Molecular evolution of algae and emergence of chloroplasts as deduced from 5S ribosomal RNA sequences.

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The cytoplasmic 5S ribosomal RNA (rRNA) sequences from seven red algae, a golden yellow alga, a diatom , five brown algae, Euglena, Cyanophora and 19 green plants have been determined. Using these sequences, a phylogenic tree of these organisms have been constructed.Three types of algae, i.e., Rhodophyta (red algae), green algae (members of green plants), and "Chromophyta" (brown algae, the diatom and the goldenyellow alga) are only remotely related to one anotherphylogenically as deduced from 5S rRNA sequences. Red algae emerged first in the eukaryotic evolution, and thereafter various fungi, green plants including green algae and Chromophyta emerged in this order. In the red algal evolution, the so-called "primitive group" of the class Bangiophyceae and "more advanced group" of the class Florideophyceae separated first from each other in an early stage of their evolution. The tree also indicates that various red algae, such as Batrachospermum, Gelidium, Gleiopeltis and Carpopeltis are phylogenically rather remote from one another. Thus, red algae are very widely and anciently separated organisms in the evolution of eukaryotes."Chromophyta" contains golden-yellow algae, diatoms and brown algae. These three groups are related to one another. During the "Chromophyta" evolution, the golden-yellow alga separated first, and thereafter the diatom and brown algae separated from each other. Although brown algae are very different in their morphology and life cycle, the 5S rRNA sequences clearly indicate that the brown algae were diversified quite recently within a short period.In the chloroplast evolution, 5S rRNA sequences of chloroplasts from green plants, Euglena and Cyanophora species show high homology to Cyanobactrial 5S rRNAs, indicating endosymbiosis of them.