Biological Classification · Lesson 7 of 10
Fungal Groups: Phycomycetes to Deuteromycetes
“Compare four traditional fungal groups by hyphal structure, spores, sexual processes, habitats and familiar species.”
• Describe the defining teaching features of four traditional fungal groups. • Distinguish zoospores, sporangiospores, conidia, ascospores and basidiospores. • Compare aseptate and septate mycelia among major fungal examples. • Explain how an ascus differs from a basidium. • Recognise why Deuteromycetes and Phycomycetes require historical context.
Why are fungi divided into groups?
Most fungi obtain nutrients by absorption, but their bodies and reproductive structures can differ considerably. Some form coenocytic hyphae, others form branched septate hyphae, and others grow as single yeast cells. Some release spores from sacs, whereas others produce them externally from specialised cells. In the traditional classification used here, fungal groups were distinguished using these characters, including the form of their mycelium and sexual fruiting bodies.
Molecular evidence has since changed fungal classification. The four groups below are therefore useful for mastering the original structural comparisons but are not four equivalent natural branches of the modern fungal tree. In particular, the old Phycomycetes category included organisms that are now placed in several different groups, including some that are not true fungi, while Deuteromycetes was a temporary grouping based on not knowing sexual stages.
Phycomycetes: a traditional grouping of water moulds and bread mould relatives
The source describes phycomycetes as organisms of aquatic habitats, moist decaying wood or parasitic environments. Their hyphae are typically aseptate and coenocytic. Many familiar bread moulds such as Rhizopus and Mucor form sporangia, inside which numerous asexual spores develop. In some traditionally included forms, motile zoospores disperse through water; in others, non-motile aplanospores are produced.
A sac-like structure in which spores are produced, often internally.
A motile, usually flagellated asexual spore produced by certain aquatic fungi and fungus-like organisms.
In bread mould relatives, sexual reproduction involves the fusion of compatible structures, followed eventually by a resistant sexual stage. The original grouping also includes Albugo, which is now classified among the oomycetes, a lineage distinct from true fungi. This is why names such as Phycomycetes are historical categories, not a reliable description of current evolutionary relationships.
Sexual interactions were also described traditionally in terms of isogamy, anisogamy and oogamy. Isogamy means fusion of similar gametes; anisogamy refers to differently sized gametes, while oogamy features a relatively large non-motile egg and smaller male gamete. These ideas help explain variation in gamete form, but individual examples differ in their actual sexual structures.
Ascomycetes: sac fungi
Ascomycetes include yeasts, many common moulds and species that form morels or truffles. Their filamentous forms generally have branched, septate hyphae. Yeasts such as Saccharomyces are often unicellular, showing that members of a broad fungal lineage need not share the same overall body shape. Ascomycetes can be decomposers, plant parasites or partners in symbioses.
Conidia and asci
Asexual conidia commonly develop externally on specialised structures called conidiophores. For sexual reproduction, the distinguishing structure is an ascus, a sac-like cell in which ascospores form. Groups of asci can be enclosed or supported within an ascocarp, a reproductive fruiting body. The number of ascospores varies among taxa, although eight spores in a typical ascus is a frequently illustrated outcome of meiosis followed by a mitotic division.
Examples include Aspergillus, Penicillium, Claviceps and Neurospora. Neurospora has been important in genetics research because investigators can use it to study heredity and biochemical pathways. Certain edible morels and truffles are also ascomycetes.
Basidiomycetes: club fungi and mushrooms
Basidiomycetes include familiar mushrooms, many bracket fungi, puffballs, rust fungi and smuts. Their hyphae are commonly branched and septate. In many basidiomycetes, the main vegetative mycelium maintains a dikaryotic state after plasmogamy. The sexual structure that gives the group its name is the basidium, typically a club-shaped cell.
