Eukaryote origin, a new insight

For many years we have believed that there are only two kinds of organisms on earth: prokaryote and eukaryote. The origin of the eukaryotes is undetermined to date, with conflict between the 2 main theories: the endosymbiosis and the autogenesis theory. However, this paper throws a whole new scenario into the mix, as it finds a morphological intermediate between prokaryotes and eukaryotes. The organism in question was isolated from the chaetae of a scale worm living in Myojin Knoll hydrothermal vents, off the coast of Japan. It was found at a depth of 1240m in 2010 and named Parakaryon myojinensis. The organism was classified using electron microscopy and structome analysis, a method which involves a three dimensional reconstruction of the microbe by taking ultra-thin slices of the organism and placing them back together to create a full picture, internally and externally, of the structure of the microbe. This method targets all microorganisms without segregation of the culturable from the non-culturable, and whilst it is time-consuming and laborious, it gives a less biased view of the microorganisms living in extreme environmental conditions, such as the deep sea.

Parakaryon myojinensis has various characteristic features that fall out of prokaryotic or eukaryotic classification. It has a large, single nucleoid membrane with an irregular shape. This whole nucleoid accounts for 41% of the entire cell and is made up of fibrous strands of DNA surrounded by a single membrane with visible pores. This lack of chromosomal organisation or double membrane rule out the eukaryotic classification. However its size of 10 by 3μm suggests a form of transport system to be present. Typically this is not seen in prokaryotic organisms; they are generally restricted to a few nm due to their diffusion-dependant metabolism.

This new microorganism also possesses three intracellular endosymbionts. One of which is large and spiral-shaped and 2 of which are smaller and rod-shaped. The function of these symbionts is currently unclarified. It is thought unlikely, though, that they are of predatory origin due to their range of diverse characteristics and their lack of cell wall to assist a structural attack. One theory instead is that there is a stable symbiotic relationship between host and endosymbionts. This interior endosymbiont structure of the microbe suggests vast similarities to modern eubacteria; however the lack of cell wall showed otherwise.

This paper provides an insight to the controversial topic that is the origin of prokaryote and eukaryote organisms, it is exactly the kind of evidence that will be needed to settle the debate once and for all. Having said that, there is still a lot of work to be done in terms of classification and phylogenetic analysis of this organism before anything can be set in stone. It would be prudent to investigate methods of DNA analyses to determine the phylogenetic relationship between P. myojinesis and its closest relatives. It would also be interesting to attempt to culture the organisms to see how it reacts biochemically, as often it can be these sorts of analyses that determine the true nature of an organism. If P. myojinesis is an intermediate stage between prokaryote and eukaryote life forms, it would be a big change to the way we think about the prokaryote and eukaryote lineage.

Yamaguchi M., Mori Y., Kozuka Y., Okada H., Uematsu K., Tame A., Furukawa H., Maruyama T., O’Driscoll Worman C., and Yokoyama K. (2012) Prokaryote or eukaryote? A unique microorganism from the deep sea. Journal of Electron Microscopy, 61, 423-431.