When they bite humans, mosquitoes can transmit dangerous diseases. For example, the Aedes aegypti mosquito spreads the viruses that cause yellow fever, Zika and dengue fever. Only the female mosquitoes feed on blood so they can obtain the nutrients they need to develop their eggs. Once they are ready, the insects lay their eggs in carefully selected sites where fresh water collects: if instead they choose places where the water is too salty, their offspring will die.
To find a suitable site, a mosquito ‘tastes’ the water by dipping in its legs and mouthparts, which activates the insect’s sensory neurons and sends signals to its brain. However, it remains unclear exactly how the mosquitoes can distinguish between fresh and salty water. To address this question, Matthews, Younger and Vosshall used a combination of genetic and imaging approaches to study female Ae. aegypti mosquitoes.
These experiments identified a gene known as ppk301 that is necessary for the mosquitoes to successfully lay their eggs in the right type of water. Mutant Ae. aegypti mosquitoes lacking the ppk301 gene did not properly lay eggs in fresh water even when given the opportunity.
Further experiments found that ppk301 was present in specific neurons in the legs and mouthparts of the mosquitoes. In leg neurons, ppk301 played a crucial role in sensing the presence of water and in stimulating the mosquitoes to lay eggs in water containing low levels of salt. However, these cells still responded to salt, even when lacking ppk301: other unidentified genes must therefore also be involved in preventing the mosquitoes from breeding in water that is too salty.
Every year, Aedes aegypti and other mosquitoes infect hundreds of millions of people and cause 500,000 deaths. Knowing exactly how mosquitoes breed could help to develop traps, repellents and other strategies to stop the insects from multiplying.