Development

In with the New

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Science  30 Sep 2011:
Vol. 333, Issue 6051, pp. 1802
DOI: 10.1126/science.333.6051.1802-b
CREDIT: CHAUDHARI ET AL., PNAS 108, 10.1073/PNAS.1112288108 © 2011 NAS, USA

Growing insects must periodically shed their hard exoskeletons. At the beginning of the molting process, epidermal cells secrete a thin nonchitinous envelope and then secrete new cuticular chitin underneath this envelope. The envelope was believed to protect the new cuticle from molting-fluid chitinase enzymes that digest the inner layer of the old cuticle before shedding of the hard outer layers. Chaudhari et al. determined the localization of the molting-fluid chitinase in the red flour beetle, Tribolium castaneum, and found surprisingly that it colocalized with chitin in both the old and newly synthesized cuticle. With the goal of discovering the source of the new cuticle stability, they examined the gene knickkopf (Knk), which is required for the laminar organization of the cuticle during embryonic development in Drosophila melanogaster. Knockdown of the T. castaneum Knk ortholog, TcKnk, led to molting defects at all developmental stages, and TcKnk protein colocalized with chitin in the new cuticle. Knockdown of TcKnk RNAi led to loss of total chitin. Although chitin levels could be restored by knockdown of two chitinase genes, this did not restore proper cuticular organization, suggesting that TcKnk organizes chitin into laminae to protect it from degradation. This mechanism of cuticle protection may be conserved in all chitinous invertebrates.

Proc. Natl. Acad. Sci. U.S.A. 108, 10.1073/pnas.1112288108 (2011).

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