WEB TUTORIAL 21.1 The Lifecycle of a Mushroom

Text Sections Section 21.3 Reproduction in Fungi, p. 354 Introduction Mushrooms have an unusual life cycle. After spores are released from mature fruit- ing bodies, they germinate to form hyphae. Collectively, the hyphae make up a branching web, called a mycelium. Hyphae derived from the spores of two differ- ent mating types then fuse to become one cell with two distinct nuclei. Thus begins a phase of life that is unique to fungi: the dikaryotic phase, in which cells in a fun- gal mycelium have two nuclei. This tutorial illustrates the relationships among these phases in the life cycle of a mushroom. Learning Objectives • Know the structures involved in each stage of a mushroom’s life cycle. • Understand why the life cycle of fungi is unique in the tree of life. Narration

The Life Cycle of a Mushroom A mushroom is a member of a group of fungi called the Basidiomycota. Let's con- sider its life cycle, beginning with the spores that are produced by the mature fruit- ing body.

A spore germinates, dividing by mitosis to produce a filament called a hypha. The hypha grows and branches to produce a filamentous network called a mycelium. The mycelium has a high surface-area-to-volume ratio, which allows the fungus to absorb nutrients efficiently.

If the hyphae of different mating types meet, they are attracted to each other and fuse, forming a cell with two nuclei. This is an unusual condition for a cell; in the normal case, one cell has one nucleus. This condition has come about because, while two cells fused, their nuclei remained separate.

Thus begins a phase of life that is unique to fungi: the dikaryotic phase, in which cells in a fungal mycelium have two nuclei. The mycelium continues to grow until the right environmental conditions trigger it to grow into a tightly packed, above- ground mass. The mature fruiting body, or mushroom, also consists of hyphae with two nuclei per cell.

The mushroom has a cap, in which the spores of the next generation are born. The spores arise on structures called gills on the underside of the cap.

The gills consist of hyphae and cells called basidia. The basidia are dikaryotic, con- taining two nuclei each, one from each mating type.

The two nuclei in a basidium fuse in preparation for meiosis. The nucleus then undergoes meiosis, producing four nuclei.

The basidium produces four spores on top of cellular extensions. The nuclei move into these spores.

The spores eventually break off from the basidium, and the life cycle repeats.

You should now be able to… • Explain what the dikaryotic phase is. • Diagram and describe the life cycle of a mushroom. • List the structures involved in spore production in a mushroom.