Transgenic silkworms comprising at least one nucleic acid encoding a chimeric silk polypeptide comprising one or more spider silk elasticity and strength motifs are disclosed. Expression cassettes comprising nucleic acids encoding a variety of chimeric spider silk polypeptides (Spider 2, Spider 4, Spider 6, Spider 8) are also disclosed. A piggyBac vector system is used to incorporate nucleic acids encoding chimeric spider silk polypeptides into the mutant silkworms to generate stable transgenic silkworms. Chimeric silk fibers having improved tensile strength and elasticity characteristics compared to native silkworm silk fibers are also provided. The transgenic silkworms greatly facilitate the commercial production of chimeric silk fibers suitable for use in a wide variety of medical and industrial applications.
