Horizontal gene transfer and gene duplication of hemicellulose degrading genes in an invasive insect pest

Insects were long thought to lack endogenous genes encoding plant cell wall degrading enzymes (PCWDEs). However, this hypothesis has been challenged since symbiotic-independent PCWDE genes and enzymes were found in several insect species. As an economically-important wood-feeding beetle, how the invasive insect emerald ash borer (Agrilus planipennis) degrades plant cell walls and utilizes their contents is of high interest. We identified five genes within the A. planipennis genome which putatively encode for α-L-arabinofuranosidases, a type of hemicellulases that degrade the plant cell wall polymer hemicellulose. Phylogenetic analysis showed that they are clustered together and exhibit close relationship with bacterial homologs, indicating that acquisition of these genes by the insect is through the horizontal gene transfer and duplication mechanism. Three genes reside in tandem on the same genomic DNA scaffold, further suggesting occurrence of the gene duplication events. To exclude the possibility that these α-L-arabinofuranosidase genes are from midgut bacterial symbionts, we provide evidence including: PCR amplification from non-symbiont hosting tissues (e.g. adult elytra and leg tissues); polyadenylation of their mRNAs in the 3’-end; and lastly, the presence of an intron in one of these five genes. Preliminary quantitative RT-PCR analysis on two genes indicated that they both were almost exclusively expressed in the larval midgut and during larval stages of development. These results together with the presence of N-terminal signal peptides in the deduced protein sequences, suggest that these gene products are secreted into the larval midgut, facilitating digestion of the host plant cell walls.