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Lichens consist mostly of fungal hyphae; the photosynthetic algae or cyanobacteria form a thin layer just under the surface. The hyphae anchor the lichen to a rock or tree, aid in the uptake and retention of water and nutrients, and produce chemicals that protect against excess light and herbivorous animals. The photosynthetic partners provide a source of reduced carbon, such as carbohydrates. Cyanobacteria living in lichens also provide a source of nitrogen, like ammonia. The two partners exchange nutrients through fungal hyphae that tightly encircle or even penetrate the walls of the photosynthetic cells. As a result, lichens are able to thrive at sites where neither partner could exist on its own. It remains an open question whether either partner can exist independently in nature. When grown in the laboratory in culture, the fungal partner produces an undifferentiated hyphal mass. So while the bulk of the lichen structure comes from the fungus, the chemical signals from the photosynthetic partner influence the form and shape of the fungus. Lichens spread asexually by fragmentation or through the formation of dispersal units consisting of a single photosynthetic cell surrounded by hyphae. Sexual reproduction by the fungi is common. The photosynthetic cells reproduce asexually by mitotic cell division. Lichens are remarkable for their ability to grow on the surfaces of rocks and tree trunks. Lichens in harsh environments grow slowly, have a high tolerance for desiccation, and can tolerate wide fluctuations in temperature and light. They are also sensitive to air pollution, particularly sulfur dioxide. For this reason, lichen growth is sometimes used as an indicator of industrial pollution Spores can form by meiotic cell division as part of sexual reproduction, and they can also form asexually. Asexual spores are formed by mitotic cell division and therefore are genetically identical to their parent. Asexual spores allow fungi to proliferate and disperse to new environments. In many species, asexual spores are produced within sporangia that form at the ends of erect hyphae, facilitating the release of the spores into the air. A close look at a moldy piece of bread reveals that the surface is covered with hyphae carrying sporangia containing asexual spores. Like plants, fungi face two challenges in completing their life cycles: Maintaining genetic diversity: In order to maintain genetic diversity, they must find other individuals to mate with. Dispersal: They must be able to disperse from one place to another. Fungi produce spores that can be carried by the wind, in water, or attached to (or within) animals. The spores of fungi that live in aquatic environments have flagella that allow them to swim. The great majority of fungi, however, live on land, and their spores have no flagella. Instead, the spores are encased in a thick wall that protects them as they are dispersed over habitats unsuitable for growth. The probability that any given spore will come to rest in a favorable habitat is low, so fungi produce huge numbers of spores. Fungal spores remain viable, if provided with an appropriate environment, for a few hours in some species to many years in others. Spores allow fungi to use resources that are patchy in time as well as in space. In fact, a shortage of resources is one of the cues that triggers spore formation. Fungi employ an astonishing array of mechanisms to enhance spore dispersal. The multicellular fruiting bodies produced by some fungi facilitate the dispersal of sexually produced spores. Mushrooms, stinkhorns, puffballs, bracket fungi, truffles, and many other well-known structures are fungal fruiting bodies. Fruiting bodies are highly ordered and compact structures compared to the mycelia from which they grow, yet they are constructed entirely of hyphae. In many cases, their mechanisms of spore dispersal demand a high degree of structural precision. The fruiting bodies of many fungi rise above the ground or grow from the trunks of dead trees, so the sexually produced spores are released high above the ground. However, elevation itself is not enough to ensure dispersal. Many fungi forcibly eject their spores, achieving velocities of more than 1 m/s. At this speed, the tiny spores can penetrate and travel beyond the layer of stagnant air that surrounds the fruiting body. Other fungi rely on external agents such as raindrops or animals to move their spores around. Like other sexually reproducing eukaryotes, fungi have life cycles that include haploid and diploid stages. The nuclei in fungal hyphae are haploid, and the fungal life cycle is therefore similar to haploid-dominant organisms. Asexual reproduction involves the production of haploid spores by mitosis, while sexual reproduction involves the