There is still no significant change in the general view on cellulose digestion in animals, which is considered to be accomplished by symbiotes in the alimentary tracts. This is in spite of many experiments during the 20th century which have suggested the presence of endogenous cellulases in animals. In 1998, the first two examples of animal endogenous cellulase genes were isolated from plant cyst-nematodes and a termite. Since then, it has been conclusively shown that members of glycoside-hydrolase family (GHF) 5 are present in nematodes, GHF 9 members are present in termites, cockroaches and crayfish, and a GHF 45 member is found in beetles. The GHF 9 members from these animals form an independent Glade from other GHF 9 members. Thus it is supposed that a GHF 9 cellulase gene originated in an ancestral species among arthropods and was carried to the present species during the course of phylogenetic development. Different from fungal and bacterial cellulases, all animal cellulases, other than some of nematode origins, are composed only of a catalytic domain, which alone, is not effective in digesting the native form of cellulose so it is supposed that the animals in question developed an unique cellulose digesting system using the help of masticating organs.
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