Lysozymes as a unifying axis of insect development, metabolism, and defense

In insects, two lysozyme types have been described: c-type lysozymes, which are well characterized at both the genetic and functional levels, and i-type lysozymes, whose functions remain comparatively less understood. Although traditionally recognized for their antibacterial activity, insect lysozymes have been implicated in a broad range of biological processes, including defense against fungi, viruses, and parasites, as well as digestion, microbiota regulation, nutrient recycling, and developmental transitions. In this review, we synthesize current knowledge on the diversity, structure, and functions of insect lysozymes across different taxa and feeding strategies. We propose that the multifunctionality of these enzymes positions them at the interface of immunity, metabolism, and development. Finally, we highlight major knowledge gaps, including the limited understanding of i-type lysozymes and the scarcity of studies in hemimetabolous insects, which constrain our understanding of lysozyme evolution and functional diversification.