Absence of fibroin H sequences and a significant divergence in the putative fibroin L homolog in Neomicropteryx cornuta (Lepidoptera, Micropterigidae) silk.

Abstract

Micropterigidae is the sister group to all other Lepidoptera, providing important insights into the evolution of Lepidoptera. However, the gene and protein profiles of silk from Micropterigidae remain unidentified. We investigated the silk components of silk cocoons of the micropterigid species Neomicropterix cornuta. We characterized the silk glands and cocoons using light and electron microscopy, X-ray microtomography, and molecular techniques. Our results showed that the protein fibroin heavy chain (FibH) is absent in the silk of N. cornuta and that the putative homolog of fibroin light chain (FibL) is also absent or severely altered. This was confirmed by transcriptome and genome searches of the conserved regions in this species. The examination of the conserved synteny around the fibH genes in several Lepidoptera and Trichoptera showed that the genomic region containing this gene was absent in another micropterigid species, Micropterix aruncella. In contrast, we found putative orthologs of fibH and fibL in the transcripts of representatives of another distinct clade, Eriocraniidae. This study showed that the loss of FibH and the loss or severe divergence of FibL occurred specifically in the family Micropterigidae and revealed dynamic evolutionary changes in silk composition during the early evolution of Lepidoptera.