General Biology of the Fungi

The kingdom Fungi encompasses heterotrophic organisms with absorptive nutrition and with chitin in their cell walls. The fungi live by absorptive nutrition: They secrete digestive enzymes that break down large food molecules in the environment, and then absorb the breakdown products. Many fungi are saprobes that absorb nutrients from dead matter, others are parasites that absorb nutrients from living hosts (Figure 31.1), and still others are mutualists that live in intimate association with other organisms.

The production of chitin, a polysaccharide, is a synapo-morphy (shared derived trait) for fungi, choanoflagellates, and animals. That is, its presence in fungi is the evidence that all fungi are more closely related to animals than any fungi are to plants. Chitin is used in the cell walls of fungi, but it is used in other ways in animals. The use of chitin in cell walls is a synapomorphy for fungi, and it allows us to distinguish between the fungi and the basal eukaryotes (protists) that resemble them. Some protists that were formerly confused with fungi include the slime molds (see Figures 28.31 and 28.32) and water molds (oomycetes; see Figure 28.23).

Fungal fruiting body

Fungal fruiting body

Zygomycetes

The alternation between gametophyte (n) and sporophyte (2n) generations that evolved in plants (see Chapter 29) is found in only the most basal group of fungi, the chytrids. The derived condition, which is found in the other three fungal clades, involves a unique state in which two haploid nuclei are present in a single cell, discussed later in this chapter. As one might expect, the chytrids, which are aquatic, possess flagellated gametes (or spores). Flagella have been lost in the terrestrial fungi.

The kingdom Fungi consists of four phyla: Chytridiomy-cota, Zygomycota, Ascomycota, and Basidiomycota. We distinguish the phyla on the basis of their methods and structures for sexual reproduction and, to a lesser extent, by criteria such as the presence or absence of cross-walls separating their cell-like compartments. This morphologically based phylogeny has proved largely consistent with phylo-genies based on DNA sequencing. The term "fungal system-atics" has an interesting anagram, "fantastic ugly mess," but we'll see that the situation isn't all that bad.

In the sections that follow, we'll consider some aspects of the general biology of the fungi, including their body structure and its intimate relationship with their environment, their nutrition, and some special aspects of their unusual sexual reproductive cycles.

Some fungi are unicellular

Unicellular forms are found in all of the fungal phyla. Unicellular members of the Zygomycota, Ascomycota, and Ba-sidiomycota are called yeasts. Yeasts may reproduce by budding, by fission, or by sexual means (Figure 31.2). Their means of reproduction help us to place them in their appropriate phyla, as we will see below.

The body of a multicellular fungus is composed of hyphae

Most fungi are multicellular. The body of a multicellular fungus is called a mycelium (plural, mycelia). It is composed of rapidly growing individual tubular filaments called

31.1 Parasitic Fungi Attack Other Living Organisms (a) The gray masses on this ear of corn are the parasitic fungus Ustilago maydis,common\y called corn smut. (b) The tropical fungus whose fruiting body is growing out of the carcass of this ant has developed from a spore ingested by the ant.The spores of this fungus must be ingested by insects before they will germinate and develop.The growing fungus absorbs organic and inorganic nutrients from the ant's body, eventually killing it, after which the fruiting body produces a new crop of spores.

Saccharomyces

Saccharomyces sp.

31.2 Yeasts Are Unicellular Fungi Unicellular members of the fungal phyla Zygomycota,Ascomycota,and Basidiomycota are known as yeasts. Many yeasts reproduce by budding—mitosis followed by asymmetrical cell division—as those shown here are doing.

Saccharomyces sp.

31.2 Yeasts Are Unicellular Fungi Unicellular members of the fungal phyla Zygomycota,Ascomycota,and Basidiomycota are known as yeasts. Many yeasts reproduce by budding—mitosis followed by asymmetrical cell division—as those shown here are doing.

hyphae (singular, hypha). Within hyphae of two clades, incomplete cross-walls called septa (singular, septum) divide the hypha into separate cells. Pores in the septa allow organelles—sometimes even nuclei—to move in a controlled way between cells (Figure 31.3). Other hyphae are coenocytic and have no septa.

Certain modified hyphae, called rhizoids, anchor chytrids and some other fungi to their substratum (the dead organism or other matter upon which they feed). These rhizoids are not homologous to the rhizoids of plants because they are not specialized to absorb nutrients and water. Parasitic fungi may possess modified hyphae that take up nutrients from their host.

The total hyphal growth of a mycelium (not the growth of an individual hypha) may exceed 1 km per day. The hy-

Coenocytic Hyphae

Nuclei Cell wall

(a) Coenocytic hypha

Nuclei Cell wall

Showing Hyphal Septa

Pore Septum

Septa are not complete: Pores allow movement of organelles and other materials between celllike compartments.

(b) Septate hypha

Pore Septum

Septa are not complete: Pores allow movement of organelles and other materials between celllike compartments.

