Recurrent invasion of mitochondrial group II introns in specimens of Pylaiella littoralis (brown alga), collected worldwide

Categorizing 161 plant (streptophyte) mitochondrial group II introns into 29 families of related paralogues finds only limited links between intron mobility and intron-borne maturases

Group II introns are common in the two endosymbiotic organelle genomes of the plant lineage. Chloroplasts harbor 22 positionally conserved group II introns whereas their occurrence in land plant (embryophyte) mitogenomes is highly variable and specific for the seven major clades: liverworts, mosses, hornworts, lycophytes, ferns, gymnosperms and flowering plants. Each plant group features "signature selections" of ca. 20-30 paralogues from a superset of altogether 105 group II introns meantime identified in embryophyte mtDNAs, suggesting massive intron gains and losses along the backbone of plant phylogeny. We report on systematically categorizing plant mitochondrial group II introns into "families", comprising evidently related paralogues at different insertion sites, which may even be more similar than their respective orthologues in phylogenetically distant taxa. Including streptophyte (charophyte) algae extends our sampling to 161 and we sort 104 streptophyte mitochondrial group II introns into 25 core families of related paralogues evidently arising from retrotransposition events. Adding to discoveries of only recently created intron paralogues, hypermobile introns and twintrons, our survey led to further discoveries including previously overlooked "fossil" introns in spacer regions or e.g., in the rps8 pseudogene of lycophytes. Initially excluding intron-borne maturase sequences for family categorization, we added an independent analysis of maturase phylogenies and find a surprising incongruence between intron mobility and the presence of intron-borne maturases. Intriguingly, however, we find that several examples of nuclear splicing factors meantime characterized simultaneously facilitate splicing of independent paralogues now placed into the same intron families. Altogether this suggests that plant group II intron mobility, in contrast to their bacterial counterparts, is not intimately linked to intron-encoded maturases.

Group II intron and repeat-rich red algal mitochondrial genomes demonstrate the dynamic recent history of autocatalytic RNAs

Background

Group II introns are mobile genetic elements that can insert at specific target sequences, however, their origins are often challenging to reconstruct because of rapid sequence decay following invasion and spread into different sites. To advance understanding of group II intron spread, we studied the intron-rich mitochondrial genome (mitogenome) in the unicellular red alga, Porphyridium.

Results

Analysis of mitogenomes in three closely related species in this genus revealed they were 3–6-fold larger in size (56–132 kbp) than in other red algae, that have genomes of size 21–43 kbp. This discrepancy is explained by two factors, group II intron invasion and expansion of repeated sequences in large intergenic regions. Phylogenetic analysis demonstrates that many mitogenome group II intron families are specific to Porphyridium, whereas others are closely related to sequences in fungi and in the red alga-derived plastids of stramenopiles. Network analysis of intron-encoded proteins (IEPs) shows a clear link between plastid and mitochondrial IEPs in distantly related species, with both groups associated with prokaryotic sequences.

Conclusion

Our analysis of group II introns in Porphyridium mitogenomes demonstrates the dynamic nature of group II intron evolution, strongly supports the lateral movement of group II introns among diverse eukaryotes, and reveals their ability to proliferate, once integrated in mitochondrial DNA.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01200-3.

Genomic Rearrangements and Sequence Evolution across Brown Algal Organelles

Organellar genomes serve as useful models for genome evolution and contain some of the most widely used phylogenetic markers, but they are poorly characterized in many lineages. Here, we report 20 novel mitochondrial genomes and 16 novel plastid genomes from the brown algae. We focused our efforts on the orders Chordales and Laminariales but also provide the first plastid genomes (plastomes) from Desmarestiales and Sphacelariales, the first mitochondrial genome (mitome) from Ralfsiales and a nearly complete mitome from Sphacelariales. We then compared gene content, sequence evolution rates, shifts in genome structural arrangements, and intron distributions across lineages. We confirm that gene content is largely conserved in both organellar genomes across the brown algal tree of life, with few cases of gene gain or loss. We further show that substitution rates are generally lower in plastid than mitochondrial genes, but plastomes are more variable in gene arrangement, as mitomes tend to be colinear even among distantly related lineages (with exceptions). Patterns of intron distribution across organellar genomes are complex. In particular, the mitomes of several laminarialean species possess group II introns that have T7-like ORFs, found previously only in mitochondrial genomes of Pylaiella spp. (Ectocarpales). The distribution of these mitochondrial introns is inconsistent with vertical transmission and likely reflects invasion by horizontal gene transfer between lineages. In the most extreme case, the mitome of Hedophyllum nigripes is ∼40% larger than the mitomes of close relatives because of these introns. Our results provide substantial insight into organellar evolution across the brown algae.

A pangenomic analysis of the Nannochloropsis organellar genomes reveals novel genetic variations in key metabolic genes

BACKGROUND

Microalgae in the genus Nannochloropsis are photosynthetic marine Eustigmatophytes of significant interest to the bioenergy and aquaculture sectors due to their ability to efficiently accumulate biomass and lipids for utilization in renewable transportation fuels, aquaculture feed, and other useful bioproducts. To better understand the genetic complement that drives the metabolic processes of these organisms, we present the assembly and comparative pangenomic analysis of the chloroplast and mitochondrial genomes from Nannochloropsis salina CCMP1776.

RESULTS

The chloroplast and mitochondrial genomes of N. salina are 98.4% and 97% identical to their counterparts in Nannochloropsis gaditana. Comparison of the Nannochloropsis pangenome to other algae within and outside of the same phyla revealed regions of significant genetic divergence in key genes that encode proteins needed for regulation of branched chain amino synthesis (acetohydroxyacid synthase), carbon fixation (RuBisCO activase), energy conservation (ATP synthase), protein synthesis and homeostasis (Clp protease, ribosome).

CONCLUSIONS

Many organellar gene modifications in Nannochloropsis are unique and deviate from conserved orthologs found across the tree of life. Implementation of secondary and tertiary structure prediction was crucial to functionally characterize many proteins and therefore should be implemented in automated annotation pipelines. The exceptional similarity of the N. salina and N. gaditana organellar genomes suggests that N. gaditana be reclassified as a strain of N. salina.

Complex repeat structures and novel features in the mitochondrial genomes of the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana

The mitochondrial genome of the raphid pennate diatom Phaeodactylum tricornutum has several novel features compared with the mitochondrial genomes of the centric diatom Thalassiosira pseudonana and the araphid pennate diatom Synedra acus. It is almost double the size (77,356 bp) due to a 35,454 bp sequence block consisting of an elaborate combination of direct repeats, making it the largest stramenopile (heterokont) mitochondrial genome known. In addition, the cox1 gene has a +1 translational frameshift involving Pro codons CCC and CCT, the first translational frameshift to be detected in an algal mitochondrial genome. The nad9 and rps14 genes are fused by the insertion of an in-frame sequence and cotranscribed. The nad11 gene is split into two parts corresponding to the FeS and molybdate-binding domains, but both parts are still on the mitochondrial genome, in contrast to the brown algae where the second domain appears to have been transferred to the nucleus. In contrast to P. tricornutum, the repeat region of T. pseudonana consists of a much smaller 4790 bp string of almost identical double-hairpin elements, evidence of slipped-strand mispairing and active gene conversion. The diatom mitochondrial genomes have undergone considerable gene rearrangement since the three lineages of diatoms diverged, but all three have kept their repeat regions segregated from their relatively compact coding regions.