Ascomycetes and Basidiomycetes

Ascomycetes (Ascomycota) and basidiomycetes (Basidiomycota) are the two largest phyla of fungi and together form the subkingdom Dikarya, the most familiar and species-rich part of the fungal kingdom. They are distinguished above all by the cell in which they form sexual spores: the sac-like ascus in ascomycetes and the club-shaped basidium in basidiomycetes.

Definition

Ascomycota are fungi that produce sexual spores (ascospores) inside a sac-like cell called an ascus, whereas Basidiomycota produce sexual spores (basidiospores) externally on a club-shaped cell called a basidium; together they constitute the subkingdom Dikarya.

Scope

This topic covers the defining features, diversity, and identification relevance of the two Dikarya phyla. It explains the ascus and basidium as diagnostic sexual structures, the position of these phyla in the molecular-phylogenetic classification of the kingdom, and the asexual (anamorph) states through which many medically and ecologically important members are encountered. It is a reference entry and does not give diagnostic or treatment guidance.

Core questions

• What structural features distinguish ascomycetes from basidiomycetes?
• Why are these two phyla grouped together as Dikarya?
• How do asexual (anamorph) states relate to the sexually defined classification?
• How does molecular phylogenetics resolve relationships within and between the two phyla?

Key concepts

• Ascus and ascospores
• Basidium and basidiospores
• Subkingdom Dikarya
• Dikaryotic (n+n) hyphal stage
• Teleomorph and anamorph states
• Yeasts versus filamentous (mould) growth forms
• Conidia and conidiogenesis

Mechanisms

In ascomycetes, karyogamy and meiosis occur within an ascus, producing ascospores that are typically borne in characteristic fruiting bodies; in basidiomycetes, these events occur in a basidium that bears basidiospores externally. Both phyla pass through a distinctive dikaryotic stage in which two compatible nuclei coexist in the same hyphal compartment before fusing, which is the unifying feature that places them in Dikarya (Hibbett et al., 2007). Many members are encountered chiefly in an asexual state producing mitospores (conidia), and matching such anamorphs to their sexual teleomorphs was a long-standing challenge that molecular tools, including ITS sequencing, helped resolve (Schoch et al., 2012).

Clinical relevance

Both phyla include fungi relevant to medicine: many common moulds and yeasts encountered in diagnostic mycology are ascomycetes, while certain basidiomycetous yeasts are also of clinical interest, and histopathology helps recognise their tissue forms (Guarner & Brandt, 2011). This entry describes the biology and classification that make such fungi recognisable; it is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The phylum-level placement of Ascomycota and Basidiomycota within Dikarya follows the molecular-phylogenetic classification of Hibbett et al. (2007) and the Deep Hypha synthesis (Blackwell et al., 2006); ITS barcoding (Schoch et al., 2012) provides the standard sequence marker for identification within both phyla.

History

The distinction between sac fungi and club fungi predates molecular biology and was originally drawn from the visible sexual structures and fruiting bodies. Recognition that both groups share a dikaryotic stage led to their union as Dikarya, and the higher-level classification of Hibbett et al. (2007) formalised this within a phylogenetic framework. The subsequent abandonment of separate names for asexual states, enabled in part by DNA barcoding (Schoch et al., 2012), further integrated anamorphic fungi into this scheme.

Key figures

• David Hibbett
• Meredith Blackwell
• Joseph W. Spatafora
• Conrad Schoch

Seminal works

• hibbett-2007
• schoch-2012
• blackwell-2006

Frequently asked questions

What is the main difference between an ascomycete and a basidiomycete?

Ascomycetes form their sexual spores inside a sac-like cell called an ascus, while basidiomycetes form theirs externally on a club-shaped cell called a basidium.

Why are these two phyla called Dikarya?

Both pass through a dikaryotic stage in which two genetically distinct nuclei share a hyphal compartment before fusing; this shared feature unites them in the subkingdom Dikarya.