Abstract
The larvae of the Japanese horned beetle, Trypoxylus dichotomus (Coleoptera: Scarabaeidae: Dynastinae), are an example of a saprophage insect. Generally, Scarabaeid larvae, such as T. dichotomus, eat dead plant matter that has been broken down by fungi, such as Basidiomycota. It is thought that β-1,3-glucan, a constituent polysaccharide in microbes, is abundant in decayed plant matter. Studies of the degradation mechanism of β-1,3-glucan under these circumstances are lacking. In the current study, we sought to clarify the relationship between the capacity to degrade polysaccharides and the food habits of the larvae. The total activities and optimum pH levels of several polysaccharide-degrading enzymes from the larvae were investigated. The foregut, midgut and hindgut of final instar larvae were used. Enzymatic activities were detected against five polysaccharides (soluble starch, β-1,4-xylan, β-1,3-glucan, pectin and carboxymethyl cellulose) and four glycosides (p-nitrophenyl (PNP)-β-N-acetylglucosaminide, PNP-β-mannoside, PNP-β-glucoside and PNP-β-xyloside). Our results indicate that the digestive tract of the larvae is equipped with a full enzymatic system for degrading β-1,3-glucan and β-1,4-xylan to monomers. This finding elucidates the role of the polysaccharide-digesting enzymes in the larvae, and it is suggested that the larvae use these enzymes to enact their decomposition ability in the forest environment.
Highlights
The investigation of digestive enzymes in the intestinal tract of insects has become increasingly important for the utilization of biomass resources
The optimal alkaline pH values observed were in the foregut and hindgut extracts for amylase and in the midgut extracts for pectinase
Amylase had the highest activity among the glycanases from the gut of T. dichotomus, its corresponding glycosidase, α-glucosidase, was not detected
Summary
The investigation of digestive enzymes in the intestinal tract of insects has become increasingly important for the utilization of biomass resources. Reviews of other polysaccharide digestive enzymes in the internal tract of the insects were reported by Terra and Ferreira, and Ni and Tokuda [3,4]. The boundary between saprophagy and phytophagy in Scarabaeid larvae is equivocal, since many species of the subfamilies, Scarabaeidae and Aphodiinae, subsist on the dung of grazers, which is thought to be a mixture of degraded plant fiber and microbe mass. Such equivocality is seen in other subfamilies: species of Rutelinae, Dynastinae and Cetoniinae have been reared on a mixture of animal dung and wood [17,18,19]. Despite the use of saprophagy in Scarabaeid larvae, research on digestive enzymes in these insects has resulted in the identification of a cellulase and a xylanase that degrade polysaccharides in plant cell walls and an amylase that degrades the starch
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