Malaria: Is crossbreeding a game changer?

To stop the spread of malaria, a disease that annually kills more than 660,000 people around the world, Gregory Lanzaro says, "it is critical that we understand the movement of genes between these two forms of the African malaria mosquito." (Credit: CDC/Wikimedia Commons)

Many of the assumptions that inform current thinking about mosquito genetics may be wrong, according to new research that shows two genetically distinct malaria mosquitoes exchange genes via crossbreeding.

For the past decade, scientists around the world have intensely studied two forms of the African malaria mosquito Anopheles gambiae. They thought the mosquitoes in the two subgroups—known as the M and S forms—rarely mated outside of their groups, and, just this year, the two were recognized as distinct species.


Developing an accurate picture of gene flow through matings within and between these two mosquito groups could prove key to preventing the spread of malaria. The deadly mosquito-transmitted disease annually kills more than 660,000 people around the world, mostly in Africa.

In this study, published in the Proceedings of the National Academy of Sciences, researchers showed that hybridization, or mating, between the M and S groups was far higher than previously assumed.

“It is critical that we understand the movement of genes between these two forms of the African malaria mosquito,” says Gregory Lanzaro, medical entomologist at the University of California, Davis.

Malaria control

Researchers need to know whether genes for traits like insecticide resistance are shared between the two malaria mosquito subgroups—important information because some of the techniques that appear to be most promising for controlling malaria involve the movement of genes from genetically modified mosquitoes into natural mosquito populations.

The findings also are significant for evolutionary biologists, confirming that the malaria mosquito is a good model for studying gene flow between groups, as well as how species evolve.

The new results show that the frequency of hybrid, or crossbred, individuals in a population ranges from 5 percent to 97 percent, compared to the earlier estimates of less than one percent.

250 mosquito generations

Although the M and S hybrids generated in the laboratory are normal in every sense, the new study demonstrates that this is not true in nature, where hybrids are common but less fit than other mosquitoes.

The study includes data gathered during 21 years from a single village in Mali. The data, representing approximately 250 mosquito generations, revealed that mating behaviors were temporarily unstable. For example, in one year, the rate of crossbreeding between the two groups jumped from the usual zero percent to 12 percent.

Analysis of the distribution of M and S genetic variations at three regions of the genome demonstrated that strong genetic selection prevented transfer of two of the three regions from one group to the other.

However, one region of the genome successfully crossed the reproductive barrier from the S subgroup into the M subgroup.  This region presumably contained a gene or genes beneficial to the M form mosquitoes.

Researchers at the Malaria Research and Training Center at the University of Bamako, Mali contributed to the study, which was funded by several grants from the National Institutes of Health.

Source: UC Davis