Taxa in disguise
Encyclopedia
In bacteriology
a taxon in disguise is a species
, genus
or higher unit of biological classification
whose evolutionary history reveals has evolved from another unit of similar or lower rank, making the parent unit paraphyletic
. This happen when rapid evolution makes a new species appear radically different from the ancestral group, so that it is not (initially) recogniced as belonging to the parent phylogenetic
group, leaving the latter an evolutionary grade
. While the term is from bacteriology, parallel examples are found throughout the tree of life
. E.g. four footed animals have evolved from piscine ancestors, yet are not generally considered fish
. The four footed animals can thus be said to be "fish in disguise". In many cases, the paraphyly can be resolved by re-classifying the taxon in question under the parent group, but in bacteriology renaming groups may have serious consequences as it may cases confusion over the identity of pathogen
s, and is generally avoided for some groups.
Bacteriology
Bacteriology is the study of bacteria. This subdivision of microbiology involves the identification, classification, and characterization of bacterial species...
a taxon in disguise is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
or higher unit of biological classification
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
whose evolutionary history reveals has evolved from another unit of similar or lower rank, making the parent unit paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
. This happen when rapid evolution makes a new species appear radically different from the ancestral group, so that it is not (initially) recogniced as belonging to the parent phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
group, leaving the latter an evolutionary grade
Evolutionary grade
In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...
. While the term is from bacteriology, parallel examples are found throughout the tree of life
Tree of life (science)
Charles Darwin proposed that phylogeny, the evolutionary relatedness among species through time, was expressible as a metaphor he termed the Tree of Life...
. E.g. four footed animals have evolved from piscine ancestors, yet are not generally considered fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
. The four footed animals can thus be said to be "fish in disguise". In many cases, the paraphyly can be resolved by re-classifying the taxon in question under the parent group, but in bacteriology renaming groups may have serious consequences as it may cases confusion over the identity of pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s, and is generally avoided for some groups.
Examples of taxa in disguise in medically-relevant microbial taxa
- The bacterial genus ShigellaShigellaShigella is a genus of Gram-negative, nonspore forming, non-motile, rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella causes disease in primates, but not in other mammals. It is only naturally found in humans and apes. During...
is the cause of bacillary dysenteryShigellosisShigellosis, also known as bacillary dysentery or Marlow Syndrome, in its most severe manifestation, is a foodborne illness caused by infection by bacteria of the genus Shigella. Shigellosis rarely occurs in animals other than humans and other primates like monkeys and chimpanzees...
, a potentially severe infection that claim the lives of over a million people annually. The genus (S. dysenteriae, S. flexneriShigella flexneriShigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become...
, S. boydii, S. sonneiShigella sonneiShigella sonnei is a species of Shigella. Together with Shigella flexneri, it is responsible for 90% of shigellosis. Shigella sonnei is named for the Danish bacteriologist Carl Olaf Sonne....
) have evolved from the common intestinal bacterium Escherichia coliEscherichia coliEscherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
, rendering that species parephyletic. E. coli itself can also cause serious dysentery, but the difference in the genetic makeup between the E. coli and Shigilla causes different medical conditions and symptoms. The Escherichia coli is a badly classified species as some strains share only 20% of their genome. Being so diverse it should be given a higher taxonomic ranking. However, due to the medical conditions associated both with E. coli itself and with Shigilla the current classification will not be changed, to avoid confusion in medical context. Shigilla will thus remain "E. coli in disguise. - In a similar way, the BacillusBacillusBacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
species of the B. cereus-group (B. anthracisBacillus anthracisBacillus anthracis is the pathogen of the Anthrax acute disease. It is a Gram-positive, spore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.It is one of few bacteria known to...
, B. cereus, B . thuringiensisBacillus thuringiensisBacillus thuringiensis is a Gram-positive, soil-dwelling bacterium, commonly used as a biological pesticide; alternatively, the Cry toxin may be extracted and used as a pesticide. B...
