In a recent study, published in Scientific Reports, we provide solid phylogenetic evidence that the enigmatic green seaweed order Palmophyllales together with the unicellular planktonic Prasinococcales (= prasinophyte clade VI) form the deepest-branching clade of the Chlorophyta. Our analyses are based on chloroplast genomic, and nuclear rDNA data.
Species of Palmophyllales typically grow in deep marine waters and other shady habitats. The group is characterized by a unique form of multicellularity, forming macroscopic plants that are composed of isolated and undifferentiated spherical cells embedded in an amorphous gelatinous matrix. In 2010, we showed that the Palmophyllales formed an early-diverging lineage of green plants, but the exact phylogenetic position could not be determined with certainty.
Our phylogenetic results improve our understanding of morphological evolution in the green algae. Until present, the early-diverging lineages of the Chlorophyta (the prasinophytes) were only known to comprise unicellular planktonic algae. Our results point to an independent origin of macroscopic growth and multicellularity outside of the core Chlorophyta.
Our study also contributes to a better understanding of plastid genome evolution in green plants. The small, compact and intronless chloroplast genome (cpDNA) of V. peltata shows striking similarities in gene content and organization with the cpDNAs of Prasinococcales and the streptophyte Mesostigma viride, indicating that cpDNA architecture has been extremely well conserved in these deep-branching lineages of green plants.
The phylogenetic distinctness of the Palmophyllales-Prasinococcales clade, characterized by unique ultrastructural features, warrants recognition of a new class of green plants, Palmophyllophyceae class. nov.
Leliaert F, Tronholm A, Lemieux C, Turmel M, et al. 2016. Chloroplast phylogenomic analyses reveal the deepest-branching lineage of the Chlorophyta, Palmophyllophyceae class. nov. Scientific Reports 6:25367. article
Leliaert F, Verbruggen H, Zechman FW. 2011. Into the deep: New discoveries at the base of the green plant phylogeny. BioEssays 33:683-692. article
Zechman FW, Verbruggen H, Leliaert F, Ashworth M, et al. 2010. An unrecognized ancient lineage of green plants persists in deep marine waters. Journal of Phycology 46:1288-1295. article