Parakaryon

Parakaryon myojinensis

Drawing showing unique cell structure with cell wall, single nuclear membrane, and a single large spiral endosymbiont (seen in section), a combination found neither in prokaryotes nor eukaryotes. Cell is 10 μm long.
Scientific classification
Higher classification:	incertae sedis
Genus:	Parakaryon Yamaguchi et al. 2012
Species:	P. myojinensis Yamaguchi et al. 2012

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Location of Myōjin Knoll off the coast of Japan, where the specimen was found

Parakaryon myojinensis, also known as the Myojin parakaryote, is a highly unusual species of single-celled organism known only from a single specimen, described in 2012. It has features of both prokaryotes and eukaryotes but is apparently distinct from either group, making it unique among organisms so far discovered. It is the sole species in the genus Parakaryon.

Etymology

The generic name Parakaryon comes from Greek παρά (pará, "beside") and κάρυον (káryon, "kernel", "nucleus"), and reflects its distinction from eukaryotes and prokaryotes. The specific name myojinensis reflects the locality where the only sample was collected: from the bristle of a scale worm collected from hydrothermal vents at Myōjin Knoll (明神海丘, 32°06.2′N 139°52.1′E), about 1,240 metres (4,070 ft) deep in the Pacific Ocean, near Aogashima island, southeast of the Japanese archipelago.

Structure

Parakaryon myojinensis has some structural features unique to eukaryotes, some features unique to prokaryotes, and some features different to both. The table below details these structures, with matching traits coloured beige.

Structure	Prokaryotes	Eukaryotes	Parakaryon myojinensis
Nucleus present	No	Yes	Yes
No. of nuclear membrane layers	—	2	1
Nuclear pores present	—	Yes	No
Ribosome location	Cytoplasmic	Cytoplasmic	Cytoplasmic and intranuclear
Endosymbionts present	No	Yes	Yes
Endoplasmic reticulum present	No	Yes	No
Golgi apparatus present	No	Yes	No
Mitochondria present	No	Usually	No
Chromosome structure	Variable	Linear	Filamentous
Cytoskeleton present	Yes	Yes	No

Classification

It is not clear whether P. myojinensis can or should be classified as a eukaryote or a prokaryote, the two categories to which all other cellular life belongs. Excluding viruses, which are non-cellular and often distinguished from cellular life, and excluding several fossils that contain disputed evidence of ancient life (nanobacteria, nanobes), P. myojinensis is the only organism to have a completely unknown position in the tree of life. Adding to the difficulties of classification, only one instance of this organism has been discovered to date, and so scientists have been unable to study it further.

British evolutionary biologist Nick Lane hypothesized in a 2015 book that the existence of P. myojinensis may be an important clue to the origins of life on Earth, perhaps as an example of the abiogenesis of simple organisms from organic compounds continuing in the present day. The fact that P. myojinensis was discovered near hydrothermal vents, which have been proposed as possible primordial reaction chambers for the earliest ancestors of prokaryotes and eukaryotes, lends credence to this idea.

See also

• Anatoma fujikurai, a species of gastropod discovered at the same location

Further reading

• Yamaguchi, Masashi; Yamada, Hiroyuki; Chibana, Hiroji (2020-09-25). "Deep-Sea Bacteria Harboring Bacterial Endosymbionts in a Cytoplasm?: 3D Electron Microscopy by Serial Ultrathin Sectioning of Freeze-Substituted Specimen". Cytologia. 85 (3): 209–211. doi:10.1508/cytologia.85.209. S2CID 224920168.
• Yamaguchi M, Yamada H, Uematsu K, Horinouchi Y, Chibana H (2018). "Electron Microscopy and Structome Analysis of Unique Amorphous Bacteria from the Deep Sea in Japan". Cytologia. 83 (3): 336–341. doi:10.1508/cytologia.83.337.{{cite journal}}: CS1 maint: multiple names: authors list (link)