Green algae and the origin of land plants

TLDR

Molecular phylogenetics over the past two decades has reshaped green‑algal taxonomy, revealing two major lineages—chlorophytes and charophytes (the latter sister to land plants)—and prompting ongoing reclassification as more data accrue. The authors used nuclear 18S rDNA, plastid rbcL, intron‑gain patterns, complete genome sequences, and mitochondrial data to conduct these analyses. These analyses have redefined classification across nearly all taxonomic levels, confirmed Mesostigma’s close affinity to charophytes, and positioned Charales as sister to embryophytes.

Abstract

Over the past two decades, molecular phylogenetic data have allowed evaluations of hypotheses on the evolution of green algae based on vegetative morphological and ultrastructural characters. Higher taxa are now generally recognized on the basis of ultrastructural characters. Molecular analyses have mostly employed primarily nuclear small subunit rDNA (18S) and plastid rbcL data, as well as data on intron gain, complete genome sequencing, and mitochondrial sequences. Molecular‐based revisions of classification at nearly all levels have occurred, from dismemberment of long‐established genera and families into multiple classes, to the circumscription of two major lineages within the green algae. One lineage, the chlorophyte algae or Chlorophyta sensu stricto, comprises most of what are commonly called green algae and includes most members of the grade of putatively ancestral scaly flagellates in Prasinophyceae plus members of Ulvophyceae, Trebouxiophyceae, and Chlorophyceae. The other lineage (charophyte algae and embryophyte land plants), comprises at least five monophyletic groups of green algae, plus embryophytes. A recent multigene analysis corroborates a close relationship between Mesostigma (formerly in the Prasinophyceae) and the charophyte algae, although sequence data of the Mesostigma mitochondrial genome analysis places the genus as sister to charophyte and chlorophyte algae. These studies also support Charales as sister to land plants. The reorganization of taxa stimulated by molecular analyses is expected to continue as more data accumulate and new taxa and habitats are sampled.