MOORE, A.M.F.; SPAGNA, J.C.; THOR, D.; LAWRENCE, B.A.: Molecular Structure and Material Properties in Tarantula Silk

One of the deepest divergences in spider evolution is between the mygalomorphs (which include tarantulas) and the araneomorphs. Whereas the araneomorphs are characterized by a variety of silk types and web-building, the mygalomorphs tend to have only one type of silk and do not build webs. Since mygalomorphs tend not to use their silk for extreme mechanical functions, we hypothesized that their silk should not display the great strength or toughness that is prevalent in silks from the araneomorphs. To test this hypothesis, we characterized the molecular and mechanical properties of silk from the Chilean common tarantula Grammastola rosea and compared it to data from the dragline silks of the araneomorphs. Grammastola silk contains only 11% alanine, or half that found in dragline silk. This lack of alanine is important because alanine-rich beta-sheet crystallites have been proposed to confer dragline’s great strength. Furthermore, our C-13 CP-MAS NMR spectroscopy on Grammastola silk shows that only half of the alanine present is in beta-sheet conformation. To find breaking strength, we stretched Grammastola silk while measuring the resistant force to +/- 5*10^-8 N. Cross-sectional area was measured by FESEM. The mean strength was 0.15 GPa, about 15% that of dragline (1.1GPa). The combined decrease in beta-sheet crystals and strength strongly corroborates the hypothesis that alanine residues in beta-conformation confer great strength to araneomorph silks. Moreover, the relative scarcity of these motifs in silks of the more basal, less diverse spider lineage suggests that their increase may have contributed to the greater diversification of Araneomorph spiders.