Parakaryon

Parakaryon myojinensis is a highly unusual single-celled organism. It belongs to the genus Parakaryon and the domain Parakarya. This species is unique because it does not fit easily into standard biological groups. Most life is divided into prokaryotes or eukaryotes. However, this organism shows features of both groups. It also possesses traits that are different from both. Scientists have only ever found one single specimen of this organism. This makes it a monotypic genus, meaning it is the only species in its group.

The structure of P. myojinensis is a complex puzzle for biologists. It possesses a nucleus, which is a central part of a cell. In most eukaryotes, the nucleus has two layers of membrane. In P. myojinensis, the nuclear membrane has only one layer. It also has nuclear pores, which are openings in the membrane. The ribosomes, or protein-making parts, are found in both the cytoplasm and the nucleus. This is different from most other cells. It also contains endosymbionts, which are smaller organisms living inside a host.

Despite these complex parts, the organism lacks several standard features. It does not have a cytoskeleton, which is a frame that provides cell shape. It also lacks an endoplasmic reticulum and a Golgi apparatus. These are parts used by eukaryotes to process materials. Additionally, it does not have mitochondria, which usually provide energy to eukaryotic cells. Instead, its chromosomes have a filamentous structure. This differs from the linear chromosomes found in eukaryotes or the variable structures in prokaryotes.

Researchers first described this species in 2012. The discovery was made by a team led by Yamaguchi. They found the specimen at Myojin Knoll in the Pacific Ocean. This location is near Aogashima island, southeast of the Japanese archipelago. The sample was taken from the bristle of a scale worm. The worm lived near hydrothermal vents on the ocean floor. The name Parakaryon comes from Greek words meaning "beside" or "near" the nucleus. The specific name myojinensis refers to the Myojin location.

Scientists have debated whether this organism is a real species or an artifact. An artifact is something created by the way a sample is prepared. Yamaguchi and his team proposed three reasons why it is a genuine species. First, they noted it is unlikely that many different bacteria attacked a host at once. They observed one large helix with a volume of 2.3 μm³. They also saw two smaller pieces with volumes of 0.2 μm³ and 0.1 μm³.

Second, the researchers observed that the cells looked healthy. The cytoplasms of the host and the endosymbionts showed orderly structures. This suggests that no digestion had occurred between them. Third, they argued that if this were a predator-host interaction, predator populations would need to be very dense. In 2016, the team found "Myojin spiral bacteria" on polychaetes in the same area. In 2020, other studies showed bacteria containing intracellular bacteria in the same region.

The discovery has major implications for evolutionary biology. It is unclear if P. myojinensis is a prokaryote or a eukaryote. Because only one specimen exists, scientists cannot perform DNA sequencing. They need more samples to understand its place in the tree of life. In 2015, biochemist Nick Lane hypothesized about its significance. He suggested it could be an example of symbiogenesis outside of eukaryotes. Symbiogenesis is when different organisms join to form a new, complex life form. This could reveal how complex life began on Earth.