Abstract
BackgroundThe insect cuticle is composed of various proteins and formed during the molt under hormonal regulation, although its precise composition and formation mechanism are largely unknown. The exhaustive catalogue of genes expressed in epidermis at the molt constitutes a massive amount of information from which to draw a complete picture of the molt and cuticle formation in insects. Therefore, we have catalogued a library of full-length cDNAs (designated epM) from epidermal cells during the last larval molt of Bombyx mori.ResultsOf the 10,368 sequences in the library, we isolated 6,653 usable expressed sequence tags (ESTs), which were categorized into 1,451 nonredundant gene clusters. Seventy-one clusters were considered to be isoforms or premature forms of other clusters. Therefore, we have identified 1,380 putative genes. Of the 6,653 expressed sequences, 48% were derived from 92 cuticular protein genes (RR-1, 24; RR-2, 17; glycine-rich, 29; other classes, 22). A comparison of epM with another epidermal EST data set, epV3 (feeding stage: fifth instar, day 3), showed marked differences in cuticular protein gene. Various types of cuticular proteins are expressed in epM but virtually only RR-1 proteins were expressed in epV3. Cuticular protein genes expressed specifically in epidermis, with several types of expression patterns during the molt, suggest different types of responses to the ecdysteroid pulse. Compared with other Bombyx EST libraries, 13 genes were preferentially included in epM data set. We isolated 290 genes for proteins other than cuticular proteins, whose amino acid sequences retain putative signal peptides, suggesting that they play some role in cuticle formation or in other molting events. Several gene groups were also included in this data set: hormone metabolism, P450, modifier of cuticular protein structure, small-ligand-binding protein, transcription factor, and pigmentation genes.ConclusionWe have identified 1,380 genes in epM data set and 13 preferentially expressed genes in epidermis at the molt. The comparison of the epM and other EST libraries clarified the totally different gene expression patterns in epidermis between the molting and feeding stages and many novel tissue- and stage-specifically expressed epidermal genes. These data should further our understanding of cuticle formation and the insect molt.
Highlights
The insect cuticle is composed of various proteins and formed during the molt under hormonal regulation, its precise composition and formation mechanism are largely unknown
In addition to cuticular proteins, we identified 290 genes encoding the amino acid sequences of their putative signal peptides, suggesting that they play some role in cuticle formation or other molting events
Construction and characterization of the epM expressed sequence tag (EST) data set To exhaustively identify the cuticular protein components and epidermal genes expressed during the larval molting stages of the silkworm, we constructed a library of fulllength cDNAs by mixing the RNAs of eight consecutive stages (C1, C2, D1, D2, D3, E1, E2, and F, according to [20])
Summary
The insect cuticle is composed of various proteins and formed during the molt under hormonal regulation, its precise composition and formation mechanism are largely unknown. The number of types of cuticular proteins included in the cuticle has not yet been resolved, nor how their coordinated expression in epidermal cells and their excretion into the cuticle are precisely controlled during molting. These issues are very important for any understanding of the mechanisms underlying the formation of the highly ordered and layered structure of the cuticle. Many other cuticular proteins lacking these motifs have structures containing other repeated structures, such as GGX or AAP(A/V) [1,11,12]
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