Taxon in disguise

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.^[1][2] This happens when rapid evolution makes a new species appear radically different from the ancestral group, so that it is not (initially) recognised 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 cause confusion over the identity of pathogens, and is generally avoided for some groups.

Examples of taxa in disguise in medically relevant microbial taxa

Shigella

The bacterial genus Shigella is the cause of bacillary dysentery, a potentially severe infection that claim the lives of over a million people annually.^[3] The genus (S. dysenteriae, S. flexneri, S. boydii, S. sonnei) have evolved from the common intestinal bacterium Escherichia coli, rendering that species parephyletic. E. coli itself can also cause serious dysentery,^[4] but the difference in the genetic makeup between the E. coli and Shigella causes different medical conditions and symptoms.^[2]

Escherichia coli is a badly classified species, since 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 Shigella the current classification will not be changed, to avoid confusion in medical context. Shigella will thus remain "E. coli in disguise".

B. cereus-group

In a similar way, the Bacillus species of the B. cereus-group (B. anthracis, B. cereus, B . thuringiensis, B. mycoides, B. pseudomycoides, B. weihenstephanensis 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.^[5] Some of the members of the group appear to have arisen from other Bacillus strains by acquisition of a protein coding plasmid and the group may thus be polyphyletic. For medical reasons (anthrax etc.), the current arrangement of separate species remain.^[5]

Examples of microbes in disguise due to large genera

• The bacterial genus Pseudomonas has enlarged through several generations of taxonomic methods, bringing the species count to alarming proportions, with around 800 species recognized by the middle of the 20th century.^[6][7] The nitrogen-fixing bacteria of the genus Azotobacter and the species Azomonas macrocytogenes have evolved from a species in the genus Pseudomonas.^[8] Due to its nitrogen fixing capabilities and deviant features, Azotobacter has been described as "Pseudomonas in disguise".^[1]
• The genus Bacillus is a genus described early on in the history of microbiology, and is as a consequence a large and gentically very diverse genus with 266 species.^[9] The two genera Paenibacillus and Brevibacillus are nested clades within the Bacillus genus.^[10] Bacillus being highly medically relevant and Paenibacillus a model organism used in research, renaming them to reflect phylogeny would result in confusion.

See also

• paraphyly, monophyly and polyphyly
• Species problem
• Evolutionary grade
• Cryptic species complex
• Synonym (taxonomy)
• Taxonomy
• LPSN, a list of accepted bacterial and archaeal names
• Cyanobacteria, a phylum of common bacteria, but poorly classified at present
• Bacterial phyla, a complex classification scheme

References