| Parakaryon | |
|---|---|
 | |
| Drawing showing uniquecell structure withcell wall, singlenuclear membrane, and a single large spiralendosymbiont (seen in section), a combination found neither inprokaryotes noreukaryotes. Cell is 10 μm long. | |
Scientific classification | |
| Domain: | incertae sedis |
| Genus: | Parakaryon Yamaguchiet al. 2012[1] |
| Species: | P. myojinensis |
| Binomial name | |
| Parakaryon myojinensis Yamaguchiet al. 2012[1] | |
Parakaryon myojinensis, also known as theMyojin parakaryote, is a highly unusual species of single-celled organism known only from a single specimen, described in 2012. It has features of bothprokaryotes andeukaryotes but is apparently distinct from either group, making it unique among organisms discovered thus far.[1] It is the sole species in the genusParakaryon.
The generic nameParakaryon comes from Greek παρά (pará, "beside", "beyond", "near") and κάρυον (káryon, "nut", "kernel", "nucleus"), and reflects its distinction from eukaryotes and prokaryotes. The specific namemyojinensis reflects the locality where the only sample was collected: from the bristle of ascale worm collected fromhydrothermal vents atMyōjin Knoll (明神海丘,[2]32°06.2′N139°52.1′E / 32.1033°N 139.8683°E /32.1033; 139.8683), about 1,240 metres (4,070 ft) deep in the Pacific Ocean, nearAogashima island, southeast of theJapanese archipelago. The authors explain the full binomial as "next to (eu)karyote from Myojin".[1]
Parakaryon myojinensis has some structural features unique to eukaryotes, some features unique to prokaryotes, and some features different from both. The table below details these structures, with matching traits coloured beige.[1][3]
| Structure | Prokaryotes | Eukaryotes | P. myojinensis |
|---|---|---|---|
| Nucleus present | No | Yes | Yes |
| No. ofnuclear membrane layers | N/a | 2 | 1 |
| Nuclear pores present | N/a | 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 |
Yamaguchiet al. proposed in their 2012 paper[1] that there were three reasons why the specimen they namedP. myojinensis was not simply a result of parasitic or predatory bacteria living within another prokaryote host, which they acknowledged is known from several examples:
In 2016, Yamaguchiet al. detailed the discovery of helical bacteria onpolychaetes collected from the same location, which they named "Myojin spiral bacteria".[4] In 2020, Yamaguchi and two others published a new short paper on their studies of the microbiota of polychaetes from Myojin Knoll. The authors stated "Among them, we often observed bacteria that contained intracellular bacteria on ultrathin sections." They studied one such specimen and concluded that the "host" bacterium was dead and its cell wall broken. The smaller bacteria could have been feeding on the larger bacterium but they also suggest "The association of the bacteria with dead bacteria could also have been artificially caused by the centrifugation steps used for the preparation of specimens for electron microscopy." In this paper, all five mentions ofP. myojinensis were as a valid taxon with no implication that it is an artifact.[5]
It is not clear whetherP. myojinensis can or should be classified as an eukaryote or a prokaryote, the two categories to which all other cellular life belongs. 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. Its discoverers suggested that additional specimens would be needed for culturing andDNA sequencing to place the organism in aphylogenetic context.[1]
British evolutionary biochemistNick Lane hypothesized ina 2015 book that the existence ofP. myojinensis could be the first known example ofsymbiogenesis outside eukaryotes, which could offer clues to the requirements for the development of complex life in general.[3]