Mechanisms Of Interactions

Interactions can be mediated: (1) at a distance; (2) following contact at the hyphal level; and (3) following contact at the mycelial level (Boddy, 2000; see below). Antagonism at a distance is effected by volatile and diffusible chemicals, including enzymes, toxins and other antifungal metabolites. Two broad types of antagonism occur at the hyphal level—hyphal interference and parasitism. At the mycelial level—often termed gross mycelial contact—there are probably many different mechanisms, but again they involve the release of enzymes, toxins and other antifungal compounds. Regardless of the mechanism of antagonism, ultimately the outcome can be deadlock where neither species gains headway, replacement where one species wrests territory from the other, partial replacement where one species captures some but not all of the antagonist's territory or mutual replacement where one species takes some of the territory formerly occupied by the other and vice versa (Figure 1). Interactions often manifest themselves in natural organic resources as 'interaction zone lines' (Hyppel, 1968; Rayner, 1977; Rayner and Todd, 1979; Figure 1). In wood, these appear as narrow, often dark-coloured, lines in cross-section. They comprise pseudosclerotial plates

Figure 1 (a and b) Interactions between mycelial systems of Phanerochaete velutina (bottom left) and Hypholoma fasciculare 12 days after contact. Mycelia are growing from wood inocula across compressed non-sterile soil in 24 x 24 cm trays, incubated at 18-20 °C. (b) Mycelia have been grazed by collembola—80 Folsomia candida added 2 days after mycelia made contact. Note the more rapid progress of P. velutina across the H. fasciculare mycelium in the grazed microcosms, the wood inoculum of the latter being colonized with subsequent replacement. (c) Resinicium bicolor (left) being replaced by H. fasciculare on malt agar. Note brown pigment production in mycelium of R. bicolor in the vicinity of the invading mycelium. (d) R. bicolor (left) replacing Phallus impudicus on malt agar. Note the zone of lysed R. bicolor mycelium, which has subsequently been overgrown by cords of R. bicolor. (e) Interaction zone lines in Fagus sylvatica logs decaying on the forest floor for 4.5 years. Scale bar 1 cm. (f) R. bicolor (right) replacing P. velutina as mycelial cords. Digital images (a)-(d) and (f) courtesy of T.D. Rotheray. (See Colour Section)

Figure 1 (a and b) Interactions between mycelial systems of Phanerochaete velutina (bottom left) and Hypholoma fasciculare 12 days after contact. Mycelia are growing from wood inocula across compressed non-sterile soil in 24 x 24 cm trays, incubated at 18-20 °C. (b) Mycelia have been grazed by collembola—80 Folsomia candida added 2 days after mycelia made contact. Note the more rapid progress of P. velutina across the H. fasciculare mycelium in the grazed microcosms, the wood inoculum of the latter being colonized with subsequent replacement. (c) Resinicium bicolor (left) being replaced by H. fasciculare on malt agar. Note brown pigment production in mycelium of R. bicolor in the vicinity of the invading mycelium. (d) R. bicolor (left) replacing Phallus impudicus on malt agar. Note the zone of lysed R. bicolor mycelium, which has subsequently been overgrown by cords of R. bicolor. (e) Interaction zone lines in Fagus sylvatica logs decaying on the forest floor for 4.5 years. Scale bar 1 cm. (f) R. bicolor (right) replacing P. velutina as mycelial cords. Digital images (a)-(d) and (f) courtesy of T.D. Rotheray. (See Colour Section)

(PSPs), which are often longitudinally extensive and completely surround the territory occupied by a decay fungus. Replacement is often evidenced by 'relic' zone lines (Figure 1f), i.e. PSPs that have been breached and partly decomposed by the invading fungus. Outcome of interactions can be inferred in natural communities, but this is fraught with difficulties, and most studies have therefore been conducted on artificial media or in microcosms in the laboratory (see below).

Which species ultimately dominate is determined by the relative abilities of the antagonists to capture and defend the resource (Rayner and Webber, 1984; Boddy, 1993, 2000). Fungi exhibit an hierarchy of combative ability (e.g. Coates and Rayner, 1985; Holmer and Stenlid, 1997a; Boddy, 2000). However, outcomes between particular combinations of species vary depending on abiotic regime, fungal strain, presence of other microorganisms (Schoeman et al., 1996) and invertebrate grazing (T.D. Rotheray, unpublished; Figure 1a and b) but also can sometimes differ under apparently identical conditions (Boddy, 2000; Woods et al. , 2005). The size of the domain occupied by individual competing fungi is also of importance (Holmer and Stenlid, 1993, 1997a), as are the state of decay and other aspects of resource quality and quantity (Boddy, 2000).

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