The core microbiome and plant-cell-wall degraders of coconut rhinocerose beetles (Oryctes rhinoceros: Coleoptera: Scarabaeidae)

Coconut rhinoceros beetle (CRB, Oryctes rhinoceros) is a serious pest on palm trees that can digest woody plant tissues that many insects cannot. This study sought to identify how this diet is digested. Chemical assays of CRB intestinal tissues suggested the presence of enzymes that can break down cellulose and hemicellulose. These were hypothesized to be produced by intestinal microbes, since expression tests found that the endogenous CRB cellulase is inactive. The apparent hindgut swelling in larvae suggests that major fermentation processes occur there. Tree-like structures on the hindgut were observed with stereomicroscopy. Scanning electron microscopy revealed that these structures are covered in conspicuous biofilms comprised of possibly symbiotic bacteria. To investigate CRB’s microbiome and find the plant-cell-wall degraders, 16S sequencing was performed. The class Clostridia was dominant in most samples. Within this class, Papillibacter cinnamivorans and Christensenella massiliensis were identified as part of the core microbiome. Metagenome sequencing and the CRB transcriptome further provided a detailed profile of all carbohydrate active enzymes (CAZymes) from the CRB holobiont. Most CAZymes, especially lignocellulose-degrading enzymes, are derived from the microbiome, identified as Clostridia mostly. This study identified a core microbiome producing lignocellulolases for CRB that may enable their survival on wood.