On the dynamic caddisfly silk H-fibroin gene: a population study in a net-spinning species

Larvae of the caddisfly Arctopsyche grandis BANKS build protective structures and spin silken capture nets in flowing water. Caddisfly H-fibroin, the major protein component of its silk fibers, has a blocky structure with repeating units defined as beginning with a [(SX)nE]m region followed by a G-rich spacer. Previous observation of H-fibroin allelic variation in haploid-resolved individuals led us to investigate allelic variation within two geographically close but separated natural populations of A. grandis. The genomes of 18 individuals were sequenced, and 34 haploid-resolved H-fibroin sequences were extracted. Twenty-four unique alleles were identified in 18 genomes, revealing the dynamic nature of the H-fibroin gene. H-fibroin length variations of at up to 25% were tolerated. The major source of the length variations were large-scale deletions and insertions of entire [(SX)nE]m blocks. Small scale indel events were numerous, nonrandomly distributed, and constrained to a few types. One, a 44 residue indel comprising two (SX)nE motifs changed m ± 2 by splitting direct tandem repeats without disrupting tertiary structure or block boundaries. The G-rich spacers are of two types, the first distinguished by repeating GLGPH pentapeptides. Indels within this spacer type occur as multiples of the GLGPH pentapeptide. The other category of G-rich spacer was confined to a narrow length distribution. Overall, the results demonstrate the rapid evolution of the caddisfly H-fibroin gene and the wide range of H-fibroin structural polymorphism tolerated in functional capture net silk. At the same time, the limited nature of the indels point to the critical structural features of H-fibroin.