The Discovery of an Organism with a Cellular Structure Intermediate between Those of Prokaryotes and Eukaryotes

Abstract

There are only two kinds of organisms on earth: prokaryotes and eukaryotes. Although eukaryotes are considered to have evolved from prokaryotes, there were no previously known intermediate forms between them. The differences in their cellular structures are so vast that the problem of how eukaryotes could have evolved from prokaryotes is one of the greatest enigmas in biology. Here we report a unique organism with cellular structures appearing to have intermediate features between prokaryotes and eukaryotes that was discovered in the deep-sea off the coast of Japan by using electron microscopy. The organism was 10 μm long and 3μm in diameter, having more than 100 times volume of Escherichia coli. It had a large ‘nucleoid’, consisting of naked DNA fibers, with a single layered ‘nucleoid’ membrane, and endosymbionts that resemble bacteria, but no mitochondria. Because this organism as appears to be a life form distinct from both prokaryotes and eukaryotes but similar to eukaryotes, we named this unique microorganism the ‘Myojin parakaryote’ with the scientific name of Parakaryon myojinensis (“next to (eu)karyote from Myojin”) after the discovery location and its intermediate morphology. The existence of this organism is an indication of a potential evolutionary path between prokaryotes and eukaryotes.