untitled

• tending or “gardening” the substrate to promote the
  growth of the fungus which will be used as a food
  source by the insects.

The Sirexwood wasp associations

Female wood wasps of the genus Sirexinject their eggs
into the wood of weakened or damaged trees by means
of an auger-like ovipositor (Fig. 13.26). Then, using the
same entrance hole, the wasp produces two or three
further tunnels at different angles, and she deposits
further eggs in these. But in the final tunnel the wasp
injects spores of a wood-rotting fungus Amylostereum
areolatum (Basidiomycota) from pouches termed
mycangiathat are located at the base of the oviposi-
tor. The mucus containing the fungal spores is toxic
and causes the foliage to wilt, while the growth of the
fungus causes the wood to dry out locally and disrupts
water movement in the trunk. This combination of
factors provides ideal nutritional conditions for the
development of the wasp larvae, which feed on the
fungus and the cellulolytic breakdown products of
the wood. After pupation, the larvae produce a new
batch of adults, and as the females emerge they col-
lect spores from the rotting wood and store them in
the mycangia, to repeat the cycle of inoculation in other
trees.
Several types of wood wasp oviposit in dead trees,
but the most significant damage to living trees is
caused by Sirex noctilio. This wasp can be an econom-
ically significant pest in pine plantations and can also
attack several other conifers. Sirex noctiliooccurs natur-
ally in the northern hemisphere, including Britain and
several Asian countries, but in the past century it has

spread to many countries in the southern hemisphere,

including parts of Africa, South America, Australia,

and New Zealand.

Slippers et al. (2002) analyzed the population struc-

ture of Amylostereumstrains associated with different

wood wasp species, as a potential basis for imple-

menting pest control and quarantine measures. Using

PCR-RFLP (polymerase chain reaction–restriction frag-

ment length polymorphisms) and analysis of nuclear

intergenic spacer DNA, it was possible to match different

isolates of the fungus to recent geographical movements

of S. noctilio. The results indicate that, since the intro-

duction of S. noctiliointo the southern hemisphere from

the northern hemisphere, around 1900, all subsequent

movement of this pest has been between countries in

the southern hemisphere and not by further intro-

ductions from the north. This provides the potential

basis of a management strategy: local quarantine mea-

sures could be applied to prevent further spread of the

pest.

Leaf-cutting ants, “gardening” termites,

and ambrosia fungi

Several insects and arthropods have co-evolved with

fungi in mutualistic associations. Three classic examples

of this are exhibited by the leaf-cutting ants of Central

and South America, the “garden-tending” termites of

Africa and South-east Asia, and wood-boring beetles

in several parts of the world (Mueller & Gerardo

2002).

The leaf-cutting ants, commonly termed attine ants,

produce large nests with many subterranean chambers.

The nests are started by a single winged female – the

future queen – who carries inoculum of a fungus in a

pouch at the back of her mouth. She deposits the

fungus on suitable plant material, and as the fungus

grows she begins to lay eggs. Worker ants bring leaf

pieces back to the nest, lick them, chew them and

defecate on them, then inoculate them with tufts

of the fungus and pack the inoculated material into

chambers. Remarkably, these “fungus gardens” consist

almost exclusively of one fungus – often a species

of the mushroom-forming genus Leucoagaricus. The

mycelium of the fungus develops a distinctive growth

form within the gardens, producing hyphae with

balloon-like swollen tips, called gongylidiaor bro-

matia, which are fed to the larvae and provide their

food source. In addition, the gongylidia are foraged by

the ants, to supplement their diet of plant sap. Recent

evidence (Ronhede et al. 2004) suggests that the fungi

in these associations provide many of the enzymes,

including proteases, laccases, pectinases, and car-

boxymethylcellulases, for degrading the leaf material.

These enzymes seem to be protected and possibly

FUNGAL SYMBIOSIS 277

Fig. 13.26The wood wasp, Sirex noctilio, boring a hole
in a weakened tree to deposit eggs and fungal spores.
(Courtesy of M.P. Coutts, J.E. Dolezal and the University
of Tasmania; see Madden & Coutts 1979.)