Parakaryon myojinensis

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.[1] It is the sole species in the genus 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[1]
Species:P. myojinensis
Yamaguchi et al. 2012[1]
Location of Myōjin Knoll off the coast of Japan, where the specimen was found

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,[2] 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.[1]

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.[1][3]

StructureProkaryotesEukaryotesParakaryon myojinensis
Nucleus presentNoYesYes
No. of nuclear membrane layers—21
Nuclear pores present—YesNo
Ribosome locationCytoplasmicCytoplasmicCytoplasmic and intranuclear
Endosymbionts presentNoYesYes
Endoplasmic reticulum presentNoYesNo
Golgi apparatus presentNoYesNo
Mitochondria presentNoUsuallyNo
Chromosome structureVariableLinearFilamentous
Cytoskeleton presentYesYesNo

    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.[1]

    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.[4]

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

    References
    1. Yamaguchi M, Mori Y, Kozuka Y, Okada H, Uematsu K, Tame A, Furukawa H, Maruyama T, Worman CO, Yokoyama K (2012). "Prokaryote or eukaryote? A unique microorganism from the deep sea". J Electron Microsc (Tokyo). 61 (6): 423–431. doi:10.1093/jmicro/dfs062. PMID 23024290.
    2. Fumitoshi, Murakami (1997). "Fumitoshi MURAKAMI, The Forming Mechanism of the Submarine Caldera on Myojin Knoll in the Northern Part of the Izu-Ogasawara (Bonin) Arc". Journal of Geography (Chigaku Zasshi). 106 (1): 70–86. doi:10.5026/jgeography.106.70.
    3. Evolution of complex life on Earth, take 2
    4. Nick Lane (2015). "Epilogue: From the Deep". The Vital Question: Energy, Evolution, and the Origins of Complex Life. W.W.Norton and Company. pp. 281–290. ISBN 978-0-393-08881-6.

    Further reading
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