A species name needs something to hold on to
A scientific name is a promise that a particular organism can be recognised again. For many fungi this promise has been hard to keep. Descriptions written long ago were often brief, the original material may be lost or too poor to study, and later authors applied the name to whatever they found that seemed similar. The result is a name that many people use but that no one can pin to a definite organism.
The paper by Walker, Schüßler, Vincent, Cranenbrouck and Declerck (2021), published in Fungal Systematics and Evolution 8:179-201, addresses exactly this problem for one widely used arbuscular mycorrhizal (AM) fungus. Its title is “Anchoring the species Rhizophagus intraradices (formerly Glomus intraradices)”. This post explains what anchoring means and why it matters to anyone who works with these fungi.
What anchoring means
To anchor a species is to tie its name firmly to a defined piece of evidence, so that the name can no longer drift. In practice this means linking the name to a specimen or living culture that can be examined, and to molecular data that characterise it. Once that link exists, other researchers can compare their own material with the anchor and decide whether it belongs to the same species or not.
Type specimens and epitypes
Under the Botanical Code, the permanent reference for a name is its type. The type is usually a preserved specimen chosen by the author of the name. If the original type is ambiguous and cannot be identified with certainty, a later author may designate an epitype. An epitype is a specimen or culture selected to serve as the interpretative type, and it fixes the meaning of the name where the original material is too poor to do so.
For fungi that can be cultured and sequenced, an epitype with associated DNA data is a very strong anchor, because it carries information that the original specimen could never have provided.
Why AM fungi pose a special challenge
AM fungi are obligate symbionts. They depend on living plant roots to complete their life cycle, so they are kept in pot cultures or in root organ cultures rather than on simple growth media. Their spores contain many nuclei, and these nuclei may carry sequence variants of the same gene. A single spore can therefore yield more than one version of a ribosomal sequence.
Combined with the limited number of structures that can be seen under the microscope, this means that species concepts based on appearance alone are fragile. A name may have been applied to several different fungi, or one fungus may have been given several names. Anchoring with a well-characterised culture and sequence data is one of the most reliable ways to bring order into such a situation.
The case of Rhizophagus intraradices
The species in the title was long known as Glomus intraradices. It became one of the most common AM fungi in research and in commercial inoculum, and a very large number of experiments, from plant nutrition to genomics, used isolates called by this name. Yet it was not always clear that all these isolates truly belonged to one species, or that the name matched the original description.
The genus Glomus itself has had to be reorganised. Many species once placed in it now belong to other genera, and Rhizophagus is one of them. The paper therefore deals with two questions at once: what exactly the species R. intraradices is, and how it relates to the well-known isolates that carried the old name.
The model strain
A particularly important isolate is DAOM197198, widely used as a model strain for experimental work and genome studies. It was long called Glomus intraradices DAOM197198. According to the information on the reference site, it is Rhizophagus irregularis DAOM197198. This matters because a very large body of literature uses the old name for this strain. A reader who looks only for R. intraradices may miss relevant work, and a reader who assumes the two names are interchangeable may draw wrong conclusions.
The data files that accompany the paper
The downloads page of the reference site offers two files from the study. One is an alignment with the extension .fas, and the other is a tree with the extension .tre.
- The alignment is a FASTA-format file in which sequences are lined up so that homologous positions fall in the same column.
- The tree file is a text file describing the phylogenetic tree built from that alignment, usually in Newick notation, which records branching order and branch lengths.
Providing both files lets readers check the work. They can open the alignment, inspect it, and run their own analyses. They can also load the tree into a viewer and see exactly how the isolates were grouped. Anyone who doubts a conclusion can test it directly with the same data. Sharing such files is a basic part of reproducible science, and it is particularly valuable in taxonomy, where a decision about a name rests on the evidence assembled for it.
Lessons from the genus Glomus
A related example comes from the genus Glomus. Its anchoring was fixed by culturing its type species, Glomus macrocarpum. Because the type species defines the genus, knowing what G. macrocarpum really is, and having it in culture, determines which species can legitimately stay in Glomus and which must be moved elsewhere.
This shows why anchoring is done at several levels. A genus is anchored through its type species, and a species is anchored through its type or epitype. When both levels are secure, a name such as Rhizophagus intraradices has a clear meaning, and the placement of the species in its genus can be defended with evidence.
What anchoring means for your own work
The idea is simple, but it changes how a careful researcher writes a methods section.
If you work with AM fungi, there are several practical consequences.
- Record the exact isolate or strain you used, not only the species name.
- Keep the culture accessible, or deposit it in a recognised collection, so that others can obtain it.
- Deposit sequences with clear information on their source.
- When reading older literature, check whether the name used then is still the accepted name now.
- Be cautious with sequences labelled with a species name that has not been anchored.
The authors of the reference site have also warned against poorly defined species, uncharacterised variability within species and biased selection of sequences in some publications. Anchoring is a direct answer to the first of these, and it also helps with the second, because a defined anchor gives a fixed point from which variation can be measured.
A caution on timing
The data on the reference site come from a project that was planned to end by the close of 2024, and they may be outdated from the formal point of view. Names and classifications can change after publication, so always compare the information with current taxonomic databases. The central message of the 2021 study is lasting, however. A species name is useful only if it points to something that can be examined, and anchoring is the step that makes this possible.
