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laying. The female beetle tunnels into the inner bark

and eats out a channel, depositing eggs along its

length – the “brood gallery.” The eggs hatch and the

young larvae eat out a series of radiating channels before

they pupate for overwintering. Meanwhile, the fungus

that killed the tree grows from the xylem into the bark

and sporulates in the beetle tunnels. In this way, the

young adult beetles that emerge from the pupae in the

following spring become contaminated with spores; they

leave the bark and fly in search of new trees, repeat-

ing the disease cycle.

The fungus–vector relationship clearly benefits both

partners, because the beetle carries the fungus to a new

host, while the beetle is ensured of a fresh supply of

breeding sites in the bark of newly killed trees – it will

not lay eggs in older, dead bark. As shown in Fig. 10.10b,

the fungus can produce a sexual stage in the bark –

a cleistothecium with a long neck which extrudes

ascospores in a mucilaginous matrix. This sexual stage

can ensure the generation of recombinant strains,

and again the beetle is involved in this because adult

beetles feed on the bark late in the season, introducing

192 CHAPTER 10

Fig. 10.8Diagrammatic representation of some coprophilous fungi. (a) Pilaira anomala(Zygomycota): the sporangio-
phore elongates to several centimeters at maturity and the spores “flop” onto the surrounding vegetation. (b) Mucor
racemosus(Zygomycota) with no special method of spore release. (c) Pilobolus(Zygomycota); see also (h) and (i).
(d) Ascobolussp. (Ascomycota); the tips of the mature asci project from the apothecium and are phototropic, shoot-
ing the ascospores towards a light source. (e) Sordariasp. (Ascomycota); the neck of the perithecium is phototropic
and the mature asci elongate up the neck to discharge the ascospores. (f) Coprinussp. (Basidiomycota). (g) Sphaerobolus
sp. (Basidiomycota); the large spore mass is shot from the cup-shaped fruitbody when the layers of this separate and
the inner layer suddenly inverts. (h,i) Pilobolus, showing how the terminal vesicle of the sporangiophore acts as a lens
to focus light and orientate the sporangiophore, and showing also the mechanism of sporangium discharge (see also
Fig. 10.7).