Pilobolus spp

A common fungus that grows on the dung of herbivorous animals is Pilobolus (Zygomycotina). The dung contains nitrogen, vitamins, growth factors and minerals and satisfies the fungus' unusual nutritional requirement of a chelated form of iron. The fungus can be grown in media incorporating a decoction of dung or on a synthetic medium containing a complex iron-containing compound called coprogen. Exposure to visible light (380 to 510 nm) triggers the formation of a large bulbous cell called a trophocyst that is embedded in the substratum. The trophocyst elongates into a stalk about 0.5 to 1 cm high called a sporangiophore. The sporangiophore enlarges upwards into a crystal-clear sub-sporangial bulb and capped by a sporangium containing asexual spores. When dung is kept under a bell jar to provide a damp atmosphere and light, in a few days it becomes covered by turgid sporangiophores. The sporangiophore bends toward the light source and the entire black sporangium containing the spores (Figure 11.3) is shot toward light with a velocity of 5 to 10 m/s. The empty sporangiophore is thrown flat on the medium by the recoil (Page, 1962). Covering the bell jar with a black paper in which a small window has been cut demonstrates the precision with which the fungus can direct the shooting of sporangium toward a light source. The released sporangia strike the inside of the bell jar in the area that receives the illumination. This phototrophic response permits

Subspc

Subspc

Pilobolus Fungi

Figure 11.3 Pilobolus kleinii. (a) Upper part of sporangiophore acting as a simple eye. (b) Sporangiophore originally developed in light from direction 1, but two hours ago illumination was altered to direction 2. (From Ingold and Hudson (1970). With permission of Kluwer Academic Publishers.)

Figure 11.3 Pilobolus kleinii. (a) Upper part of sporangiophore acting as a simple eye. (b) Sporangiophore originally developed in light from direction 1, but two hours ago illumination was altered to direction 2. (From Ingold and Hudson (1970). With permission of Kluwer Academic Publishers.)

the sporangiophores to grow out of dung; the sporangia strike a blade of grass and adhere to it. When a grazing animal eats grass, the spores within each sporangium pass unharmed through its alimentary canal and are voided with the dung wherein they germinate.

Klein (1948) subjected Pilobolus grown on dung-decoction agar to light-dark (LD) cycles. He found that although sporangium formation occurred predominantly at the end of a dark period, a dark period is essential to establish periodicity of growth and maturation of fruiting bodies. Periodicities other than those observed in nature can be established by artificial illumination. Among these were light-dark cycles (in hours) not only of 12-12 but 16-16, 15-9 and 9-15. When subjected to continuous darkness, the rhythm synchronized by the previous light cycle persists and cultures initiate a self-sustained rhythm under circadian control (Uebelmesser, 1954).

The Canadian mycologist A.H. Reginald Buller suggested that the subsporangial vesicle acts as a lens, focusing the rays of light at the base of the orange-colored vesicle (Figure 11.3). The curvature results from an increase in growth in this region. A fresh crop of sporangiophores is formed daily and the discharge occurs around noon. To determine the region of the sporangiophore that is sensitive to light, Robert Page (1962) made a series of photomicrographs at intervals following unilateral illumination. The tip (about 0.5 mm) of the sporangiophore curved sharply toward a light souce in 10 minutes and reached its maximum curvature in about one hour. To respond in this way, some chemical substance must perceive light. E. Bunning (1960) compared the phototropic response of various wavelengths (action spectrum) of light by means of glass and liquid filters with the absorption spectrum of extracted pigments. From the close resemblance between the action and absorption spectrum, he suggested that the photosensitive pigment is a caro-tenoid. Page (1962) exposed sporangiophores to wavelengths of light dispersed by a prism and caught the shot sporangia on a glass plate (Figure 11.4). The distribution of the sporangia gave an action spectrum for phototropism. The response was strong between 410 and 420 nm. Based on the similarity of the action and the absorption spectrum, the

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