Pelomyxa
Encyclopedia
Pelomyxa are giant amoeba
e, usually 500-800 μm
but occasionally up to 5 mm in length. One notable species is P. palustris; other described species may be synonyms, or have been moved to the unrelated genus Chaos
. They are found in mud at the bottom of freshwater streams and ponds.
,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 vacuole
s containing whatever particles that happen across, both food such as diatom
s and debris such as sand.
Pelomyxa lack mitochondria
, as well as several other organelles usually found in eukaryote
cells (notably, peroxisome
s 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."
In 1982, Lynn Margulis created the subclass Caryoblastea (or Pelobiontidae) for "anaerobic ameobas that lack undulipodia," and assigned Pelomyxa to it as the only member of the group. The following year, Cavalier-Smith included the genus with several other "primitive" amitochondriate amoeboids in a new taxonomical group: the Archamoebae
. The Archamoebae were, in turn, recruited to the new kingdom of Archezoa
, along with other amitochondriate eukaryotes, the Metamonad
s and the Microsporidia
.
The primitivity of Pelomyxa came into doubt in 1988, when Joe I. Griffin published a structural study of Pelomyxa palustris showing that the species does, after all, possess rudimentary flagella, and that it does mitose. Griffin concluded that "Pelomyxa is neither primitive nor different from related forms, once it is realized that its relatives are amoeboid flagellates." In 1995, the case against Pelomyxas primitivity became stronger still, when molecular analysis revealed that the ancestors of Pelomyxa palustris had most probably possessed mitochondria. By the end of the decade, it was clear that all members of Cavalier-Smith's Archamoebae were descended from mitochondriate cells. In other words, they were not early-branching or "primitive" eukaryotes at all, but rather "degenerate protists" that had lost organelles their ancestors had possessed.
Consequently, Pelomyxa and the other Archamoeba were reassigned to the phylum Amoebozoa
, under the subphylum Conosa
(shared with the Mycetozoa
n slime moulds). Kingdom Archezoa was eliminated entirely.
In 2004 a new species was added to the genus: Pelomyxa corona. According to its discoverer, it resembles Pelomyxa palustris but features, among other differences, a distinct hyaline crown on the anterior pseudopod.
Amoebozoa
The Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
e, usually 500-800 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
but occasionally up to 5 mm in length. One notable species is P. palustris; other described species may be synonyms, or have been moved to the unrelated genus Chaos
Chaos (amoeba)
Chaos is a genus of giant amoebae. The largest and best known species, Chaos carolinensis, can reach lengths of 5mm, although most specimens fall between 1 and 3 mm....
. They are found in mud at the bottom of freshwater streams and ponds.
Characteristics
Pelomyxa have multiple nucleiCell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
,which can number from two to several thousand in rare cases. A moving cell is cylindrical in shape, with a single hemispherical pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
at the front and a semipermanent projection called a uroid at the back, which is covered in tiny non-motile flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
. They consume a wide variety of food, and have many vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s containing whatever particles that happen across, both food such as diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s and debris such as sand.
Classification
The classification of Pelomyxa has been the subject of considerable discussion, in recent decades.Pelomyxa lack mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
, as well as several other organelles usually found in eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cells (notably, peroxisome
Peroxisome
Peroxisomes are organelles found in virtually all eukaryotic cells. They are involved in the catabolism of very long chain fatty acids, branched chain fatty acids, D-amino acids, polyamines, and biosynthesis of plasmalogens, etherphospholipids critical for the normal function of mammalian brains...
s and dictyosomes). At one time, they were also believed to lack flagella and to be incapable of mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
. 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
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
, 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."
In 1982, Lynn Margulis created the subclass Caryoblastea (or Pelobiontidae) for "anaerobic ameobas that lack undulipodia," and assigned Pelomyxa to it as the only member of the group. The following year, Cavalier-Smith included the genus with several other "primitive" amitochondriate amoeboids in a new taxonomical group: the Archamoebae
Archamoebae
The Archamoebae are a group of Amoebozoa distinguished by the absence of mitochondria. They include genera that are internal parasites or commensals of animals . A few species are human pathogens, causing diseases such as amoebic dysentery...
. The Archamoebae were, in turn, recruited to the new kingdom of Archezoa
Archezoa
Archezoa was a kingdom proposed by Thomas Cavalier-Smith for eukaryotes that diverged before the origin of mitochondria. At various times, the pelobionts and entamoebids , the metamonads, and the Microsporidia were included here. These groups appear near the base of eukaryotic evolution on rRNA trees...
, along with other amitochondriate eukaryotes, the Metamonad
Metamonad
The metamonads are a large group of flagellate protozoa. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well...
s and the Microsporidia
Microsporidia
The microsporidia constitute a phylum of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals...
.
The primitivity of Pelomyxa came into doubt in 1988, when Joe I. Griffin published a structural study of Pelomyxa palustris showing that the species does, after all, possess rudimentary flagella, and that it does mitose. Griffin concluded that "Pelomyxa is neither primitive nor different from related forms, once it is realized that its relatives are amoeboid flagellates." In 1995, the case against Pelomyxas primitivity became stronger still, when molecular analysis revealed that the ancestors of Pelomyxa palustris had most probably possessed mitochondria. By the end of the decade, it was clear that all members of Cavalier-Smith's Archamoebae were descended from mitochondriate cells. In other words, they were not early-branching or "primitive" eukaryotes at all, but rather "degenerate protists" that had lost organelles their ancestors had possessed.
Consequently, Pelomyxa and the other Archamoeba were reassigned to the phylum Amoebozoa
Amoebozoa
The Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
, under the subphylum Conosa
Conosa
Conosa is a grouping of Amoebozoa. It is subdivided into two infraphyla – Mycetozoa and Archamoebae.Conosa includes the species Dictyostelium discoideum and Entamoeba histolytica, among others....
(shared with the Mycetozoa
Mycetozoa
Mycetozoa is a grouping of slime molds.-Classification:It can be divided into dictyostelid, myxogastrid, and protostelid groups.The mycetozoan groups all fit into the unikont supergroup Amoebozoa, whereas most other slime molds fit into various bikont groups .-Utility in research:Dictyostelids are...
n slime moulds). Kingdom Archezoa was eliminated entirely.
In 2004 a new species was added to the genus: Pelomyxa corona. According to its discoverer, it resembles Pelomyxa palustris but features, among other differences, a distinct hyaline crown on the anterior pseudopod.