The Evolution of Cellulose Digestion in Insects [and Discussion]

Despite the abundance and diversity of species that include living or dead plant tissue in their diets, the ability to digest cellulose is rare in insects and is restricted to a small number of orders and families. In this paper it is argued that cellulolytic capacity is uncommon in insects simply because it is a trait that is rarely advantageous to possess. Although there is a growing body of evidence for the occurrence of symbiont-independent cellulose digestion in cockroaches and in higher termites from the subfamily Nasutitermitinae, cellulose digestion in insects is usually mediated by microorganisms. It is proposed that non-cellulolytic omnivorous scavengers and detritivores may be preadapted to evolve symbiont-mediated cellulolytic mechanisms because of the prevalence of mutualistic associations between such species and the microorganisms that normally reside in their hindguts. A scenario is proposed for the evolution of symbiont-mediated cellulolytic capacity in roaches and lower termites. Finally, it is suggested that biochemical studies of insect cellulases might provide crucial insights that would greatly advance our understanding of the evolution of cellulose digestion in insects.