Hindgut Protozoa

Digestion in Cryptocercus is comparable to that of lower termites in all respects. The hindgut is a fermentation chamber filled to capacity with a community of interacting symbionts, including flagellates, spirochetes, and bacteria that are free in the digestive tract, attached to the gut wall, and symbiotic with resident protozoans. Included are uricolytic bacteria, cellulolytic bacteria, methano-

Fig. 9.6 Scanning electron micrographs of flagellates from the hindgut of Cryptocercus punctu-latus. (A) The hypermastigote Trichonympha sp., scale bar = 25 ^m. (B) The oxymonad Sacci-nobaculus sp., scale bar = 5 ^m. Images courtesy of Kevin J. Carpenter and Patrick J. Keeling.

gens, and those capable of nitrogen fixation, as well as bacteria that participate in the biosynthesis of volatile fatty acids (Breznak et al., 1974; Breznak, 1982; Noirot, 1995).

The common possession of oxymonad and hyper-mastigid hindgut flagellates in Cryptocercus and lower termites (Fig. 9.6) is often a focal point in discussions of the evolutionary origins of termites. These protozoans are unusually large, making them good subjects for a variety of experimental investigations; some in the gut of Cryptocercus are 0.3 mm in length and visible to the unaided eye (Cleveland et al., 1934). They are unusually intricate, with singular morphological structures and a complex of bacterial symbionts of their own (e.g., Noda et al., 2006). They are unique; most are found nowhere in nature but the hindguts of these two groups (Honigberg, 1970). Finally, and of most interest for termite evolutionary biology, most are cellulolytic and interdependent with their hosts. For many years these flagellates were thought to be not only the sole mechanism by which dictyopteran wood feeders digested cellulose, but also the proximate cause of termite eusociality. Currently, however, neither of these hypotheses is fully supported, despite misconceptions that still abound in the literature.

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