Smooth loop-like mitochondrial nucleus in the primitive red alga cyanidioschyzon merolae revealed by drying treatment

It is thought that mitochondria were generated by the symbiosis of autonomous a-proteobacteria and a eukaryote-like organism derived from an archaeon of the species Sulfolobus. Soon after the symbiosis, most of the genome of the a-proteobacterium, which was required for autonomy, was lost. Many genes were transferred into the host genome. However, a small amount of DNA-the mitochondrial genome (mt-genome, mtDNA)- remained in the symbiotic organelle. The primitive eukaryotic cells increased the mtDNA copy number and formed a mitochondrial nucleus (mt-nucleus). The primitive unicellular eukaryote evolved into organisms with one mitochondrion containing multiple mtDNA copies per cell, and organisms with multiple mitochondria with a small number of mtDNA copies in each cell. There have been many studies on the mitochondria and mt-genomes of amoeba, plants, and animals which contain many mitochondria per cell, but only a few studies have reported morphological characteristics of the mitochondria and their genomes in primitive unicellular organisms that have only a single mitochondrion per cell. Here, we show that centrally located mt-nuclei in the primitive red alga Cyanidioschyzon merolae form smooth rings following the application of a drying method that produces slight cell swelling. We discuss regulatory mechanisms for genome function in endosymbiotic organelles on the basis of the differences between the copy number of mtDNA in smooth-ring shaped mt-nuclei and plastid DNA in beadshaped plastid nuclei.