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 phylogeny1. 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.