Evolution of Highly Repetitive Silk Genes in the Luna Moth, Actias luna.

Gene duplications are a major driver of molecular diversification and phenotypic evolution. Arthropod silk genes provide an excellent model for studying these processes due to their extensive internal repeats and rapid evolutionary rates. In Lepidoptera, the Fibroin heavy chain (fibH) gene encodes the primary structural protein for silk fibers, contributing largely to their mechanical strength. This inner fibroin core is surrounded by an outer coating composed primarily of sericins. Sericins are a group of highly repetitive, serine-rich proteins that modulate silk fiber properties. Although sericins in the domestic silkworm (Bombyx mori) have been associated with life stage-specific variation in silk characteristics, their evolution and function remain poorly understood. Here, we provide a detailed molecular characterization of sericin genes in the Luna moth (Actias luna) known for forming dense, robust, silk-woven cocoons. We identified eight sericin genes that (i) include two clusters of closely related paralogs, (ii) exhibit considerable variation in repeat number and amino acid composition, and (iii) display distinct gene expression patterns across life stages. A comparison of sericin genes between A. luna and three other moths of the superfamily Bombycoidea reveals evidence for convergent subfunctionalization. These findings suggest that sericin gene duplications enable dynamic shifts in silk composition both within and between species, potentially reflecting adaptive responses to ecological and functional demands.