doi:10.1038/nindia.2012.175 Published online 27 November 2012
Through genetic modification, researchers have created a new breed of silkworms resistant to a deadly viral infection which accounts for more than half of the world's silk cocoon crop losses1.
"The research will have great impact in the sericulture industry," says Javaregowda Nagaraju of the Centre for DNA Fingerprinting and Analysis (CDFD) in Hyderabad who led the team that included scientists from France and Andhra Pradesh State Sericulture Research & Training Institute.
The domesticated silkworm, Bombyx mori is a primary producer of silk, especially in Asia. The baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) affects its rearing and hampers silk cocoon production to an extent that in India alone more than 50 per cent of silk cocoons are lost due to this infection. Effective treatment against the virus has been elusive due to its sturdy nature and till date no enduring therapy exists to combat the problem.
Nagaraju and his team now report having generated a breed of genetically transformed (transgenic) silkworms resistant to the BmNPV1. They accomplished this by exploiting the so-called RNA interference (RNAi) — a mechanism by which cells 'silence' the expression of foreign genes.
Transgenic silkworm lines capable of silencing one or other of the essential genes of the baculovirus had been created in the past by other groups. But these attempts for targeting a single gene to block baculovirus proliferation resulted in incomplete gene silencing and only transitory inhibition of viral infection. "Hence, in the present study, we assayed the efficiency of the simultaneous inhibition of several key viral genes," the authors say. The scientists reasoned that this approach will stop virus multiplication and confer the silkworms long-term natural defence against the virus. They were proved right.
The researchers selected three baculoviral genes that are essential for DNA replication and transcription, and another gene necessary for infectivity. They constructed vectors incorporating short sequences of all these four genes and introduced these RNAi producing 'transgenes' into the germline of high yielding, baculovirus-susceptible commercial strain, CSR2 which is currently in use in sericulture. While almost all the nontransgenic CSR2 lines succumbed to infection before reaching cocoon stage, more than 75% of the transgenic CSR2 lines survived and reproduced successfully demonstrating that this approach conferred stable protection against high doses of baculovirus infection to the transformed individuals and to their progeny.
"To our knowledge, this is the first report where multiple genes of the same pathogen are being simultaneously used for effective RNAi mediated suppression of virus replication/infectivity in insects," the researchers report. They said that "targeting simultaneously several viral genes from a single transgene construct is possible since only a few hundred-long double-stranded RNA sequence is sufficient to elicit RNA interference in the silkworm."
The researchers succeeded in getting high yielding silkworm lines protected from baculovirus infection that could be harnessed for the benefit of sericulture industry. "The contained multilocational trials of the baculovirus resistant transgenic silkworms are currently underway," Nagaraju told Nature India.