Characterization and Evolutionary Analyses of Silk Fibroins from Two Insect Orders: Embioptera and Lepidoptera
Silk production has independently evolved in several insect lineages, such as Hymenoptera (bees, ants and wasps), Siphonaptera (fleas), and Archeognatha (bristletails and silverfish). Primarily composed of proteins, silks are used for functions, such as protection, prey capture, dispersal, and reproduction. Because of the ecological significance of silks, natural selection can act on silk fibers and thus the underlying silk proteins (fibroins). Lepidoptera (moths and butterflies) and Embioptera (webspinners) are two insect orders that utilize silk mostly for protection. Characterizing fibroins from distantly related insect lineages may reveal convergent evolutionary patterns and provide insight into the functional elements of their fibroins. Within a lineage, conserved characteristics may yield a better understanding of silk evolution and fiber formation.
I obtained fibroin sequence data through silk gland cDNA libraries of Lepidoptera and Embioptera. This was accomplished through construction of species-specific silk gland libraries for the lepidopteran Hepialus californicus (ghost moth), and five embiopteran species: Antipaluria urichi, Archembia n. sp., parthenogenic Haploembia solieri, Oligotoma nigra, and Saussurembia davisi. After screening the libraries, transcripts for fibroins were identified for each species. Hepialus californicus heavy chain fibroin and light chain fibroin were compared to other lepidopteran and trichopteran heavy and light chain fibroins. Analyses revealed that both heavy and light chain fibroins have historically experienced stabilizing selection and light chain fibroin has potential as a phylogenetic marker. For the embiopteran fibroins, many similar features were found to be shared across species, such as glycine/serine rich, repetitive motifs, and a short, non-repetitive carboxyl-terminal region. Conservation of these elements suggests that these regions are important for fiber formation and mechanical properties. Furthermore, the conserved carboxyl-terminal regions imply that embiopteran fibroins have undergone stabilizing selection. Additional characterizations were performed on embiopteran silks to determine amino acid composition, secondary structural conformations, and mechanical properties. These features were found to have a high degree of similarity across taxa, suggesting that embiopteran silk has remained largely unchanged since it arose over 100 million years ago.