Olisthodiscus represents a new class of Ochrophyta

Abstract

The phylogenetic diversity of Ochrophyta, a diverse and ecologically important radiation of algae, is still incompletely understood even at the level of the principal lineages. One taxon that has eluded simple classification is the marine flagellate genus Olisthodiscus. We investigated Olisthodiscus luteus K‐0444 and documented its morphological and genetic differences from the NIES‐15 strain, which we described as Olisthodiscus tomasii sp. nov. Phylogenetic analyses of combined 18S and 28S rRNA sequences confirmed that Olisthodiscus constitutes a separate, deep, ochrophyte lineage, but its position could not be resolved. To overcome this problem, we sequenced the plastid genome of O. luteus K‐0444 and used the new data in multigene phylogenetic analyses, which suggested that Olisthodiscus is a sister lineage of the class Pinguiophyceae within a broader clade additionally including Chrysophyceae, Synchromophyceae, and Eustigmatophyceae. Surprisingly, the Olisthodiscus plastid genome contained three genes, ycf80, cysT, and cysW, inherited from the rhodophyte ancestor of the ochrophyte plastid yet lost from all other ochrophyte groups studied so far. Combined with nuclear genes for CysA and Sbp proteins, Olisthodiscus is the only known ochrophyte possessing a plastidial sulfate transporter SulT. In addition, the finding of a cemA gene in the Olisthodiscus plastid genome and an updated phylogenetic analysis ruled out the previously proposed hypothesis invoking horizontal cemA transfer from a green algal plastid into Synurales. Altogether, Olisthodiscus clearly represents a novel phylogenetically distinct ochrophyte lineage, which we have proposed as a new class, Olisthodiscophyceae.

A cryptic plastid and a novel mitochondrial plasmid inLeucomyxa plasmidiferagen. and sp. nov. (Ochrophyta) push the frontiers of organellar biology

“…To this end we constructed a supermatrix of 30 plastid genome-encoded proteins, sampling selected ochrophytes (to have represented all classes with plastid genome data available) and several non-ochrophyte algae as an outgroups. Analyses of the data (6,627 aligned amino acid positions) using the maximum likelihood method and the complex model LG+C60+F+G yielded a tree topology (figure 5) that is generally congruent with recent phylogenetic analyses based on multiple plastid genome-encoded proteins (Ševčíková et al 2019; Han et al 2019; Kim et al 2020; Barcytė et al 2021; Di Franco et al 2022). However, thanks to new plastid genome data that became available only recently, our tree is more comprehensive in terms of the main lineages of ochrophytes represented, and exhibits several notable features.…”

“…Plastid genome sequences have proven instrumental for reconstructing the phylogenetic relationships within ochrophytes (Barcytė et al 2021; Di Franco et al 2022). Hence, the availability of the L. salinum plastid genome sequence prompted us to employ it for casting more light on the phylogenetic position of this organisms in ochrophytes.…”

Olisthodiscophyceae, the 17th heterokont algal class

“…1992) or the Chrysophyceae (Vesk and Moestrup 1987), until the work of Barcytė et al. (2021), that is highlighted here.…”

“…For the first time, gene sequences of Olisthodiscus were included in a large phylogenetic analysis by Barcytė et al. (2021). Olisthodiscus was clearly separate from the Xanthophyceae, Chrysophyceae, or Raphidophyceae, where it had been previously classified, and represented an independent lineage within the heterokont algae that is sister to the Pinguiophyceae.…”

“…Overall, the work of Barcytė et al. (2021) is an important step forward, not only for the classification and taxonomic clarity of the heterokont algae, but because this study forms the foundation for subsequent evolutionary analyses. The investigation of older literature visits to type localities when possible, and the careful establishment of a link between traditional and modern taxonomy remains a crucial task for future generations of scientists.…”