Agriculture Reference
In-Depth Information
Breznak and Brune (1994) consider it unlikely that termites can substantially degrade the
aromatic core of lignin molecules.
Almost all termite food materials are of very low protein content (Section IV.5.2.1)
and the problem for termites in meeting their metabolic N requirements has been met at
least partially through the acquisition of N-fixing symbionts which reside in the hind gut.
N fixation has been reported from all termite families although not from the fungus-
cultivating species of the sub-family Macrotermitinae (Slaytor and Chappell, 1994).
Tayasu et al. (1994) estimated that from 30 to more than 60 % of the N in the tissues
of the dry wood termite Neotermes koshunensis was derived from fixation. Bentley
(1987) estimated that N fixation accounted for nearly 25 % of the dietary requirements
of certain Venezuelan higher termites and considered that N-fixation rates were
inversely proportional to food N concentrations. Further, N-conserving mechanisms
have been described including the recycling of uric acid for possible reuse by the ter-
mites through the medium of the symbiotic gut biota, digestion of symbiotic intestinal
organisms and such behavioural traits as oophagy, cannibalism and necrophagy
(Breznak and Brune, 1994).
Digestive processes. Although ithasbeen claimed that much of the breakdown of
cellulose in termite food materials occurs through symbiotic associations with micro-
organisms (see, for example, Cleveland, 1923; Breznack, 1984; Rouland et al., 1988a, b
and Figure III.50), work by Slaytor (1992) and his colleagues has cast considerable doubt
on the generality of this view. All of the lower and higher termites studied by Slaytor
(1992) could survive on crystalline cellulose and it was concluded that the necessary
cellulases were secreted by the salivary glands, the foregut and midgut tissues. A range
of other hydrolytic enzymes (amylase, maltase, invertase, hemicellulase) are also secreted
in the same areas. Further, appreciable cellulolytic activity is largely absent from
the hindgut except in the lower termites where it is associated with the presence of
cellulolytic protists (Breznak and Brune, 1994).
Wood-feeding species occur in both the higher and lower termites, although digestive
mechanisms differ between the two groups. In the lower termites, resistant materials
appear to be digested partly through mutualistic associations with specialised protists;
these obligate anaerobes inhabit a specialised region of the hind gut, although other
prokaryotes also occur there. Higher termites lack the key glycolytic enzyme pyruvate
dehydrogenase which is needed to convert glucose to acetyl CoA or acetate which is the
major cellulose derivative that can be absorbed by the termite. The role of the bacteria
may perhaps be to further break down materials from the initial stages of cellulose
dissimulation and to mediate the conversion of enzymic degradation products to acetate
(Breznak and Brune, 1994). In the many species of wood-feeding termites that feed on
wood or stored grasses previously attacked by saprotrophic fungi, it is not known whether
enzymes acquired from the ingested fungi play a functional role in the insect gut, if the
fungi are used solely as a dietary item or whether there is an energetic advantage to
the termites in attacking partly-degraded materials. A number of species appear to be
nutritionally dependent on certain fungi (Sands, 1970).
The workers of soil-feeding termites have mandibles specialised for crushing and
a substantial gut flora of prokaryotes. Adoption of the soil-feeding habit has lead to large
changes in the structure of the intestinal tract and a system of digestion different from
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