Abstract
The details of the lignocellulose deconstruction processes in the digestive systems of wood-feeding insects remain elusive. This study aimed to examine the biochemical conversion of lignocellulose in the digestive system of a wood-feeding anobiid beetle, Nicobium hirtum, one of the most important pests of wooden products in Japan. To this end, N. hirtum larvae were fed with Japanese red pine (softwood) and Japanese beech (hardwood) sapwood diets, as well as an artificial diet containing Shorea wood (hardwood) sapwood sawdust. The structural differences between the original and digested (feces) lignocellulose samples were examined using wet-chemical and two-dimensional (2D) nuclear magnetic resonance (NMR) methods. Cellulose and hemicelluloses, especially mannan in the softwood diet, were preferentially degraded over lignin in the larval digestive system. As a result, lignin was enriched in the digested lignocellulose residues. Lignin compositional analyses based on thioacidolysis and 2D NMR determined that the proportions of oxidized lignin aromatic units were notably increased after digestion. Further, the 2D NMR analyses revealed the accumulation of aldehyde and hydroxypropiovanillone/syringone end-unit structures in lignin, indicating that oxidative and/or reductive modifications of lignin polymers occur in the larval digestive system. Such structural alterations of lignin may facilitate the dissociation of the lignin barrier, thereby liberating polysaccharides for subsequent enzymatic conversion for assimilation and energy.
Highlights
Wood-feeding or xylophagous insects are mainly distributed in the three major orders of Insecta, i.e., Coleoptera, Blattodea and Hymenoptera [1]
Physiological characteristics of N. hirtum larvae fed on lignocellulose diets We recorded larvae survival, the decrease in the mass of the lignocellulose diet, and the growth of the larvae fed on three different lignocellulose diets, i.e., plain sapwood blocks of Japanese red pine and Japanese beech, and an artificial diet block consisting of Shorea sapwood sawdust, yeast extract, and starch
Our preliminary experiments indicated that this Shorea woodbased artificial diet, which was originally developed for feeding powder-post beetles (Lyctus species) in the laboratory [20,21,22], is suitable for the maintenance of N. hirtum larvae [19]
Summary
Wood-feeding or xylophagous insects are mainly distributed in the three major orders of Insecta, i.e., Coleoptera (beetles), Blattodea (cockroaches and termites) and Hymenoptera (ants, bees, and wasps) [1]. Recent studies employing pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) [9,10,11,12,13] and/or two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy [13,14,15] have detected differential degradation patterns of cell wall polysaccharides, i.e., cellulose and hemicelluloses, as well as evidence of previously overlooked chemical alterations of lignin polymers during their passage through the insect digestive systems. These structural studies are still limited to a small number of insect species, mostly termites (Blattodea). We close this gap by analyzing the digestive processes of the anobiid beetle, Nicobium hirtum (Coleoptera: Ptinidae)
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