Why the rank of a group is a real question
Arbuscular mycorrhizal (AM) fungi live inside the roots of most land plants, where they exchange soil nutrients for plant-derived carbon. They have a long association with plants and form a distinct lineage of fungi. Few biologists doubt that they belong together. What is disputed is how the group should be ranked: as a phylum, called Glomeromycota, or as a subphylum, called Glomeromycotina, within a larger phylum.
This may sound like a matter of labels, but rank carries meaning. A phylum is expected to be a major, ancient division of life, comparable in depth to other fungal phyla. If AM fungi are given a lower rank, the classification implies they are a branch of something bigger. If they keep phylum rank, it implies they are one of several coequal main lineages.
Two competing proposals
The first proposal treats AM fungi as a phylum. This was the standard view when the group was first separated from the traditional Zygomycota on the basis of molecular data. The second proposal places the fungi in the subphylum Glomeromycotina, grouping them with other early-diverging fungal lineages in a single larger phylum, so that several basal lineages are merged into one.
The position taken on this site
The authors of this site do not adopt the placement of AM fungi in Glomeromycotina. Their reasoning is that the phylum level may be resurrected, that is, restored if further evidence supports it. They also see a problem of consistency, which is explained below.
How the group came to be recognised
Before molecular methods, AM fungi were placed among the zygomycetes because of features such as their coenocytic hyphae, which lack regular cross walls, and their large multinucleate spores. Gene sequences showed that this grouping was artificial. The AM fungi formed their own lineage, separate from the other fungi that had been called zygomycetes. That discovery is what led to the idea of a separate phylum in the first place, and it also explains why the debate about rank keeps returning: the position of the lineage on the tree is clear, but the best label for it is not.
The argument from consistency
The main point is about comparability. Among the fungi, the Dikarya comprise the Ascomycota and the Basidiomycota. Each of these is a large, well-defined phylum with a long evolutionary history. If many high-ranking basal lineages were merged into a single phylum, that phylum would not be comparable with the Ascomycota or Basidiomycota. The result would be a system in which ranks are applied by different standards in different parts of the tree.
Ranks above species are conventions, not natural units. Still, a convention is useful only if it is applied evenly. Comparing “phyla” is meaningful only if they represent roughly similar levels of divergence. Putting several ancient lineages into one phylum while keeping the two main groups of Dikarya separate undermines that logic.
A possible compromise: a superphylum
The authors suggest an alternative. A superphylum, perhaps called Zygomycota, could host several ancestral fungal phyla. Zygomycota is a discarded taxon, since the old group has been shown not to be a single natural lineage. Using the name in a higher rank, however, is a way to respect classical usage while accepting the molecular results.
In such a scheme the AM fungi would remain zygomycetes in the broad, traditional sense. That is a compromise with the way the group has long been described in textbooks and the older literature, and it would help users who still rely on that terminology. It also keeps the phylum rank available for the lineages that deserve it.
- The phylum level is preserved for deep, distinct lineages.
- The classical term “zygomycetes” retains a place, though at a higher rank.
- The comparison with Ascomycota and Basidiomycota stays consistent.
It is worth adding that a name is not the same as a hypothesis. Whatever label is chosen, the same fungi are included, and the same sequences place them on the tree. Disagreement about rank can therefore be discussed calmly, without suggesting that either side has misunderstood the data.
What the Botanical Code says, and does not say
An important part of this discussion is nomenclature. The Botanical Code, which governs the names of fungi as well as plants and algae, does not regulate all ranks in the same way. Names above the rank of family are not governed by priority. In other words, there is no rule that the oldest name must be used for a phylum or a subphylum. Up to and including family, the Code is binding.
This difference has two consequences. First, the choice between Glomeromycota and Glomeromycotina is not a matter that the Code can settle. It is a scientific and practical decision, which depends on arguments about the structure of the tree and on consistency. Second, nobody can claim that a particular name is mandatory because of priority. Authors who defend one option over another should offer reasons, not appeal to rules that do not exist.
Sticking to the Code and the expert system
For names that are governed by the Code, the authors follow it strictly and consult Index Fungorum as an expert system. They do not follow what is sometimes called taxa inflation, in which authors set their own rules and ignore parts of the Code. The result of such behaviour is a literature in which the same group has several competing names, and in which it is difficult for students or non-specialists to know what is valid.
Caution about formal status
Readers should note that this site is scheduled to close by the end of 2024, and that its data may be outdated from the formal point of view. A statement about rank here reflects the view of the authors at the time and is not a final ruling. Where the question matters for a publication, the most recent literature and nomenclatural databases should be consulted.
What this means for students and researchers
The debate does not alter the biology. The fungi in question are still obligate symbionts of plants that form arbuscules in root cortical cells, and their evolutionary position is still being refined with new data. The choice of name changes how the group is presented, not what it is.
For practical work, a few habits help:
- State which classification you follow, and cite it.
- When you read papers, check whether Glomeromycota or Glomeromycotina is used, and do not assume the authors disagree about the biology.
- Use family names and genus names with the Code in mind, since these are the ranks where the rules are binding and where confusion is most costly.
- Treat high ranks as hypotheses about the shape of the fungal tree, which can be revised when better data become available.
Looking ahead
The question of rank will probably remain open until the relationships among the early-diverging fungal lineages are better resolved. If these relationships become clearer, the case for keeping a separate phylum, or for merging, will be easier to judge. Until then, a cautious approach is reasonable: avoid changes that weaken the consistency of the system, keep the classical usage understandable, and accept that a rank is a tool, not a discovery.
