Why Genes That Make Mosquitoes Glow Can Help Reduce Vector-Borne Disease
By Ed Ricciuti
Fireflies they are not, but glow they do. Not in the dark, to be sure, but mosquitoes genetically modified in the laboratory for an emerging approach to reducing the threat of vector-borne disease look like miniature neon signs when subjected to ultraviolet light.
To produce genetically modified, or transgenic, mosquitoes, scientists stich together a construct of mosquito DNA that endows them with a trait that kills off females before they can reproduce, eventually suppressing the surrounding population. Two genes are inserted together into the modified DNA. One is a self-limiting device that prevents female mosquitoes from maturing to adults. The other is a marker that makes the mosquitoes that possess it glow under certain wavelengths of ultraviolet light, facilitating identification when they are collected in monitoring efforts. Scientists at North Carolina State University (NCSU) are trying to give that glow more pizzazz, according to a new study published in July in the Journal of Medical Entomology.
Just this spring, the Florida Keys were the site of the first test release in the United States of transgenic mosquitoes, in this case Aedes aegypti, an invasive species in the Western Hemisphere and vector of yellow fever, dengue, and Zika viruses. After release, the genetically engineered male mosquitoes mate with wild females, whose offspring inherit the self-limiting gene. The male offspring mate with wild females and the cycle continues. If all goes according to plan, the effort gradually reduces the mosquito population over a number of generations.
The release of these millions of mosquitoes has drawn plenty of news coverage, especially since genetic modification has its vocal critics and approval was preceded by a long public battle. Behind the headlines, however, the release of transgenic organisms depends on exactingly precise, exceedingly nuanced laboratory research during which scientists continually tinker with DNA to find genes that make transgenic techniques more efficient.
“Transgenic insects are made by injecting DNA into very young embryos before cellularization,” says Maxwell Scott, Ph.D, professor of entomology at NC State and senior author of the paper. Injection into eggs containing the target embryos is via a tiny glass needle, its point microscopic, testifying to the mini-world in which the process is carried out.
Equipping transgenic mosquitoes with fluorescence that labels them as transgenic is necessary for monitoring them in the field, an essential step for determining the effectiveness of the method. Since it is tough to tell one mosquito from another, genes that manufacture proteins that glow under UV light make it easier for scientists to measure the presence of the transgenic mosquitoes in the monitoring samples they collect.
The brighter, bigger, and more long lasting the glow, the better. The glowing markers are produced through the gene expression, the process by which instructions stored in DNA are copied and transcribed into a form that cells can use to make proteins and other essentials with which they function. As far as transgenic technology is concerned, finding ways to enhance gene expression is akin to building a better mousetrap.
Scott and colleague Sophia Webster, Ph.D., postdoctoral researcher at NC State, sought to express the red fluorescent protein DsRed and green fluorescent protein ZsGreen at highly visible levels throughout the mosquito body at all life stages. To do so, they had to identify a suitable Aedes gene promoter; a promoter is a hunk of DNA, usually at the beginning of a gene and which switches expression on or off.
The promoter that Webster and Scott show works well with the fluorescent genes goes by the term Aeahsp83 (short for Ae. aegypti heat shock protein 83). Says Scott, “Some 10 years ago, Zach Adelman [professor of entomology at Texas A&M University] published that the Aedes ubiquitin gene promoter can be used to drive high levels of expression in most cells at most stages. The Aeahsp83 gene promoter is another promoter that can be used for strong expression throughout development. So, this promoter just provides another option for building gene constructs.”
Easy to see in all stages of a mosquito, “the Aeahsp83 promoter is a useful addition to the molecular toolbox for manipulating gene expression in transgenic Ae. aegypti,” the researchers conclude.
Journal of Medical Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.