(a) Coenocytic hypha

(b) Septate hypha

Grass cells

Grass cells

Grass Cells Images

Fungal hyphae

31.3 Most Hyphae Are Incompletely Divided into Separate Cells

(a) Coenocytic hyphae have no septa between their nuclei. (b) Even in septate hyphae, the septa do not block the movement of organelles within the hypha.

phae may be widely dispersed to forage for nutrients over a large area, or they may clump together in a cottony mass to exploit a rich nutrient source. Sometimes, when sexual spores are produced, the mycelium becomes reorganized into a fruiting (reproductive) structure such as a mushroom.

The way in which a parasitic fungus attacks a plant illustrates the absorptive role of fungal hyphae (Figure 31.4). The hyphae of a fungus invade a leaf through the stomata, through wounds, or in some cases, by direct penetration of epidermal cells. Once inside the leaf, the hyphae form a mycelium. Some hyphae produce haustoria, branching projections that push into the living plant cells, absorbing the nutrients within the cells. The haustoria do not break through the plant cell plasma membranes; they simply press into the cells, with the membrane fitting them like a glove. Fruiting structures may form, either within the plant body or on its surface.

Fungal hyphae

31.4 A Fungus Attacks a Leaf The white structures in the micrograph are hyphae of the fungus Blumeriagraminis,which is growing on the dark surface of the leaf of a grass.

Fungi are in intimate contact with their environment

The filamentous hyphae of a fungus give it a unique relationship with its physical environment. The fungal mycelium has an enormous surface area-to-volume ratio compared with that of most large multicellular organisms. This large ratio is a marvelous adaptation for absorptive nutrition. Throughout the mycelium (except in fruiting structures), all the hyphae are very close to their environmental food source.

Another characteristic of some fungi is their tolerance for highly hypertonic environments (those with a solute concentration higher than their own; see Chapter 5). Many fungi are more resistant than bacteria to damage in hypertonic surroundings. Jelly in the refrigerator, for example, will not become a growth medium for bacteria because it is too hypertonic to the bacteria, but it may eventually harbor mold colonies. Their presence in the refrigerator illustrates another trait of many fungi: tolerance of temperature extremes. Many fungi tolerate temperatures as low as 5-6°C below freezing, and some tolerate temperatures as high as 50°C or more.

Fungi are absorptive heterotrophs

All fungi are heterotrophs that obtain food by direct absorption from their immediate environment. The majority are saprobes, obtaining their energy, carbon, and nitrogen directly from dead organic matter through the action of enzymes they secrete. However, as we've learned already, some are parasites, and still others form mutualistic associations with other organisms.

Saprobic fungi, along with bacteria, are the major decomposers of the biosphere, contributing to decay and thus to the recycling of the elements used by living things. In the forest, for example, the mycelia of fungi absorb nutrients from fallen trees, thus decomposing their wood. Fungi are the principal decomposers of cellulose and lignin, the main components of plant cell walls (most bacteria cannot break down these materials). Other fungi produce enzymes that decompose keratin and thus break down animal structures such as hair and nails.

Because many saprobic fungi are able to grow on artificial media, we can perform experiments to determine their exact nutritional requirements. Sugars are their favored source of carbon. Most fungi obtain nitrogen from proteins or the products of protein breakdown. Many fungi can use nitrate (NO3) or ammonium (NH4+) ions as their sole source of nitrogen. No known fungus can get its nitrogen directly from nitrogen gas, as can some bacteria and plant-bacteria associations (see Chapter 37). Nutritional studies also reveal that most fungi are unable to synthesize their own thiamin (vitamin B:) or biotin (another B vitamin) and must absorb these vitamins from their environment. On the other hand, fungi can synthesize some vitamins that animals cannot. Like all organisms, fungi also require some mineral elements.

Nutrition in the parasitic fungi is particularly interesting to biologists. Facultative parasites can attack living organisms but can also be grown by themselves on artificial media. Obligate parasites cannot be grown on any available medium; they can grow only on their specific living hosts, usually plants. Because their growth is limited to living hosts, they must have specialized nutritional requirements.

Some fungi have adaptations that enable them to function as active predators, trapping nearby microscopic protists or animals. The most common strategy is to secrete sticky substances from the hyphae so that passing organisms stick tightly to them. The hyphae then quickly invade the prey, growing and branching within it, spreading through its body, absorbing nutrients, and eventually killing it.

A more dramatic adaptation for predation is the constricting ring formed by some species of Arthrobotrys, Dacty-laria, and Dactylella (Figure 31.5). All of these fungi grow in soil. When nematodes (tiny roundworms) are present in the soil, these fungi form three-celled rings with a diameter that just fits a nematode. A nematode crawling through one of these rings stimulates the fungus, causing the cells of the ring to swell and trap the worm. Fungal hyphae quickly invade and digest the unlucky victim.

