The complex gene evolution of sericins, a group of lepidopteran silk fiber coating proteins

Silk is a highly variable natural protein fiber with broad ranges of utility, yet the diversity of its composition and function remains poorly understood across the tree-of-life. Silk in Lepidoptera consists of a inner core consisting of fibroins and an outer coating of supplementary proteins that mostly consists of sericins. Both fibroins and sericins are large, repeat-rich proteins, and these characteristics likely play a role in the strength and elasticity of the silk fiber. Fibroins are generally considered the most important proteins in lepidopteran silk. However, they are represented by only a single copy in most species, and can thus not explain the variation in silk fiber properties observed within a species. On the other hand, most lepidopterans exhibit many sericin copies that are differentially expressed across life stages, and recent data suggests sericins can modify silk fiber properties. The large size, highly-repetitive nature, and fast evolutionary speeds of sericins complicates the study of their evolution. By combining genomics and transcriptomics approaches, we identified sericins across multiple lineages of Lepidoptera, and for the first time provide a broad scale picture of the evolution of this fascinating protein family.