Table 1.

Plant cell wall degrading enzymes identified in the A. glabripennis genome assembly by manual annotation

Gene family	Putative function	Genes total	Pseudogenes
Cellulose/hemicellulose degradation
GH9	Endo-β-1,4-glucanase	1	0
GH45	Endo-β-1,4-glucanase	2	0
GH5 subfamily 2	Endo/exo-β-1,4-glucanase	6	0
GH48	Reducing end-acting cellobiohydrolase	2	0
GH1	β-Glucosidase (myrosinase, cyanogenic β-glucosidase)	57	3
Pectin degradation
GH28	Polygalacturonase	18	0

Genes encoding GH9 cellulases have an ancient origin in animals [26]. The other beetle-derived GH families involved in plant cell wall digestion have a more recent origin and were putatively obtained via HGT from bacteria or fungi. GH5 subfamily 2 genes were likely acquired via HGT from Bacteroidetes [27]. GH45 genes were likely acquired by the last common ancestor (LCA) of the Phytophaga (the sister beetle superfamilies Chrysomeloidea and Curculionoidea) via HGT from a fungus [28, 29]. Amino acid sequences of beetle GH48 cellulases are similar to bacterial cellobiosidases, but their function(s) remain unclear; they may have evolved to scavenge nitrogen by degrading chitin in the gut or diet [81], e.g., from host plant tissues containing fungi, or from fungi resident in the gut (e.g., yeasts, Fusarium solani) which are thought to concentrate nitrogen and synthesize essential amino acids [9, 30, 35]. GH48s are constitutively highly expressed in A. glabripennis larvae (Fig. 5), and their induction in larvae feeding in a nutrient-poor environment (reported herein) is consistent with a putative role in nutrient scavenging. They were most likely acquired by the LCA of the Phytophaga via HGT from a bacterial donor [28, 30]. GH28 genes were likely acquired by the LCA of the Phytophaga via HGT from an ascomycete fungus and subsequently expanded and diversified, but lost in the longhorned beetle subfamily Lamiinae (which includes A. glabripennis). After this loss, a GH28 gene was apparently re-acquired by Lamiinae via HGT from a fungal donor [10]