Two compatible fungal hyphae can fuse cytoplasm; their haploid nuclei coexist for a period. In the developing basidium, karyogamy creates a diploid nucleus and meiosis produces haploid nuclei. Commonly four basidiospores develop externally on one basidium. Basidia are found on surfaces of a basidiocarp such as the familiar mushroom fruiting body.
Agaricus is a typical mushroom example. Puccinia includes plant rust fungi, while Ustilago includes smut fungi. The edible mushroom is a fruiting structure, not the entire organism: the mycelium normally extends through the substrate.
Deuteromycetes: a category based on incomplete knowledge
Deuteromycetes were called imperfect fungi because only the asexual stage had been observed. These fungi could reproduce with conidia and often displayed septate, branched mycelia. As sexual stages were discovered or DNA comparisons clarified relationships, many species were reassigned to established groups such as Ascomycetes or Basidiomycetes. A missing observed sexual stage is not proof that an organism lacks all possible sexual processes.
Traditional examples include Alternaria, Colletotrichum and Trichoderma. Many species occur as litter decomposers, with others acting as plant pathogens or interacting with roots and other organisms. This category shows how scientific groupings change when investigators gain new evidence.
| Traditional group | Hyphae | Asexual structures | Sexual structures or status | Examples |
|---|---|---|---|---|
| Phycomycetes | Typically coenocytic in the described forms | Zoospores or aplanospores inside sporangia | Different fusion structures; traditional zygospore examples | Mucor, Rhizopus; historically Albugo |
| Ascomycetes | Mostly branched and septate; some yeasts unicellular | Conidia on conidiophores | Ascospores inside asci; ascocarps | Aspergillus, Neurospora, Saccharomyces |
| Basidiomycetes | Branched and septate | Fragmentation common; asexual spores variable | Basidiospores outside basidia; basidiocarps | Agaricus, Puccinia, Ustilago |
| Deuteromycetes | Typically septate and branched | Conidia | Sexual stage not known when placed in this historical group | Alternaria, Colletotrichum, Trichoderma |
Albugo belongs to the oomycetes rather than true fungi, and Deuteromycetes is not accepted as one natural evolutionary group. Learn the structural descriptions and examples while recognising the reason for later reclassification.
Problem
A filamentous fungus makes sexual spores inside sacs. Another produces spores on the outside of club-shaped cells. Identify the most likely traditional groups.
- 1.The sac-forming fungus is consistent with Ascomycetes because its sexual spores form in asci.
- 2.The second fungus is consistent with Basidiomycetes because basidiospores form externally on basidia.
- 3.Confirm with additional features such as the anatomy of the fruiting body and molecular evidence when precise identification is required.
Quick Quiz
Quiz
Which traditional group commonly has coenocytic, aseptate hyphae?
Where are ascospores produced?
What is the defining sexual spore-bearing cell in basidiomycetes?
Why was the Deuteromycetes group called imperfect?
Which organism is a common mushroom example?
Why does Albugo require a historical clarification?
Practice Problems
- Compare phycomycetes and ascomycetes using hyphae and spore formation.
- Explain the relationship among a basidium, basidiospores and a basidiocarp.
- Describe how conidia differ from ascospores.
- Explain why the category Deuteromycetes became difficult to maintain as a natural classification.
- Prepare a four-row comparison using the groups, their examples, nutritional modes and sexual or asexual reproductive structures.
Traditional phycomycetes often have aseptate, coenocytic hyphae and produce spores in sporangia; ascomycetes usually have septate hyphae, conidia as common asexual spores and ascospores within asci.
Key Takeaways
Fungi were traditionally grouped by hyphal structure and reproductive characters. Phycomycetes: often coenocytic; sporangia and varied gamete fusion. Ascomycetes: asci with internal ascospores and often conidia. Basidiomycetes: basidia bearing external basidiospores, often arranged in basidiocarps. Deuteromycetes: historical grouping for forms with no known sexual stage; many were later reclassified. Historical grouping names and contemporary evolutionary relationships are not always identical.