Isolating blood-feeding and non-biting genes in mosquitoes

Led by University of Oregon biologists William E. Bradshaw and Christina M. Holzapfel, a nine-member team of scientists at five institutions methodically sorted out 902 genes related to blood feeding and 478 genes linked to non-blood feeding from the mosquito Wyeomyia smithii.

The species, commonly known as pitcher plant mosquitoes, is found in swamps and bogs along the east coast of North America from north Florida into Canada. The species completes its pre-adult life cycle in the water of pitcher plants.

The approach used in isolating the genes -- reported in a study published Dec. 18 in the online Early Edition of the Proceedings of the National Academy of Sciences -- will be pursued in other species. The goal is to isolate universal non-biting genes across multiple disease vectors, said Bradshaw and Holzapfel, who are members of the UO's Institute of Ecology and Evolution.

The next species to come under focus will be: common house mosquitoes (Culex pipiens), which spread many encephalitis diseases, West Nile virus and heartworm; Asian tiger mosquitoes (Aedes albopictus), which is spreading rapidly in the United States and carries, among other viruses, dengue, Zika and yellow fever; and the African malaria mosquito Anopheles gambiae.

"We'll see what comparable genes pop out of these other species and identify commonalities," Bradshaw said. "We will continue this process, the end point of which will be the identification of universal non-biting mosquito genes."

Doing that could provide pharmaceutical targets for non-toxic inhibitors that could be developed to turn off biting genes but also allow mosquito populations to thrive and keep their place in the food chain, the researchers said.

"We are seeking the genes that are in the transition between biting and non-biting," Holzapfel said. "The reason we are seeking those genes is because if we can figure out how to incapacitate biting genes, that would mitigate vector-borne disease worldwide. If there is no bite, there is no disease transmission, period."

The research initially targeted Wyeomyia smithii because it is the only known species to have females that bite to obtain blood and are obligate non-biters -- those that don't seek blood. Bradshaw and Holzapfel said that they had realized the possibility 20 years ago that genes guiding these lifestyle differences existed and had evolved in nature, but the technology was not yet developed to isolate these genes.

Females are the blood-feeders in mosquito populations and, thus, the vectors of diseases; males feed on nectar.

In the project, funded by two National Science Foundation grants, the research team 21,618 potential genes in the pitcher plant mosquito. Over seven generations, they identified and extracted 1,380 genes that were determined to have direct effects on differentiating the biting and non-biting lifestyles.

The step-by-step method involved strong, directional gene selection on a low-biting Florida population. By saving and mating only females about to blood-feed, researchers created an avid, voracious biting line. A group of disinterested non-biters also was developed from the same population by eliminating all females that bit or attempted to bite.

The research team also examined known metabolic pathways of the isolated genes -- knowledge that, when further understood, will be helpful for future efforts by pharmaceutical companies to harness a control approach that nature already has established, Holzapfel said.

"This study shows the power of combining well-designed breeding experiments and genomics data to gain insights into the biology of disease vectors," said co-author Michael E. Pfrender, director of the Genomics & Bioinformatics Core Facility at the University of Notre Dame, which processed the genetic data for the study.