Friday, January 2, 2009 - 07:51
Mosquito-borne diseases, like Dengue and Malaria, are serious problems in many parts of the world. While some people are working on treatments for mosquito-carried disease, others are looking at ways to treat the mosquitoes.
Figure 1. Image of Aedes aegypti from the Public Health Library
Controlling insect activities by messing with their genomes isn't a new idea. In many parts of the world, screwworms are controlled by damaging their genomes on a large scale by treating the flies with X-rays and releasing the sterile males out into the environment. The sterile males aren't as healthy or as successful at mating, but they are sent out in such large numbers that they outnumber their fertile counterparts, making this program a success. Unfortunately, for a number of reasons, this approach isn't a good method for controlling mosquitoes.
Now, a group from Australia has found another way to modify mosquitoes.
They injected Wolbachia into mosquito eggs and created new strains of mosquitoes with half the lifespan of a normal mosquito (1, 2). This is important because many disease-causing organisms need to go through a developmental stage in the mosquito. Parasites, like those that cause malaria, and viruses, like Dengue, are picked up when a mosquito bites someone. They must reproduce in the gut and move into the salivary glands before they can be injected during a mosquito bite. Since this process takes about two weeks, a mosquito with a shorter life span will have less time to spread infections.
Figure 2. An insect embryo containing Wolbachia (green) (3).
Wolbachia are bacteria that normally live as symbionts in many species of mosquitoes, as well as fruit flies, and nematodes. They just haven't been found in Aedes aegypti. Wolbachia pipientis wMelPop, also shortens the life time of infected fruit flies and it modifies the ability of its host to reproduce. If a fly or mosquito is female, the Wolbachia are inherited by the fly's offspring. If an infected fly is male, and the female is uninfected, the embryos usually die, through a phenomenon known as cytoplasmic incompatibility (CI) (3).
Cytoplasmic incompatibility is also useful because it makes it less likely that Aedes aegypti mosquites will become resistant to Wolbachia infection. According to the Science paper, evolution of resistance is also less likely because the mosquitos will experience less selection pressure. And no signs of resistance have appeared in the ten years that people have been working with infected laboratory strains of D. melanogaster.
- C. J. McMeniman, R. V. Lane, B. N. Cass, A. W.C. Fong, M. Sidhu, Y.-F. Wang, S. L. O'Neill (2009). Stable Introduction of a Life-Shortening Wolbachia Infection into the Mosquito Aedes aegypti Science, 323 (5910), 141-144 DOI: 10.1126/science.1165326
- A. F. Read, M. B. Thomas (2009). MICROBIOLOGY: Mosquitoes Cut Short Science, 323 (5910), 51-52 DOI: 10.1126/science.1168659
- S. Zabalou (2004). Wolbachia-induced cytoplasmic incompatibility as a means for insect pest population control Proceedings of the National Academy of Sciences, 101 (42), 15042-15045 DOI: 10.1073/pnas.0403853101