Pelomyxa
| Pelomyxa | |
|---|---|
 |
|
| Pelomyxa palustris | |
| Scientific classification | |
| Domain: | Eukaryota |
| Phylum: | Amoebozoa |
| Subphylum: | Conosa |
| Class: | Archamoebae |
| Order: | Pelobiontida |
| Family: | Pelomyxidae |
| Genus: |
Pelomyxa Greef 1874 |
| Species | |
Pelomyxa is a genus of giant flagellar amoeboids, usually 500-800 μm but occasionally up to 5 mm in length, found in anaerobic or microaerobic bottom sediments of stagnant freshwater ponds or slow-moving streams.
The genus was created by R. Greeff, in 1874, with Pelomyxa palustris as its type species. In the decades following the erection of Pelomyxa, researchers assigned numerous new species to it. However, in the last quarter of the 20th century, investigators reduced the genus to a single species, Pelomyxa palustris, which was understood to be a highly changeable organism with a complex life cycle, whose various phases had been mistaken for separate species. All described species were relegated to the status of synonyms, or moved to the unrelated genus Chaos.
Since 2004, four new Pelomyxa species have been described, and two older species have been redescribed and confirmed as valid members of the genus. These developments have raised new questions about the nature of Pelomyxa palustris itself.
Pelomyxa have multiple nuclei, which can number from two to several thousand in rare cases. A moving cell is cylindrical in shape, with a single hemispherical pseudopod at the front and a semipermanent projection called a uroid at the back, which is covered in tiny non-motile flagella. They consume a wide variety of food, and have many vacuoles containing both food, such as diatoms, and debris such as sand.
The classification of Pelomyxa has been the subject of considerable discussion, in recent decades.
Pelomyxa lack , as well as several other organelles usually found in eukaryote cells (notably, peroxisomes and dictyosomes). At one time, they were also believed to lack flagella and to be incapable of mitosis. As nucleated cells that lacked "nearly every other cell-inclusion of eukaryotes",Pelomyxa were, for a time, regarded as surviving "proto-Eukaryotes", standing somewhere between the bacteria and the modern cell. In 1973, it was proposed that the ancestors of Pelomyxa palustris had branched off from the eukaryote line before the advent of mitochondria In 1976, Jean M. Whatley wrote that Pelomyxa palustris "may justly be considered the most primitive eukaryotic organism living today." As such, the organism was potentially a modern analogue of the ancestral eukaryote that, according to the theory of serial endosymbiosis, internalized the bacterial symbiont that later evolved into the mitochondria of the modern cell. The species was known to host several bacterial symbionts. While the function of these was unclear, Whatley argued that they might provide a useful evolutionary example, indicating the "ways in which a bacterial mitochondrial transformation might have been attained."
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