Mosquitoes which produce 95% male offspring have been created by scientists, in a bid to help control the spread of malaria.
Malaria is exclusively transmitted by mosquitoes. Although reductions in the disease have been controlled by spraying homes with insecticides and using nets, the disease continues to kill hundreds of thousands on an annual basis, particularly in sub-Saharan Africa.
The idea of using a genetic defect to control pest populations was first proposed about six decades ago, however this is the first instance where it has been demonstrated practically.
The research team, led by Dr Nikolai Windbichler and Professor Andrea Crisanti from Imperial College London, moved a gene from a slime mould into an African malaria mosquito, Anopheles gambiae. This particular gene produces an enzyme, called endonuclease, which is responsible for chopping up DNA when a particular sequence is recognised.
Prof Crisanti and his team were aware of a coincidence where the target sequence of the endonuclease is found in huge quantities on the X chromosome of the mosquito.
When sperm is produced in a normal fashion, whether in humans or mosquitoes, it is split equally between the X and Y chromosomes. When fusion with the egg occurs, the male and female embryos are produced.
In the new mosquitoes that have been produced, the X-attacking endonuclease is activated during sperm formation. This results in the production of more male than female offspring. It ensures that in excess of 95% of the offspring is male.
This causes the population to decline due to the shortage of females, which causes the number of malaria-spreading mosquitoes to decline.
The most important factor about this forced change is that it is passed on to the male mosquitoes’ progeny. This implies that if the artificial strain is released into a lab-driven or wild population, it will spread until most of the males are only producing male offspring. This will eventually eliminate the population completely.
The researchers found that in five test cages that commenced with 50 males and 50 females, the female numbers declined drastically after four generations, after they introduced 150 of the sex-distorter males.
Prof Crisanti explained the benefits of both effects, as only the females bite humans and cause the spread of malaria. If there is a decline in the numbers of females, it will slow down the spread of malaria, and a crash in the overall population could break the cycle of the transmission of the disease.
It is believed that if the sex-distorter strain is used in the wild, it may not spread for an indefinite period of time and may need to be re-instated on a regular basis.
Dr Alphey stated that for overall success, it would be necessary for a system to be expressed on the Y chromosome. This would be a more powerful method as fewer individuals would need to be released, since the males will have inherited the gene from their fathers and will pass it on to their sons, which means the effect would not be diluted at all.
Prof Crisanti added that if it was on the Y chromosome, a single individual could wipe out an entire population. Although this has not been fully assembled, the pieces are already in place to do so.
Image Credit: NIAID