, B. mycoidesBacillus mycoidesBacillus mycoides is a bacterium of the genus Bacillus.Gram positiveRhizoid formationimmobileCasein breakdown positive Gelatin breakdown positiveCatalase positiveGlucose fermentation positiveLactose fermentation negative...
, B. pseudomycoides, B. weihenstephanensisBacillus weihenstephanensisBacillus weihenstephanensis is a soil-dwelling, Gram-positive, rod-shaped, beta hemolytic bacterium. It is differentiated by its psychrotolerant growth . -Pathogenisis:...
and B. medusa) have 99-100% similar 16S rRNA sequence (97% is a commonly cited adequate species cut-off) and should be considered a single species. Some of the members of the group appear to have arisen from other Bacillus strains by acquisition of a protein coding plasmidPlasmidIn microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
and the group may thus be polyphyletic. For medical reasons (anthraxAnthraxAnthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals...
etc.), the current arrangement of separate species remain.
Examples of microbes in disguise due to large genera
- The bacterial genus PseudomonasPseudomonasPseudomonas is a genus of gammaproteobacteria, belonging to the family Pseudomonadaceae containing 191 validly described species.Recently, 16S rRNA sequence analysis has redefined the taxonomy of many bacterial species. As a result, the genus Pseudomonas includes strains formerly classified in the...
has been through several generations of taxonomic methods, bringing the species counts to alarming proportions with around 800 species recogniced around the middle of the 20th century. The nitrogene fixing bacteria of the genus AzotobacterAzotobacterAzotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes which play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is...
and the species Azomonas macrocytogenes have evolved from from a species in the genus PseudomonasPseudomonasPseudomonas is a genus of gammaproteobacteria, belonging to the family Pseudomonadaceae containing 191 validly described species.Recently, 16S rRNA sequence analysis has redefined the taxonomy of many bacterial species. As a result, the genus Pseudomonas includes strains formerly classified in the...
. Due to its nitrogen fixing capabilities and deviant features, Azotobakter has bee described as "Pseudomonas in disguise". - The genus Bacillus is a genus described early on in the history of microbiologyMicrobiologyMicrobiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...
, and is as a consequence a large and gentically very diverse genus with 266 species. The two genera PaenibacillusPaenibacillus'Paenibacillus' is a genus of Gram-positive, facultative anaerobic, endospore-forming bacteria, originally included within the genus Bacillus and then reclassified as a separate genus in 1993. Bacteria belonging to this genus have been detected in a variety of environments such as: soil, water,...
and BrevibacillusBrevibacillusThe Brevibacillus are a genus of Gram positive bacteria related to Bacillus and Paenibacillaceae. Todar's Online Textbook of Bacteriology http://textbookofbacteriology.net/Bacillus.html...
are nested cladeCladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s within the Bacillus genus. Bacillus being highly medically relevant and Paenibacillus a model organismModel organismA model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
used in research, renaming them to reflect phylogeny would result in confusion.
See Also
- paraphylyParaphylyA group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
, monophylyMonophylyIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
and polyphylyPolyphylyA polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded... - Species problemSpecies problemThe species problem or species concept is a mixture of difficult, related questions that often come up when biologists identify species and when they define the word "species"....
- Evolutionary gradeEvolutionary gradeIn alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...
- Cryptic species complexCryptic species complexIn biology, a cryptic species complex is a group of species which satisfy the biological definition of species—that is, they are reproductively isolated from each other—but whose morphology is very similar ....
- Synonym (taxonomy)Synonym (taxonomy)In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
- TaxonomyTaxonomyTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
- LPSNLPSNList of Prokaryotic names with Standing in Nomenclature is an online database that maintains and provides accurate name and related information of prokaryotes according to the International Code of Nomenclature of Bacteria , curated by prof...
, list of accepted bacterial and archaeal names - Cyanobacteria, a phyla of common bacteria but poor classified at present
- Bacterial phylaBacterial phylaThe bacterial phyla are the major lineages of the domain Bacteria.In the scientific classification established by Carl von Linné, each bacterial strain has to be assigned to a species , which is a lower level of a hierarchy of ranks...
, a complicated classification