Turmel, M. | Search bioRxiv

bioRxiv2019

Tracing the evolution of the plastome and mitogenome in the Chloropicophyceae uncovered convergent tRNA gene losses and a variant plastid genetic code

The tiny green algae belonging to the Chloropicophyceae play a key role in marine phytoplankton communities; this newly erected class of prasinophytes comprises two genera (Chloropicon and Chloroparvula) containing each several species. We sequenced the plastomes and mitogenomes of eight Chloropicon and five Chloroparvula species to better delineate the phylogenetic affinities of these taxa and to infer the suite of changes that their organelle genomes sustained during evolution. The relationships resolved in organelle-based phylogenomic trees were essentially congruent with previously reported rRNA trees, and similar evolutionary trends but distinct dynamics were identified for the plastome and mitogenome. Although the plastome sustained considerable changes in gene content and order at the time the two genera split, subsequently it remained stable and maintained a very small size. The mitogenome, however, was remodeled more gradually and showed more fluctuation in size, mainly as a result of expansions/contractions of intergenic regions. Remarkably, the plastome and mitogenome lost a common set of three tRNA genes, with the trnI(cau) and trnL(uaa) losses being accompanied with important variations in codon usage. Unexpectedly, despite the disappearance of trnI(cau) from the plastome in the Chloroparvula lineage, AUA codons (the codons recognized by this gene product) were detected in certain plastid genes. By comparing the sequences of plastid protein-coding genes from chloropicophycean and phylogenetically diverse chlorophyte algae with those of the corresponding predicted proteins, we discovered that the AUA codon was reassigned from isoleucine to methionine in Chloroparvula. This noncanonical genetic code has not previously been uncovered in plastids.

genomics↗

bioRxiv2017

The Plastid Genome In Cladophorales Green Algae Is Encoded By Hairpin Plasmids

Virtually all plastid (chloroplast) genomes are circular double-stranded DNA molecules, typically between 100-200 kb in size and encoding circa 80-250 genes. Exceptions to this universal plastid genome architecture are very few and include the dinoflagellates where genes are located on DNA minicircles. Here we report on the highly deviant chloroplast genome of Cladophorales green algae, which is entirely fragmented into hairpin plasmids. Short and long read high-throughput sequencing of DNA and RNA demonstrated that the chloroplast genes of Boodlea composita are encoded on 1-7 kb DNA contigs with an exceptionally high GC-content, each containing a long inverted repeat with one or two protein-coding genes and conserved non-coding regions putatively involved in replication and/or expression. We propose that these contigs correspond to linear single-stranded DNA molecules that fold onto themselves to form hairpin plasmids. The Boodlea chloroplast genes are highly divergent from their corresponding orthologs. The origin of this highly deviant chloroplast genome likely occurred before the emergence of the Cladophorales, and coincided with an elevated transfer of chloroplast genes to the nucleus. A chloroplast genome that is composed only of linear DNA molecules is unprecedented among eukaryotes and highlights unexpected variation in the plastid genome architecture.

genomics↗