Evolutionary significance of the ring-like plastid nucleus in the primitive red alga Cyanidioschyzon merolae as revealed by drying

• Published in: Protoplasma Vol. 257; no. 4; pp. 1069 - 1078
• Main Authors: Kuroiwa, Tsuneyoshi, Ohnuma, Mio, Imoto, Yuuta, Yagisawa, Fumi, Misumi, Osami, Nagata, Noriko, Kuroiwa, Haruko
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.07.2020; Springer Nature B.V
• Subjects: Algae; Biological Evolution; Biomedical and Life Sciences; Cell Biology; Cell Nucleus - genetics; Cell size; Chlorophyta; Cyanidioschyzon merolae; Deoxyribonucleic acid; DNA; Drying; Genomes; Life Sciences; Nuclei; Original Article; Phaeophyceae; Plant Sciences; Plastids; Plastids - genetics; Rhodophyta; Rhodophyta - genetics; Zoology; Brown algae; Plastid nucleus; Secondary endocytosis; Symbiont; Cyanidioschyzon merolae
• ISSN: 0033-183X; 1615-6102
• DOI: 10.1007/s00709-020-01496-y

Primary plastids originated from a free-living cyanobacterial ancestor and possess their own genomes—probably a few DNA copies. These genomes, which are organized in centrally located plastid nuclei (CN-type pt-nuclei), are produced from preexisting plastids by binary division. Ancestral algae with a CN-type pt-nucleus diverged and evolved into two basal eukaryotic lineages: red algae with circular (CL-type) pt-nuclei and green algae with scattered small (SN-type) pt-nuclei. Although the molecular dynamics of pt-nuclei in green algae and plants are now being analyzed, the process of the conversion of the original algae with a CN-type pt-nucleus to red algae with a CL-type one has not been studied. Here, we show that the CN-type pt-nucleus in the primitive red alga Cyanidioschyzon merolae can be changed to the CL-type by application of drying to produce slight cell swelling. This result implies that CN-type pt-nuclei are produced by compact packing of CL-type ones, which suggests that a C. merolae– like alga was the original progenitor of the red algal lineage. We also observed that the CL-type pt-nucleus has a chain-linked bead-like structure. Each bead is most likely a small unit of DNA, similar to CL-type pt-nuclei in brown algae. Our results thus suggest a C. merolae –like alga as the candidate for the secondary endosymbiont of brown algae.