Insights from comparative transcriptomics into the evolution of silk

Silk is one of the most spectacular organic materials in nature and is produced by a wide variety of arthropods. It has an almost unlimited potential for use as a biomaterial, but its function and composition have only been studied in depth in a limited number of model taxa. This severely limits what we know about both its natural variety and its evolution, and as a consequence its potential applications. The orders Trichoptera and Lepidoptera contain the majority of all silk-producing species, and as such also exhibit the greatest diversity of silk use, including for instance using silk to spin large nets, make cases, and generate underwater anchor lines. Most trichopteran larvae exhibit an entirely aquatic lifestyle while most lepidopterans are terrestrial, but lineages of both groups have switched between these environments. Similarly, silk has shifted multiple times between being used above and under water. This allows for a comparative approach to study how natural selection shapes silk fiber evolution. In this study, we focused on several lineages of Lepidoptera that contain both species with a terrestrial and species with an aquatic or semi-aquatic lifestyle. We characterized the silk transcriptome using RNA sequencing, performed genome sequencing for species for which a genome was not yet available, and mechanically characterized the silk for each studied species. Combining these data with natural history information allows us to compare the relationship between the molecular composition of silk and its natural function and to provide new insights into how silk functions evolve.