Parakaryon

Lore & Background

The generic name Parakaryon comes from Greek παρά (pará, 'beside', 'beyond', 'near') and κάρυον (káryon, 'nut', 'kernel', 'nucleus'), reflecting its distinction from eukaryotes and prokaryotes. The specific name myojinensis reflects the locality where the only sample was collected: from the bristle of an unidentified scale worm collected from hydrothermal vents at Myōjin Knoll, about 1,240 meters deep in the Pacific Ocean.

Parakaryon myojinensis has some structural features unique to eukaryotes, some features unique to prokaryotes, and some features different from both. Its discoverers proposed three reasons why the specimen was not simply a result of parasitic or predatory bacteria living within another prokaryote host, including the difficulty of multiple bacterial species attacking a host simultaneously, the orderly cellular structures showing no digestion, and the need for dense predator and host populations for such an interaction.

In 2016, Yamaguchi et al. detailed the discovery of helical bacteria on polychaetes from the same location, named 'Myojin spiral bacteria'. A 2020 paper noted that among microbiota from Myojin Knoll, they often observed bacteria containing intracellular bacteria, but one such specimen's host bacterium was dead and its cell wall broken, possibly due to centrifugation steps. In that paper, all mentions of P. myojinensis treated it as a valid taxon.

Reader's Guide

Parakaryon myojinensis is significant because it challenges the fundamental division of cellular life into prokaryotes and eukaryotes. With only one specimen ever discovered, scientists have been unable to culture or sequence its DNA, leaving its phylogenetic placement unresolved. Its discoverers suggested that additional specimens would be needed for culturing and DNA sequencing to place the organism in a phylogenetic context.

British evolutionary biochemist Nick Lane hypothesized in a 2015 book that the existence of P. myojinensis could be the first known example of symbiogenesis outside eukaryotes, potentially offering clues to the requirements for the development of complex life in general. However, some researchers have raised the possibility that the specimen might be an artifact—for example, a dead bacterium with smaller bacteria feeding on it, or an artifact of centrifugation—though the original authors and later papers continue to treat it as a valid taxon. The uncertainty surrounding its classification and the lack of additional specimens mean that its true evolutionary significance remains an open question.

Did You Know?

• Parakaryon myojinensis is known only from a single specimen, described in 2012.
• The specimen was collected from the bristle of an unidentified scale worm at Myōjin Knoll, about 1,240 meters deep in the Pacific Ocean.
• It has structural features unique to eukaryotes, some unique to prokaryotes, and some different from both.
• British evolutionary biochemist Nick Lane hypothesized it could be the first known example of symbiogenesis outside eukaryotes.