The simplest multicellular organism unveiled A four-celled living fossil, the “happiest alga” for 200 million years
It is widely accepted that multicellular organisms such as humans evolved from single-celled life forms. It has been inferred that the transition from single to multicellular organism occurred in multiple different eukaryotic organisms, but the mechanism by which it took place remains a mystery. The green algal group colonial Volvocales is a model lineage for the study of this transition, because it includes forms from single-celled to an intermediate stage to becoming a multicellular organism.
A research group of the Department of Biological Sciences, Graduate School of Science, the University of Tokyo (graduate student Yoko Arakaki, Dr. Hisayoshi Nozaki and colleagues) has unveiled morphological traits that are fundamental to multicellular organisms in the four-celled species named the “happiest alga” (Tetrabaena socialis, called the “happiest alga” because it resembles a four-leaf clover). This alga is one of the colonial volvocalean greens and might have diverged in the earliest period of the colonial volvocalean evolution 200 million years ago. The research group demonstrated that the “happiest alga” has cells that can function as a part of the four-celled individual and forms protoplasmic bridges between daughter protoplasts during embryogenesis in order to determine and maintain the beautiful four-leaf clover shape of the four-celled individual.
This alga is the simplest multicellular organism in the eukaryotes and represents a living fossil of the earliest multicellular species in the 200 million years’ evolution of the colonial Volvocales. Further morphological and molecular studies of the “happiest alga” will provide insight into the fundamental bases of the evolution of multicellularity and complexity of the individuals in the model lineage, the colonial Volvocales.
Press release (Japanese)
Yoko Arakaki, Hiroko Toyooka, Yuki Hamamura, Tetsuya Higashiyama, Akira Noga, Masafumi Hirono, Bradley J. S. C. Olson, Hisayoshi Nozaki,
“The Simplest Integrated Multicellular Organism Unveiled”,
PLOS ONE Online Edition: 2013/11/11 (Japan time), doi: 10.1371/journal.pone.0081641.