear from the chloroplast genome sequence that E. viridis and E. gracilis were very closely related and that the majority of euglenoid chloroplast evolution probably occurred before the divergence of the genus Euglena.

The chloroplast genome of Eutreptia viridis Perty, a basal taxon in the photosynthetic euglenoid lineage, was sequenced and compared with that of Euglena gracilis Ehrenberg, a crown species. Several common gene clusters were identified and gene order, conservation, and sequence similarity was assessed through comparisons with Euglena gracilis. Significant gene rearrangements were present between Eutreptia viridis and Euglena gracilis chloroplast genomes. In addition, major expansion has occurred in the Euglena gracilis chloroplast accounting for its larger size. However, the key chloroplast genes are present and differ only in the absence of psaM and roaA in Eutreptia viridis, and psaI in Euglena gracilis, suggesting a high level of gene conservation within the euglenoid lineage. Further comparisons with the plastid genomes of closely related green algal taxa have provided additional support for the hypothesis that a Pyramimonas-like alga was the euglenoid chloroplast donor via secondary endosymbiosis.