Abstract
Despite more than 40 years of intense study, essential features of the silkmoth chorion (eggshell) are still not fully understood. To determine the precise structure of the chorion locus, we performed extensive EST analysis, constructed a bacterial artificial chromosome (BAC) contig, and obtained a continuous genomic sequence of 871,711 base pairs. We annotated 127 chorion genes in two segments interrupted by a 164 kb region with 5 non-chorion genes, orthologs of which were on chorion bearing scaffolds in 4 ditrysian families. Detailed transcriptome analysis revealed expression throughout choriogenesis of most chorion genes originally categorized as “middle”, and evidence for diverse regulatory mechanisms including cis-elements, alternative splicing and promoter utilization, and antisense RNA. Phylogenetic analysis revealed multigene family associations and faster evolution of early chorion genes and transcriptionally active pseudogenes. Proteomics analysis identified 99 chorion proteins in the eggshell and micropyle localization of 1 early and 6 Hc chorion proteins.
Highlights
The secreted chorion of B. mori is organized as a fibrous helicoidal array, yielding a lamellate structure which is constructed in four sequential morphogenetic modes[9]
From the fcP8 cDNA library, 643 ESTs were assigned to chorion gene transcripts, of which early, middle and late chorion transcripts comprised 27.5%, 35.8% and 36.7%, respectively
To provide an estimate of the timing of insertion of the non-chorion region into the chorion locus, we investigated the conservation of synteny of orthologs of these non-chorion genes and several surrounding chorion genes in the genomes of B. mori and 4 other Lepidoptera, Danaus plexippus, Heliconius melpomene, Manduca sexta and Plutella xylostella (Supplementary Table 3)
Summary
The secreted chorion of B. mori is organized as a fibrous helicoidal array, yielding a lamellate structure which is constructed in four sequential morphogenetic modes[9]. Two types of C/EBP (a CCAAT/enhancer-binding protein) sites are found in the chorion locus, an early type associated with early and early-middle chorion genes, and a late type for middle-late and late chorion genes, and a BmC/EBP factor has been shown to act both as an activator and a repressor during choriogenesis[17] These few regulatory components are too limited to account for the complex developmental patterns observed in choriogenesis. We conducted a direct analysis of eggshell protein components by liquid chromatograph-mass spectrometry (LC-MS) This is the first time that a complete chorion locus and its expression has been established on all of these levels, laying the foundation for a more comprehensive view of the developmental regulation of this complex multigene system
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