Land Plants' Closest Algal Relatives Evolved Multicellular Growth Repeatedly, Genetic Study Finds

A group of freshwater algae that look, to the casual eye, like pond scum has been handed a larger role in the story of how plants came to land. A team led by the University of Göttingen, working with the universities of Maastricht and Innsbruck and the Museo Nacional de Ciencias Naturales, has published a genetic analysis in PLOS Biology of the Zygnematophyceae, the closest living algal relatives of land plants.

The group includes filamentous algae familiar to anyone who has looked at a pond, Spirogyra among them, along with many species that consist of just a single cell. Their appearance is modest. Their importance comes from their position on the family tree, which makes them a window onto what the algal ancestors of the first land plants might have been like.

The obstacle, until now, was data. Earlier work had already established that these algae are the nearest living algal kin of land plants, but the finer relationships inside the group were hard to resolve because genomic information existed for only a small number of species. To fill the gap, the researchers generated 43 new transcriptomes, which are collections of the genes active in cells, from strains spanning a broad range of zygnematophyte diversity. They combined these with publicly available sequences. The resulting dataset covered more than 100 species and 2,000 genes.

The analysis points to a clear picture. The common ancestor of all Zygnematophyceae was probably a single-celled alga. Filamentous growth then arose several times, independently, in different lineages. In other words, multicellular growth in this group did not come from one evolutionary transition. It emerged repeatedly from simpler forms.

Dr. Maaike Bierenbroodspot, who did the work for her Ph.D. at Göttingen and is now a postdoctoral researcher at Maastricht, said many zygnematophytes look deceptively simple, yet their genomes reveal a deep and complex evolutionary history. She said studying only a handful of model species leaves some evolutionary events invisible.

The genes tell a second story. Cäcilia Kunz of Göttingen noted that these algae carry genes belonging to what researchers call the land plant heritage. Some molecular components tied to important plant functions, she said, have far deeper evolutionary roots than land plants themselves. Yet the way those genes are spread across algal lineages shows that this toolkit was dynamic rather than fixed.

Professor Jan de Vries of Göttingen framed the result as a better map of the evolutionary landscape around the origin of land plants. In his words, the more diversity the team included, the less evolution looked like a simple march from single-celled algae to increasingly complex plants. Different lineages, he said, tried different body plans and molecular solutions over hundreds of millions of years, and understanding that variety is essential to reconstructing the biology of the algae from which land plants emerged.

The authors are clear about the limits. Even this expanded framework covers only a fraction of the more than 4,000 described zygnematophyte species. The work argues for broad sampling, and adding more species is the obvious next step, one that could further reshape the picture of where land plants came from.