Genome sequence of the ultrasmall unicellular red alga Cyanidioschyzon merolae 10D

TLDR

Ultrasmall unicellular algae possess compact genomes that reveal the essential genes supporting photosynthetic eukaryotes. We sequenced the 16.5‑Mb, 20‑chromosome genome of *Cyanidioschyzon merolae* 10D, identifying 5,331 genes (86.3 % expressed) with minimal introns, only three rDNA units, two dynamin genes, and a mosaic Calvin cycle enzyme set, and the absence of myosin and actin indicates a simplified cytokinesis system, making this alga a valuable model for studying eukaryotic cell origin and evolution.

Abstract

Small, compact genomes of ultrasmall unicellular algae provide information on the basic and essential genes that support the lives of photosynthetic eukaryotes, including higher plants1,2. Here we report the 16,520,305-base-pair sequence of the 20 chromosomes of the unicellular red alga Cyanidioschyzon merolae 10D as the first complete algal genome. We identified 5,331 genes in total, of which at least 86.3% were expressed. Unique characteristics of this genomic structure include: a lack of introns in all but 26 genes; only three copies of ribosomal DNA units that maintain the nucleolus; and two dynamin genes that are involved only in the division of mitochondria and plastids. The conserved mosaic origin of Calvin cycle enzymes in this red alga and in green plants supports the hypothesis of the existence of single primary plastid endosymbiosis. The lack of a myosin gene, in addition to the unexpressed actin gene, suggests a simpler system of cytokinesis. These results indicate that the C. merolae genome provides a model system with a simple gene composition for studying the origin, evolution and fundamental mechanisms of eukaryotic cells.