The green algae closest to land plants hold clues to the evolutionary changes that preceded plants’ move onto land. A peer-reviewed study by Maaike J. Bierenbroodspot and colleagues expands the genetic map of these algae, revealing deep divisions within the group and multiple evolutionary shifts in growth form.
The researchers combined 43 newly sequenced transcriptomes with existing data to build a phylogenomic tree using 104 transcriptomes and 2,243 genetic loci. The study, published in PLOS Biology, focuses on Zygnematophyceae, a diverse class of streptophyte algae that previous large-scale genetic studies have identified as the closest algal relatives of land plants.
That relationship does not mean today’s algae are direct ancestors of plants. Rather, they are close relatives whose shared evolutionary history can help researchers investigate which traits and genetic systems were present in the lineage from which land plants emerged.
The expanded tree supports a five-order classification of Zygnematophyceae and resolves some relationships among its branches. In particular, it places Zygnematales and Serritaeniales as sister groups. It also shows a deep split between Spirogyrales—which includes the genus Spirogyra—and the unicellular Desmidiales. The researchers conclude that the group’s history includes substantial genetic divergence and ancient radiations that were difficult to discern from less diverse sampling.
One striking feature is the long genetic branch leading to living Spirogyrales: it is 6.2 times longer than the branch leading to Desmidiales. That pattern does not reveal a single cause. It could reflect extinction or unsampled diversity, faster molecular evolution, or biases related to selection, genetic composition or transcriptome data. The authors say the tree alone cannot distinguish among these possibilities, so ideas about when Spirogyrales acquired traits such as spiral chloroplasts and distinctive cell-division machinery remain hypotheses.
The team also used the tree to estimate how growth forms changed over time. Their analysis supports a unicellular last common ancestor for Zygnematophyceae, with a posterior probability of 0.988. It identifies seven transitions to filamentous growth in the group, including ancient transitions in the lineages leading to Spirogyrales and the genus Zygnema. These are model-based reconstructions from the sampled algae; the study says the body plan of the common ancestor shared by land plants and Zygnematophyceae could not be robustly inferred.
The genetic comparisons add another layer to the evolutionary picture. Genes homologous to plant PYR/PYL/RCAR receptors were detected in two Zygnematophyceae orders, Serritaeniales and Zygnematales, but not in the other sampled orders. The authors suggest possible lineage-specific losses, while emphasizing that broader sampling and more complete genome assemblies are needed to confirm their absence elsewhere.
Other gene families show different patterns. Zygnematophyceae CAD homologs form diverse groups distinct from the established land-plant classes, but their precise enzyme functions remain experimentally untested. The study also finds extensive, lineage-dependent expansions of histidine kinases, proteins involved in cellular signaling. Together, these results suggest that genes shared across algae and land plants have followed varied histories of retention, loss and expansion.
The work matters because a more representative evolutionary tree helps researchers interpret those histories and ask which features may have belonged to ancient algal relatives of plants. The dataset is substantial, but it still covers only a fraction of the more than 4,000 described Zygnematophyceae species. Further sampling and genomic data will be needed to test the proposed evolutionary scenarios and sharpen the picture of the group’s past.