Phylogenomics defines Streptofilum as a novel deep branch of streptophyte algae

Streptophytes constitute a major organismal clade comprised of land plants (embryophytes) and several related green algal lineages. Their seemingly well-studied phylogenetic diversity was recently enriched by the discovery of Streptofilum capillaum , a simple filamentous alga forming a novel deep streptophyte lineage in a two-gene phylogeny ¹ . A subsequent phylogenetic analysis of plastid genome-encoded proteins resolved Streptofilum as a sister group of nearly all known streptophytes, including Klebsormidiophyceae and Phragmoplastophyta (Charophyceae, Coleochaetophyceae, Zygnematophyceae, and embryophytes) ² . In a stark contrast, another recent report, published in Current Biology by Bierenbroodspot et al. ³ , presented a phylogenetic analysis of 845 nuclear loci resolving S. capillatum as a member of Klebsormidiophyceae, nested among species of the genus Interfilum . Here we demonstrate that the latter result is an artefact stemming from an unrecognized contamination of the transcriptome assembly from S. capillatum by sequences from Interfilum paradoxum . When genuine S. capillatum sequences are employed in the analysis, the position of the alga in the nuclear genes-based tree fully agrees with the plastid genes-based phylogeny. The “intermediate” phylogenetic position of S. capillatum predicts it to possess a unique combination of derived and plesiomorphic traits, here exemplified, respectively, by the “Rho of plants” (ROP) signaling system and the cyanobacteria-derived plastidial transfer-messenger ribonucleoprotein complex (tmRNP). Our results underscore S. capillatum as a lineage pivotal for the understanding of the evolutionary genesis of streptophyte, and ultimately embryophyte, traits.