Aedes aegypti is the mosquito that is the scourge behind outbreaks of the deadly yellow, dengue and even chikungunya fevers that kill thousands of people in Africa, South America and Asia every year. At least 2.5 billion people—more than one third of the human population—are at risk of infection from dengue fever alone. Now an international consortium of researchers has sequenced the majority of the Aedes genome, offering hope that specific genes can be identified that would allow scientists to combat these tropical diseases in their carrier.
"The concern is that for dengue fever there is no vaccine," like there is for yellow fever, says Vishvanath Nene, a molecular biologist at the J. Craig Venter Institute in Rockville, Md., and lead author of the paper presenting the genome, published in the online edition of Science. "There is no other method of control other than trying to target the mosquito itself."
Unfortunately, Aedes has proved to have a technically complex and extremely large genome—one third the size of the human genome. The researchers used the so-called shotgun sequencing technique: splintering the DNA in the genetic makeup of specially inbred mosquitoes then painstakingly reassembling it. Aedes has many repetitive sequences and transposable elements (sections that can wander freely within the genome), making it difficult to accurately reassemble the entire set of DNA sequences.
The researchers were able to positively identify 15,419 genes that encode for specific proteins and speculatively identify another 15,396 putative genes. "The genome may be bigger, it may be slightly smaller," Nene says, "but we're in the ballpark."
To get a better read, scientists will have to undertake more repetitive sequencing (if sequencing costs continue to decline) or target specific areas of interest that were inadequately covered in this first go-round. Efforts to sequence the closely related common "house" mosquito Culex pipiens quinquefasciatus may also help resolve some of these questions when completed, potentially as early as the end of this year. "Since the Culex genome is much smaller, we hope to get a much better genome assembly," Nene says.
Aedes has already been shown to possess extra genes for receptors that detect odors, for proteins that help it purge pesticides from its system, and for controlling the makeup of its outer shell. Researchers now plan to focus on identifying the specific genes associated with the mosquito's other unique traits, from its ability to resist dengue fever to its ability to lay eggs that can last for up to a year without water. "Throw them into water and they hatch into larvae within minutes," Nene notes. And next up: sequencing the genome of the Lyme disease–spreading deer tick, Ixodes scapularis, which is twice the size of Aedes's and could prove to be even more complex.