key concept 29.3 Sex in Fungi Involves Multiple Mating Types

Major fungal groups were originally defined by their structures and processes for sexual reproduction and also, to a lesser extent, by other morphological differences. Although fungal life cycles are even more diverse than was once realized, specific types of life cycles generally distinguish the six major groups of fungi: microsporidia, chytrids, zygospore fungi (Zygomycota), arbuscular mycorrhizal fungi (Glomeromycota), sac fungi (Ascomycota), and club fungi (Basidiomycota). Figure 29.11 diagrams the evolutionary relationships of these groups as they are understood today.

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Figure 29.11 A Phylogeny of the Fungi Microsporidia are reduced, parasitic fungi whose relationships among the fungi are uncertain. They may be the sister group of most other fungi, or they may be more closely related to particular groups of chytrids or zygospore fungi. The dashed lines indicate that chytrids and zygospore fungi are thought to be paraphyletic; the relationships of the lineages within these two informal groups (see Table 29.1) are not yet well resolved. The sac fungi and club fungi together form the clade Dikarya.

Activity 29.1 Fungal Phylogeny

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focus your learning

  • In most fungi, sexual reproduction involves genetically determined distinctions between two or more mating types.

  • Microsporidia are parasitic fungi that lack true mitochondria.

  • Chytrids have flagellated gametes and spores.

  • Club fungi produce spectacular fruiting structures called basidiomata.

The chytrids and the zygospore fungi may not represent monophyletic groups, as they each consist of several distantly related lineages that retain some ancestral features. The clades that are thought to be monophyletic within these two informal groupings are listed in Table 29.1. Recent evidence from DNA analyses has established the placement of the microsporidia among the fungi, the likely paraphyly of the chytrids and the zygospore fungi, the independence of arbuscular mycorrhizal fungi from the other fungal groups, and the monophyly of sac fungi and club fungi.

table 29.1 Classification of the Fungi
Group Common name Features
Microsporidia Microsporidia Intracellular parasites of animals; greatly reduced, among smallest eukaryotes known; polar tube used to infect hosts
Chytrids (paraphyletic)a Chytrids Mostly aquatic and microscopic; zoospores and gametes have flagella
Chytridiomycota
Neocallimastigomycota
Blastocladiomycota
Zygomycota (paraphyletic)a Zygospore fungi Reproductive structure is a unicellular zygospore with many diploid nuclei; hyphae coenocytic; no fleshy fruiting body
Entomophthoromycotina
Kickxellomycotina
Mucoromycotina
Zoopagomycotina
Glomeromycota Arbuscular mycorrhizal fungi Form arbuscular mycorrhizae in plant roots; only asexual reproduction is known
Ascomycota Sac fungi Sexual reproductive saclike structure known as an ascus, which contains haploid ascospores; hyphae septate; dikaryon
Basidiomycota Club fungi Sexual reproductive structure is a basidium, a swollen cell at the tip of a specialized hypha that supports haploid basidiospores; hyphae septate; dikaryon

aThe formally named groups within the chytrids and Zygomycota are each thought to be monophyletic, but their relationships to one another (and to microsporidia) are not yet well resolved.