Dependence on Flagellates for Cellulase

All termites and all cockroaches examined to date produce their own cellulases, which are distinct from and unrelated to those produced by the hindgut flagellates (Watanabe et al., 1998; Lo et al., 2000; Slaytor, 2000; Tokuda et al., 2004). The common possession of a certain family of cellulase genes (GHF9) in termites, cockroaches, and crayfish suggest that these enzymes were established in the Dictyopteran lineage long before flagellates took up permanent residence in the hindguts of an ancestor of the termite- Cryptocercus clade (references in Lo et al., 2003b). At present, Cryptocercus and lower termites are considered to have a dual composting system (Nakashima et al., 2002; Ohkuma, 2003); cellulose is degraded by the combined enzymes of the host and the hindgut flagellates. Nonetheless, these hosts are dependent on the staggeringly complex communities of mutually interdependent co-evolved organisms from the Ar-chaea, Eubacteria, and Eucarya in their digestive systems. The interactions of the microbes with each other and with their hosts are still poorly understood; however, exciting inroads are being made by the laboratories actively studying them, and the field is advancing quickly (e.g., Tokuda et al., 2004,2005; Inoue et al., 2005; Watanabe et al., 2006). Products of cellulose degradation by gut protozoans may indirectly benefit the insect host by providing energy for anaerobic respiration and nitrogen fixation in gut bacteria (Bignell, 2000a; Slaytor, 2000). A comparison of gene expression profiles among castes of the termite Reticulitermes flavipes suggests that cellulases produced by the symbionts may be particularly important in incipient colonies (Scharf et al., 2005). This supports the idea that gut microbes may supply a metabolic boost at crucial points in host life history.

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