fusion of haploid cells to form a diploid zygote, which undergoes meiosis as its first division. However, sexual reproduction in fungi differs from all other haploid-dominant organisms in one important respect: In fungi, the fusion of haploid cells is not immediately followed by the fusion of their nuclei. In most fungi, the sexual phase of the life cycle involves the fusion of hyphal tips rather than specialized reproductive cells, or gametes. For mating to occur, two hyphae grow together and release enzymes that digest their cell wall at the point of contact. The cell contents of the two hyphal cells merge, forming a single cell with two haploid nuclei. Many individual fungi of a species look similar but have different mating types. The mating type of an individual is determined by a mating-type gene. Fertilization can take place only between individuals that have different alleles at the mating type gene. In most sexually reproducing organisms, when two gametes merge, their nuclei fuse almost instantly to form a diploid zygote. In fungi, however, the cytoplasmic union of two cells (plasmogamy) is not always followed immediately by the fusion of their nuclei (karyogamy). Instead, the haploid nuclei retain their independent identities, resulting in what is referred to as a herterokaryotic stage. In the heterokaryotic stage, a cell has nuclei from two parental hyphae, but the nuclei remain distinct. The heterokaryotic stage ends with nuclei fusion (karyogamy), which leads to the formation of a diploid zygote. The zygote divides by meiotic cell division, giving rise to sexually produced haploid spores. In some groups, the heterokaryotic stage consists of only a single, multinucleated cell. In other groups, plasmogamy is followed by mitosis, which produces hyphae in which each cell contains two haploid nuclei, one from each parent. The resulting dikaryote stage can be limited to a small number of cells. The edible mushrooms found in markets may consist entirely of dikaryotic cells. Fungi that lack sexual reproduction have other means of generating genetic diversity. Asexual reproduction occurs in most groups of fungi. About 20% of fungi appear to lack sexual reproduction altogether (e.g., Penicillin and Aspergillus). Fungal species that lack an observable sexual cycle have another mechanism of generating genetic diversity: the crossing over of DNA during mitosis. These species are described as parasexual. The parasexual cycle is not a coordinated cycle but rather a series of four events. Two hyphae fuse, forming a heterokaryotic cell. Karyogamy produces a diploid nucleus from two genetically distinct haploid nuclei. During mitosis, crossing over may occur between the two sets of chromosomes. (Crossing over is common in meiosis but rare in mitosis.) The haploid chromosome number is restored not by meiotic division, but by the progressive loss of chromosomes. The key difference between parasexuality and sexual reproduction is that parasexual species do not undergo meiosis. Fungi are highly diverse. About 75,000 species have been formally described, but estimates of true diversity run as high as 5 million. Among eukaryotes, the only organism more diverse are the animals. The availability of DNA sequence data has greatly advanced our understanding of phylogenetic relationships within the fungi. The phylogenetic tree shown here shows how the characters present in familiar mushrooms accumulated through the course of evolution: first, chitinous cell walls, then hyphae, then regularly placed septa, and finally the complex multicellular reproductive bodies we call mushrooms. This phylogeny also shows that the numbers of species are not spread evenly across the phylogeny. Instead, more ancient groups include less than 2% of known species, while the two dikaryotic groups include more than 98% of known fungi. Dikaryotic fungi are well adapted to many different habitats, including other organisms, both living and dead. Fungi are opisthokonts, members of the eukaryotic superkingdom that includes animals. Among fungi, the Chytridiomycota (or chytrids) lie at the base of the phylogenetic tree. This is a small group of about 1000 species found in aquatic or moist environments. Many chytrids are single cells with walls of chitin. They may also form short multinucleated structures, but they lack the well-defined hyphae characteristic of other fungi. Chytrids do not form a true mycelium, although in some species they form elongated outgrowths called rhizoids that penetrate into organic substrates. Rhizoids anchor the organism in place and absorb food molecules. There is substantial life-cycle diversity within the group. Chytrids lack a heterokaryotic stage but form flagellated gametes that swim through their aqueous environment. Most chytrids are decomposers. A few of them are pathogenic.