Origin and evolution of cellulose digestion in insects: insights from the transcriptomic analyses of the apterygotan insect Thermobia domestica

Plant cell walls (PCW) are essential food sources for insects thriving on wood, foliage, and detritus. Degradation of PCW involves the action of enzymes collectively named, PCW degrading enzymes (PCWDE). Originally, these enzymes were considered exclusive to cellulolytic bacteria, fungi and protozoans, and thus, it was assumed that plant-feeding insects lack PCWDE. However, cloning and characterization of genes encoding insect endogenous PCWDE, have finally demonstrated symbiotic-independent digestion of the PCW in insects. Insect PCWDE include cellulases [Glycosyl Hydrolase Family (GHF) 5, GHF9, and GHF45], hemicellulases (GHF11 and GHF16), and pectinases (GHF28). Nevertheless, the irregular distribution of these genes in insects, has questioned their evolutionary origins. Some evidence suggests that genes encoding endogenous PCWDE, such as GHF11 hemicellulases and GHF45 cellulases, were acquired through horizontal gene transfer, but most of the available evidence suggests that the genes encoding GHF9 cellulases, were present in the last common ancestor of the hexapods. Wingless basal hexapods known as Apterygota, represent key models to understand the evolution of insects. Therefore, in order to clarify the evolutionary origin of PCW digestion in insects, we studied the enzymes involved in the digestive process of PCW of the apterygotan insect Thermobia domestica (Thysanura), by using transcriptomic analyses from midgut and salivary gland tissues. We also conducted a comprehensive analysis of enzymatic activities involved in PCW digestion in T. domestica. Our results indicated that several transcripts of genes encoding GHF9 endoglucanases, GHF16 β-1,3-endoglucanases, GHF30 β-1,6-glucanases, and  GHF5 β-endomannanases, were expressed in T. domestica. The highest levels of enzymatic activity, were against carboxymethyl-cellulose,  lichenin, laminaribiose and cellobiose, which correspond to the highest expressed transcripts. These results provide key evidence to demonstrate that the ancestral living insects, Thysanura, possess their own enzymes to digest PCW, thereby confirming that some genes encoding PCWDE were present in the last common ancestor of Hexapoda.