Genome engineering and parthenocloning in the silkworm, Bombyx mori

Genetic engineering of the silkworm, Bombyx mori, opens door to the production of new kinds of silk and to the use of silkworms as proteosynthetic bioreactors. The insertion of foreign genes into silkworm genome and the control of their expression by diverse promoters have become possible but are not yet efficient enough for commercial use. Several methods of gene targeting are being developed to minimize position effect on transgene expression and facilitate cloning. Parthenocloning can be exploited to conserve genetic traits and improve selection and amplification of clones containing genes of interest. Some silkworm clones have been bred for decades as genetically stable female stocks whose unfertilized eggs are induced to develop by heat-shock treatment. Any exclusively female generation contains exact copies of the maternal clone-founder genome. Ovaries transplanted in either direction between the standard and the parthenogenetic genotypes yield eggs capable of parthenocloning. In addition, use of male larvae as ovary recipients eliminates diapause in eggs produced in the implants. Unfertilized eggs of some silkworm clones respond also to the cold-shock treatment by producing homozygous fertile sons; cloned females can be crossed with their parthenogenetic sons to obtain progeny homozygous for the transgene in both sexes. Rational exploitation of available parthenozygous pools and the use of parthenocloning methods enable rapid fixation and maintenance of the desired genotypes.