Duplication, Loss, and Feeding Ecology of Lysozyme Gene Copy Number in Insects

Lysozymes are ubiquitous enzymes traditionally associated with immune defense via bacterial cell wall degradation. In some insect lineages, particularly Diptera with bacteria-rich diets, these enzymes have been secondarily recruited for digestion, but how widespread this shift is across other insect groups remains unclear. Here we investigate the evolutionary dynamics and functional roles of C-type and I-type lysozymes across 275 insect genomes, inferring patterns of gene duplication and loss and testing associations between lysozyme copy number and dietary habits. Phylogenetic Generalized Least Squares (PGLS) method revealed few significant associations: a weak link between larval detritivory and C-type copy number, and a more consistent one between an omnivorous diet and I-type copy number across life stages. Duplication analysis revealed multiple independent expansions of C-type lysozymes within Brachycera. In Muscoidea, C-type lysozymes were highly expressed in the Musca domestica larval midgut, consistent with digestion, whereas in Blattodea, moderate C-type copy numbers and salivary gland-biased expression in Periplaneta americana adults instead suggest a primarily antiseptic role. I-type lysozymes lack the catalytic residues required for canonical muramidase activity, yet remain conserved throughout Insecta, including in lineages that lost C-type lysozymes, suggesting alternative, non-canonical functions. Together, these findings are more consistent with a predominantly lineage-specific pattern of gene family evolution, largely uncoupled from a single ecological driver, than with a broad, diet-driven adaptive signal. Digestive recruitment of C-type lysozymes may extend beyond the well-characterized cases of Musca domestica and Drosophila melanogaster to additional expanded lineages across Brachycera, although direct functional evidence remains lacking.