A deep genetic structure phylogenomically frames the closest algal relatives of land plants

Discerning the nature of the last common ancestor of land plants (embryophytes) and algae allows to retrace the adaptations that ushered in the conquest of land by plants. For this, the closest streptophyte algal relatives of embryophytes hold key information. Albeit initially coming as a surprise due to their plain body plans, all major phylogenomic efforts have concluded that the Zygnematophyceae are the algal sisters to land plants. The Zygnematophyceae are the streptophyte algal class with the greatest species richness and while we now have ample genome information on a few select members of zygnematophytes, our understanding of the genomic divergence as well as genetic potential for key traits is limited by a lack of phylodiverse data that accounts for this diversity and integrates it into a phylogenomic framework. We here sequenced 43 new transcriptome datasets for the Zygnematophyceae and built a phylogenomic tree based on a total of 104 zygnematophyceaen transcriptomes and 2243 loci. We recover a deep genetic structure for the Zygnematophyceae: Despite the deep split between Spirogyrales and their unicellular sister group Desmidiales, most Spirogyrales emerged after pronounced genetic divergence, accommodating the attainment of multicellularity and divergent traits such as unique cell and plastid division. Overall, our data capture signatures of massive ancient radiations that could only be revealed thanks to a phylodiverse sampling. Recognizing the deep genetic divergences and a vast evolutionary history of the Zygnematophyceae are key to illuminate the nature of the algal progenitors of land plants.