Abstract
The diamondback moth Plutella xylostella is a serious pest of crucifers. It has high reproductive potential and is resistant to many insecticides. Typically, the last-instar larvae of P. xylostella, before pupation, move to the lower or outer plant leaves to make a loose silk cocoon and pupate inside for adult formation. To better understand this pivotal stage we studied the cocoon-spinning behavior of P. xylostella and measured three successive phases by video-recording, namely the selection of a pupation site, spinning a loose cocoon and padding the scaffold cocoon. Subsequently, we cloned three fibroin genes related to cocoon production, i.e., fibroin light chain (Fib-L), fibroin heavy chain (Fib-H), and glycoprotein P25. A spatio-temporal study of these three fibroin genes confirmed a high expression in the silk glands during the final larval instar silk-producing stage. In parallel, we did an exogenous treatment of the insect molting hormone 20-hydroxyecdysone (20E), and this suppressed fibroin gene expression, reduced the normal time needed for cocoon spinning, and we also observed a looser cocoon structure under the scanning electron microscope. Hence, we demonstrated that the expression levels of key genes related to the synthesis of 20E [the three Halloween genes Spook (Spo), Shadow (Sad), and Shade (Shd)] decreased significantly during spinning, the expression of the 20E receptor (EcR and USP) was significantly lower during spinning than before spinning, and that the expression levels of CYP18-A1 related to 20E degradation were significantly up-regulated during spinning. The significance of the cocoon and the effects of 20E on the cocoon-spinning behavior of P. xylostella are discussed.
Highlights
Cocoon spinning behaviors have been described in many insect species before like, the domesticated silkworm Bombyx mori (Yokoyama, 1951; Kiyosawa et al, 1999; Chen et al, 2012a,b; Guo et al, 2016), the giant silkworm Hyalophora cecropia (Van der Kloot and Williams, 1953), and the Chinese oak silkworm Antheraea pernyi (Lounibos, 1975, 1976)
The fibrous silk core is generally comprised of a three-protein complex, including fibroin light chain (Fib-L), fibroin heavy chain (Fib-H) and glycoprotein P25 (Sehnal and Zurovec, 2004; Sutherland et al, 2010)
The synthesis and secretion of silk fibroin have been studied in detail in model insects like B. mori, the cocoon behavior, these silk genes and the hormonal regulation have not been uncleared in the diamondback moth, which is an agricultural pest that causes high damage to cruciferous vegetables
Summary
Cocoon spinning behaviors have been described in many insect species before like, the domesticated silkworm Bombyx mori (Yokoyama, 1951; Kiyosawa et al, 1999; Chen et al, 2012a,b; Guo et al, 2016), the giant silkworm Hyalophora cecropia (Van der Kloot and Williams, 1953), and the Chinese oak silkworm Antheraea pernyi (Lounibos, 1975, 1976) These authors described the major features of the stereotyped spinning movements and the effects of environmental conditions on the morphology, structure and mechanical properties. In C. cephalonica, studies revealed that 20E modulates the expression of the Fib-L, Fib-H, and P25 genes at the mRNA level These reports indicated that larval development and silk gland function in these insects are regulated by 20E. It has a high reproductive potential, a wide distribution in the world, and is resistant to many insecticides
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