Unit Four. The Evolution and Diversity of Life
18.3. Reproduction and Nutrition of Fungi
How Fungi Reproduce
Fungi reproduce both asexually and sexually. All fungal nuclei except for the zygote are haploid. Often in the sexual reproduction of fungi, individuals of different “mating types” must participate, much as two sexes are required for human reproduction.
Sexual reproduction is initiated when two hyphae of genetically different mating types come in contact, and the hyphae fuse. What happens next? In animals and plants, when the two haploid gametes fuse, the two haploid nuclei immediately fuse to form the diploid nucleus of the zygote. As you might by now expect, fungi handle things differently. In most fungi, the two nuclei do not fuse immediately. Instead, they remain unmarried inhabitants of the same house, coexisting in a common cytoplasm for most of the life of the fungus! A fungal hypha that has two nuclei is called dikaryotic. If the nuclei are derived from two genetically different individuals, it is called a het-erokaryon (Greek, heteros, other, and karyon, kernel or nucleus). A fungal hypha in which all the nuclei are genetically similar is said to be a homokaryon (Greek, homo, one).
When reproductive structures are formed in fungi, complete septa form between cells, the only exception to the free flow of cytoplasm between cells of the fungal body. There are three kinds of reproductive structures: (1) gametangia form haploid gametes, which fuse to give rise to a zygote that undergoes meiosis; (2) sporangia produce haploid spores that can be dispersed; and (3) conidiophores produce asexual spores called conidia that can be produced quickly and allow for the rapid colonization of a new food source.
Spores are a common means of reproduction among the fungi. The puffball fungus in figure 18.4 is releasing spores in a somewhat explosive manner. Spores are well suited to the needs of an organism anchored to one place. They are so small and light that they may remain suspended in the air for long periods of time and may be carried great distances. When a spore lands in a suitable place, it germinates and begins to divide, soon giving rise to a new fungal hypha.
Figure 18.4. Many fungi produce spores.
Spores explode from the surface of a puffball fungus.
All fungi obtain their food by secreting digestive enzymes into their surroundings and then absorbing back into the fungus the organic molecules produced by this external digestion. Many fungi are able to break down the cellulose in wood, cleaving the linkages between glucose subunits and then absorbing the glucose molecules as food. That is why fungi are so often seen growing on trees.
Just as some plants like the Venus flytrap are active carnivores, so some fungi are active predators. For example, the edible oyster fungus Pleurotus ostreatus, shown growing on a tree in figure 18.5, attracts tiny roundworms known as nematodes that feed on it—and secretes a substance that anesthetizes the nematodes. When the worms become sluggish and inactive, the fungal hyphae envelop and penetrate their bodies and absorb their contents, a rich source of nitrogen (always in short supply in natural ecosystems).
Figure 18.5. The oyster mushroom.
This species, Pleurotus ostreatus, immobilizes nematodes, which the fungus uses as a source of food.
Key Learning Outcome 18.3. Fungi reproduce both asexually and sexually. They obtain their nutrients by secreting digestive enzymes into their surroundings and then absorbing the digested molecules back into the fungal body.