Zygomycetes and Other Fungal Divisions

The zygomycetes are a historical grouping of fast-growing, mostly filamentous fungi traditionally defined by the formation of a thick-walled sexual spore called a zygospore. Molecular phylogenetics has shown that the old phylum 'Zygomycota' is not a single natural lineage, and its members have been reassigned to the phylum Mucoromycota and several allied phyla, alongside other early-diverging divisions of the fungal kingdom.

Definition

Zygomycetes are coenocytic (largely non-septate) filamentous fungi historically grouped by their production of zygospores; the grouping is now recognised as non-monophyletic, its members distributed mainly among Mucoromycota and allied early-diverging phyla rather than a single phylum Zygomycota.

Scope

This topic covers the zygomycete fungi and other non-Dikarya divisions of the kingdom Fungi. It explains the zygospore as the historical defining feature, the reasons the group was broken up by genome-scale phylogenetics, and the position of related early-diverging lineages. The clinically notable order Mucorales is treated as the principal MeSH anchor. The entry is a reference overview and gives no diagnostic or treatment guidance.

Core questions

• What is a zygospore and why did it define the historical zygomycetes?
• Why was the phylum 'Zygomycota' abandoned as a natural group?
• How are former zygomycetes distributed among Mucoromycota and allied phyla today?
• How do the early-diverging fungal divisions differ from the Dikarya?

Key concepts

• Zygospore
• Coenocytic (largely aseptate) hyphae
• Sporangia and sporangiospores
• Order Mucorales
• Phylum Mucoromycota
• Non-monophyly of 'Zygomycota'
• Early-diverging (basal) fungal lineages
• Wide, ribbon-like non-septate hyphae in tissue

Mechanisms

Classical zygomycetes form a zygospore when two compatible hyphae fuse, and they typically reproduce asexually by sporangiospores produced inside a sporangium; their hyphae are broad and largely without cross-walls (coenocytic). Genome-scale phylogenetic analysis showed that taxa sharing these features do not form a single clade, prompting their reclassification into the phylum Mucoromycota and several allied phyla rather than one phylum Zygomycota (Spatafora et al., 2016), refining the earlier higher-level scheme of Hibbett et al. (2007). The broad, sparsely septate hyphae of these fungi give them a distinctive appearance in tissue sections (Guarner & Brandt, 2011).

Clinical relevance

Members of the order Mucorales are the agents of mucormycosis, an invasive infection whose biology helps explain why it is recognised as difficult to manage (Katragkou et al., 2014), and the characteristic wide, ribbon-like, sparsely septate hyphae aid its recognition in histopathology (Guarner & Brandt, 2011). This entry describes the classification and morphology that make these fungi identifiable and is not a source of individual diagnostic or therapeutic recommendations.

Evidence & guidelines

The current classification of former zygomycetes follows the genome-scale phylum-level revision of Spatafora et al. (2016), set against the earlier kingdom-wide scheme of Hibbett et al. (2007). Clinical and histopathologic context for the Mucorales is drawn from Guarner and Brandt (2011) and Katragkou et al. (2014).

History

For much of the twentieth century the phylum Zygomycota united fungi that produced zygospores, with the Mucorales as its best-known order. As DNA-based phylogenies accumulated, the group proved to be an artificial assemblage of several distinct lineages; the higher-level classification of Hibbett et al. (2007) already flagged this instability, and the genome-scale analysis of Spatafora et al. (2016) formally replaced 'Zygomycota' with Mucoromycota and allied phyla.

Debates

Is 'Zygomycota' a valid phylum?

Genome-scale phylogenetics indicates that the zygospore-forming fungi do not share a single common ancestor to the exclusion of others, so the phylum was dissolved and its members reassigned to Mucoromycota and allied phyla; the older name persists informally.

Key figures

• Joseph W. Spatafora
• Gerald L. Benny
• David Hibbett
• Jason E. Stajich

Seminal works

• spatafora-2016
• hibbett-2007

Frequently asked questions

Is 'Zygomycota' still a recognised phylum?

No. Genome-scale studies showed it was not a single natural lineage, and its members have been reassigned mainly to the phylum Mucoromycota and several allied early-diverging phyla, though the term zygomycetes remains in informal use.

What distinguishes zygomycete hyphae?

They are typically broad and coenocytic, meaning they have few or no cross-walls (septa), which gives them a characteristic wide, ribbon-like appearance compared with the regularly septate hyphae of Dikarya.