Evolution of green plants and their relationship with other photosynthetic eukaryotes as deduced from 5S ribosomal RNA sequences

Abstract

The nucleotide sequence of cytoplasmic 5S ribosomal RNAs from three gymnosperms, Pinus contorta, Taxus baccata and Juniperus media and from one fern, Pteridium aquilinum, have been determined. These sequences were aligned with all hitherto known cytoplasmic 5S ribosomal RNA sequences of photosynthetic eukaryotes. A dendrogram based on that set of sequences was constructed by a distance matrix method and the resulting tree compared with established views concerning plant and algal evolution. The following monophyletic groups of photosynthetic eukaryotes are recognizable: the Rhodophyta, a group consisting of Phaeophyta, Bacillariophyta and Chrysophyta, and the green plants, the latter comprising green algae, Bryophyta, Pteridophyta and Spermatophyta. According to our 5S ribosomal RNA tree, green plants may have originated from some type of a green flagellated organism such as Chlamydomonas. The land plants seem to have originated from some form of charophyte such as Nitella. 5S ribosomal RNA seems to be less appropriate to estimate dissimilarities between species which have diverged relatively recently, like the angiosperms. Therefore, a precise evolutionary process is difficult to reconstruct for members of this group