Abstract
Raspy crickets (Orthoptera: Gryllacrididae) are unique among the orthopterans in producing silk, which is used to build shelters. This work studied the material composition and the fabrication of cricket silk for the first time. We examined silk-webs produced in captivity, which comprised cylindrical fibers and flat films. Spectra obtained from micro-Raman experiments indicated that the silk is composed of protein, primarily in a beta-sheet conformation, and that fibers and films are almost identical in terms of amino acid composition and secondary structure. The primary sequences of four silk proteins were identified through a mass spectrometry/cDNA library approach. The most abundant silk protein was large in size (300 and 220 kDa variants), rich in alanine, glycine and serine, and contained repetitive sequence motifs; these are features which are shared with several known beta-sheet forming silk proteins. Convergent evolution at the molecular level contrasts with development by crickets of a novel mechanism for silk fabrication. After secretion of cricket silk proteins by the labial glands they are fabricated into mature silk by the labium-hypopharynx, which is modified to allow the controlled formation of either fibers or films. Protein folding into beta-sheet structure during silk fabrication is not driven by shear forces, as is reported for other silks.
Highlights
The ability to produce silk has evolved in at least 23 groups of insects [1], in spiders [2] and in several other arthropods [3,4]
The molecular arrangement in spider and silkworm silks is the result of shear forces and controlled dehydration acting on highly concentrated silk protein solutions as they pass through a hardened aperture known as a spinneret [6,7]
The insect ceased to produce fibers if the shelter was undisturbed for long periods, whilst removal of a shelter resulted in the construction of a replacement
Summary
The ability to produce silk has evolved in at least 23 groups of insects [1], in spiders [2] and in several other arthropods [3,4]. Silk research has focused on silkworm cocoon and spider dragline silks, which have independently evolved a number of convergent features. Spider and silkworm silks consist of long, repetitive proteins that fold predominantly into beta-sheets, with the protein backbone parallel to the fiber axis [2]. Protein backbones in silks made by glow-worms and adult lacewings are orientated perpendicular instead of parallel to the fiber axis [8]; the silks of fleas, bees and lacewing larvae contain proteins arranged in alpha-helices instead of beta-sheets [8,9]; and the fibrous proteins in some silks are an order of magnitude smaller than spider dragline and silkworm cocoon silk proteins [10]. Further characterisation of silks in addition to spider and silkworm silks will allow a comparative approach to understanding the complex molecular arrangements found in silk
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