Chapter 18, Problem 27P

Researchers at the University of California at San Diego have designed a strategy, alternatively called the mutagenic chain reaction (MCR) or gene drive, that can introduce rapidly a designed mutation into almost all of the chromosomes within an entire interbreeding population. Their idea was surprisingly simple, and it depends

on plasmids such as that in the diagram that follows. In these MCR constructs, genes that can express high levels of Cas9 protein (gray) and an sgRNA (green) for a particular target in the genome are flanked by sequences that surround the target site in the genome (blue).

Suppose you make a recombinant MCR plasmid in which the plasmid contains sequences for the X-linked yellow body color gene in Drosophila. The Cas9 gene and sgRNA genes in the plasmid replace a protein-coding exon of the yellow gene that is needed for yellow gene function. The sgRNA is specific for a site within the wild-type yellow gene.

a.	The researchers injected this plasmid into a wild-type male embryo, where it became incorporated into some germ-line cells by homologous recombination. The sperm that developed from these germ-line cells fertilized a wild-type egg. The females that developed from these fertilized eggs were yellow-bodied, which was surprising because loss-of-function alleles of yellow are recessive to wild-type alleles. Explain (includediagrams) the genesis of these yellow-bodied females. (Hint: Think about the name mutagenic chain reaction.)
b.	When a single such yellow-bodied female was introduced into a population of 100 wild-type flies, within a couple of generations almost every fly had yellow bodies. Explain this result.
c.	Researchers are now trying to use the gene drive system to prevent Anopheles stephensi mosquitoes from spreading malaria, a disease caused by a protozoan called Plasmodium that parasitizes mosquitoes and humans. This use of the technology is based on the availability of DNA sequences, from mice resistant to malaria, that encode an antibody that interrupts the Plasmodium life cycle. Describe how this system would work to control the spread of malaria.
d.	In 2016, an expert panel convened by the National Academies of Science, Engineering, and Medicine released a report that cautioned against the release to the environment of Anopheles mosquitoes engineered as in part (c). Why was this panel so concerned about using MCR to control malaria?