Klebsormidium flaccidum Genome Reveals Genome Evolution for Plant Terrestrial Adaptation

The colonization of land by plants was a key event in the evolution of life, making the modern terrestrial environment habitable by supplying various nutrients and sufficient atmospheric oxygen. During the colonization of land, the transition species of aquatic algae must have acquired a range of adaptive mechanisms to cope with the harsh features of terrestrial environments, such as drought, high-intensity light, and UV radiation. It is generally accepted that the ancestor(s) of current terrestrial plants was closely related to present-day charophytes. The charophytic algae Klebsormidium is an early diverging lineage of charophytes, and usually consist of multicellular and non-branching filaments without differentiated or specialised cells. Klebsormidium species therefore have primitive body plans, and most species that have adapted to land also can survive in fresh water.

We completed the draft genome sequence of K. flaccidum (1,814 scaffolds, 104 Mb) to elucidate primary factors for the plant colonization of land, and identified and annotated 16,215 protein coding genes in the nuclear and organellar genomes. We compared protein set of K. flaccidum with other algae and land plants by analyses of gene families and protein domains.

These results suggested that K. flaccidum acquired many genes specific to land plants, and have intermediate biological systems between aquatic algae and terrestrial plants. Although these plant-related systems in K. flaccidum are primitive, they may facilitate various responses to harsh environmental stresses on land.