Abstract
The insect cuticle is a critical protective shell that is composed predominantly of chitin and various cuticular proteins and pigments. Indeed, insects often change their surface pigment patterns in response to selective pressures, such as threats from predators, sexual selection and environmental changes. However, the molecular mechanisms underlying the construction of the epidermis and its pigmentation patterns are not fully understood. Among Lepidoptera, the silkworm is a favorable model for color pattern research. The black dilute (bd) mutant of silkworm is the result of a spontaneous mutation; the larval body color is notably melanized. We performed integument transcriptome sequencing of the wild-type strain Dazao and the mutant strains +/bd and bd/bd. In these experiments, during an early stage of the fourth molt, a stage at which approximately 51% of genes were expressed genome wide (RPKM ≥1) in each strain. A total of 254 novel transcripts were characterized using Cuffcompare and BLAST analyses. Comparison of the transcriptome data revealed 28 differentially expressed genes (DEGs) that may contribute to bd larval melanism, including 15 cuticular protein genes that were remarkably highly expressed in the bd/bd mutant. We suggest that these significantly up-regulated cuticular proteins may promote melanism in silkworm larvae.
Highlights
Epidermis of B. mori contains a reduced amount of melanin; the number of urate granules, which accumulate in the dermis to maintain an opaque white epidermis, is increased[14]
A previous study suggested that the tyrosine hydroxylase (TH) gene, which is upstream of the melanin synthesis pathway, may contribute to differences in body color between B. mandarina and B. mori[15]
High titers of ecdysone will cause molting or metamorphosis[26], and the characteristics exhibited during feeding activity and HCS are important indications of silkworm molting[25]
Summary
Epidermis of B. mori contains a reduced amount of melanin; the number of urate granules, which accumulate in the dermis to maintain an opaque white epidermis, is increased[14]. A previous study suggested that the tyrosine hydroxylase (TH) gene, which is upstream of the melanin synthesis pathway, may contribute to differences in body color between B. mandarina and B. mori[15]. Previous research has indicated that coloration results from both regulatory and structural genes, with many of the structural genes encoding enzymes that are involved in the biochemical pathways that generate pigments[17]. The key genes in this pathway include tyrosine hydroxylase (TH), DOPA decarboxylase (DDC), N-acetyltransferase (AANAT), NBAD synthase (ebony), NBAD hydrolase (tan), dopachrome-conversion enzyme (yellow-f, yellow-f2), and melanism-promoting genes, yellow, laccase 220,21(Additional file 2: Fig. S2). Studying the changes that occur in gene expression during molting is an important approach to understanding the molecular mechanisms underlying molting and coloration. We identified the genes that contribute to the bd/bd color pattern by comparing the transcriptomes of wild-type Dazao and bd mutant specimens
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