Two other kinds of relationships between fungi and other organisms have nutritional consequences for the fungal partner. These relationships are highly specific, symbiotic (the partners live in close, permanent contact with one another), and mutualistic (the relationships benefit both partners). Lichens are associations of a fungus with a cyanobacterium, a unicellular photosynthetic protist, or both. Mycorrhizae (singular, mycorrhiza) are associations between fungi and the roots of plants. In these associations, the fungus obtains organic com-

Roundworm

Fungal loop /

Roundworm

Fungal loop /

Nematode Trapping Fungi
31.5 Some Fungi Are Predators A nematode (roundworm) is trapped in sticky loops of the soil-dwelling fungus Arthrobotrys anchonia.

pounds from its photosynthetic partner, but provides it with minerals and water in return, so that the partner's nutrition is also promoted. In fact, many plants could not grow at all without their fungal partners. We will discuss lichens and my-corrhizae more thoroughly later in this chapter.

Most fungi reproduce both asexually and sexually

Both asexual and sexual reproduction are common among the fungi. Asexual reproduction takes several forms:

► The production of (usually) haploid spores within structures called sporangia.

► The production of naked spores (not enclosed in sporangia) at the tips of hyphae; such spores are called conidia (from the Greek konis, "dust").

► Cell division by unicellular fungi—either a relatively equal division (called fission) or an asymmetrical division in which a small daughter cell is produced (called budding).

► Simple breakage of the mycelium.

Asexual reproduction in fungi can be spectacular in terms of quantity. A 2.5-centimeter colony of Penicillium can produce as many as 400 million conidia. The air we breathe contains as many as 10,000 fungal spores per cubic meter.

Sexual reproduction in many fungi features an interesting twist. There is often no morphological distinction between female and male structures, or between female and male individuals. Rather, there is a genetically determined distinction between two or more mating types. Individuals of the same mating type cannot mate with one another, but they can mate with individuals of another mating type within the same species. This distinction prevents self-fertilization. Individuals of different mating types differ genetically from one another, but are often visually and behaviorally indistinguishable. Many protists also have mating type systems.

Fungi reproduce sexually when hyphae (or, in the chytrids, motile cells) of different mating types meet and fuse. In many fungi, the zygote nuclei formed by sexual reproduction are the only diploid nuclei in the life cycle. These nuclei undergo meiosis, producing haploid nuclei that become incorporated into spores. Haploid fungal spores, whether produced sexually in this manner or asexually, germinate, and their nuclei divide mitotically to produce hy-phae. This type of life cycle, called a haplontic life cycle, is also characteristic of many protists (see Figure 28.27).

The presence of a dikaryon is a synapomorphy of three phyla

Certain hyphae of some Zygomycota, Ascomycota, and Ba-sidiomycota have a nuclear configuration other than the familiar haploid or diploid states. In these fungi, sexual repro duction begins in an unusual way: The cytoplasms of two individuals of different mating types fuse (plasmogamy) long before their nuclei fuse (karyogamy), so that two genetically different haploid nuclei coexist and divide within the same hypha. Such a hypha is called a dikaryon ("two nuclei"). Because the two nuclei differ genetically, such a hypha is also called a heterokaryon ("different nuclei").

Eventually, specialized fruiting structures form, within which the pairs of genetically dissimilar nuclei—one from each parent—fuse, giving rise to zygotes long after the original "mating." The diploid zygote nucleus undergoes meio-sis, producing four haploid nuclei. The mitotic descendants of those nuclei become spores, which give rise to the next generation of hyphae.

The reproduction of such fungi displays several unusual features. First, there are no gamete cells, only gamete nuclei. Second, there is never any true diploid tissue, although for a long period the genes of both parents are present in the dikaryon and can be expressed. In effect, the hypha is neither diploid (2n) nor haploid (n); rather, it is dikaryotic (n + n). A harmful recessive mutation in one nucleus may be compensated for by a normal allele on the same chromosome in the other nucleus. Dikaryosis is perhaps the most significant of the genetic peculiarities of the fungi.

Finally, although zygomycetes, ascomycetes, and basid-iomycetes grow in moist places, their gamete nuclei are not motile and are not released into the environment. Therefore, liquid water is not required for fertilization.

Some fungi are pathogens

Although most human diseases are caused by bacteria or viruses, fungal pathogens are a major cause of death among people with compromised immune systems. Most people with AIDS die of fungal diseases, such as the pneumonia caused by Pneumocystis carinii or the incurable diarrhea caused by some other fungi. Candida albicans and certain other yeasts also cause severe diseases in individuals with AIDS and in individuals taking immunosuppressive drugs. Such fungal diseases are a growing international health problem. Our limited understanding of the basic biology of these fungi still hampers our ability to treat the diseases they cause. Various fungi cause other, less threatening human diseases, such as ringworm and athlete's foot.

In plants, the situation is reversed. Fungi are by far the most important plant pathogens, causing crop losses amounting to billions of dollars. Major fungal diseases of crop plants include black stem rust of wheat and other diseases of wheat, corn, and oats. Bacteria and viruses are less important as plant pathogens.

The fungus that causes root and butt rot in pine trees is an important forest pathogen with an interesting, recently dis

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