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

Abstract

The mitochondrial genome of a filamentous brown alga Pylaiella littoralis (strain CCMP 1907) has been reported to contain four group IIB introns in the LSU rRNA gene and three group IIA introns in the cox1 gene. We found extreme variability in the number of group II introns for these two genes by analyzing eight P. littoralis specimens collected at worldwide habitats. The first intron of the LSU rRNA gene from a specimen collected in France and the fourth intron from a specimen harvested in Japan exhibited an exceptionally long evolutionary distance when compared with the cognate introns found in P. littoralis specimens. Moreover, these introns harbored an intact or nearly intact tripartite ORF, suggesting they are the result of a recent invasion of cognate introns. Based on the fact that many of the target sites were intronless, we propose that opportunity of intron infection is the bottleneck step of the group II intron cycle which consists of invasion, degeneration, and complete loss from the target site.

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

“…A similar group IIA intron has also been observed in the cox1 genes of diatoms and the brown alga Pylaiella litoralis [55]. Several lines of evidence indicate that these introns are a result of independent insertion events [56,57]. The unique presence of a group IIA intron in N. oculata but not the other three Nannochloropsis species reinforces this hypothesis.…”

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

“…This made the overall genome size 58,507 bp, considerably larger than that of other brown algae or most heterokonts (Oudot-Le Secq et al, 2001). However, it has recently been found that different isolates of this species from wide-spread geographic locations have from 1 to 6 introns, with and without intronic ORFs, suggesting that these introns have been acquired by recent invasions (Ikuta et al, 2008).…”

Multiple Intraspecific Variations of Mitochondrial Genomes in the Green-Tide Forming Alga, Ulva compressa Linnaeus (Ulvophyceae, Chlorophyta)

“…Both group I and II introns from the same insertion site were homologous among the Ulva mitogenomes, indicating that cognate introns should have descended from a common progenitor (Ikuta et al, 2008). For the cognate introns of both group I and II, the RNA secondary structures of ribozyme components were highly conserved, while the intronic orf (LHE or RTM) might have been degenerated and even completely lost in some introns ( Table 3).…”