[NEW] Diplomonada | diplomonadida – NATAVIGUIDES

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A Microbial Biorealm page on the Diplomonada

Classification

Higher order taxa:

Eukaryota; Diplomonadida group

Species:

NCBI: Taxonomy Genome

Description and Significance

Diplomonads are unicellular eukaryotes that have flagella, most of which are parasitic. Diplomonads used to be considered one of the most primitive eukaryotes and some thought that they might have been the link between eukaryotes and prokaryotes because though diplomonads are very characteristic eukaryotes, one main difference is that they lack mitochondria. Now they are known to possess mitochondrial relics called mitosomes.

Genome Structure

A group of diplomonads has been found to use a genetic code in which TAA and TAG encode glutamine rather than termination.

Cell Structure and Metabolism

Diplomonads have two haploid nuclei (each with four associated flagella, arranged symmetrically about the body’s main axis) that give the cells a face-like appearance.

Diplomonads do not possess mitochondria, and thus they cannot perform respiration and instead must obtain their energy from fermentative processes. Diplomonads are able to ferment sugars such as glucose to produce energy, and they are also capable of fermenting the amino acid arginine as a means of obtaining energy.

The diplomonads also lack peroxisomes and lysosomes. Lysosomes are membrane-enclosed sacs that hold digestive enzymes used in the digestion of macromolecules such as fats, polysaccharides and proteins. The lack of these enzymes might cause an indirect effect on the production of cellular components. The lack of peroxisomes might have a similar impact. Peroxisomes produce hydrogen peroxide from the reduction of oxygen gas, and also synthesize bile salts that absorb and digest fats. Once again, the lack of these bile salts to digest fats may affect the production of such macromolecules. The diplomonads have a relative simple cytoskeleton that is composed of a crossing of microtubules. This cytoskeleton is the structural component of the organism.

Ecology

Diplomonads found in fish (SEM). Originally published in Poynton, S L & Sterud, E; (2002), ‘Guidelines for species descriptions of diplomonad flagellates from fish’, Journal of Fish Diseases, 25:1, 15-31.

Some diplomonads are free-living and may be common in stagnant fresh water, but most are commensal in the intestines of animals. Some are parasitic and cause disease; in humans, the diplomonad infects the intestine and can cause diarrhea (a disease known as giardiasis, or “hiker’s diarrhea”). 50330 is a known parasite in salmon. Diplomonads are very diverse, including a wide variety of other pathogens and several free-living forms. Here are some diplomonads found in fish.

References

“Diplomonad.” © 2005 BiologyDaily.com.

Keeling PJ, Doolittle WF. “Widespread and ancient distribution of a noncanonical genetic code in diplomonads.” 1997 Sep;14(9):895-901.

Margulis, L., Corliss, J.O., Melkonian, M. and Chapman, D.J. (eds.) 1991. . Jones and Bartlett, Boston.

Mitchel, Beau; Sarah Faaborg; Robyn Seyfert; and Stephanie Wallin. “The Diplomonads.”

The identification of ancestral traits is essential to understanding the evolution of any group. In the case of parasitic groups, this helps us understand the adaptation to this lifestyle and a particular host. Most diplomonads are parasites, but there are free-living members of the group nested among the host-associated diplomonads. Furthermore, most of the close relatives within Fornicata are free-living organisms. This leaves the lifestyle of the ancestor unclear. Here, we present metabolic maps of four different diplomonad species. We identified 853 metabolic reactions and 147 pathways present in at least one of the analyzed diplomonads. Our study suggests that diplomonads represent a metabolically diverse group in which differences correlate with different environments (e.g., the detoxification of arsenic). Using a parsimonious analysis, we also provide a description of the putative metabolism of the last Diplomonadida common ancestor. Our results show that the acquisition and loss of reactions have shaped metabolism since this common ancestor. There is a net loss of reaction in all branches leading to parasitic diplomonads, suggesting an ongoing reduction in the metabolic capacity. Important traits present in host-associated diplomonads (e.g., virulence factors and the synthesis of UDP-N-acetyl-d-galactosamine) are shared with free-living relatives. The last Diplomonadida common ancestor most likely already had acquired important enzymes for the salvage of nucleotides and had a reduced capacity to synthesize nucleotides, lipids, and amino acids de novo, suggesting that it was an obligate host-associated organism.

IMPORTANCE
Diplomonads are a group of microbial eukaryotes found in oxygen-poor environments. There are both parasitic (e.g., Giardia intestinalis) and free-living (e.g., Trepomonas) members in the group. Diplomonads are well known for their anaerobic metabolism, which has been studied for many years. Here, we reconstructed whole metabolic networks of four extant diplomonad species as well as their ancestors, using a bioinformatics approach. We show that the metabolism within the group is under constant change throughout evolutionary time, in response to the environments that the different lineages explore. Both gene losses and gains are responsible for the adaptation processes. Interestingly, it appears that the last Diplomonadida common ancestor had a metabolism that is more similar to extant parasitic than free-living diplomonads. This suggests that the host-associated lifestyle of parasitic diplomonads, such as the human parasite G. intestinalis, is an old evolutionary adaptation.