Culex pipiens f. molestus is a form of mosquito found in cities all over the world
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A form of mosquito associated with the London Underground evolved to live in human environments long before subterranean railways sprung up in the 19th century.
Culex pipiens f. molestus is found in cities all over the world, but it became widely known as the London Underground mosquito after the second world war, when it was the scourge of Londoners sheltering in tube stations during the Blitz.
It is closely related to a bird-biting form of the same species known as Culex pipiens f. pipiens, and biologists thought the molestus form had evolved in urban environments within the past few centuries.
To learn more about its origins, Lindy McBride at Princeton University and her colleagues analysed the DNA of 790 mosquitoes from 44 countries around the world, including the molestus and pipiens forms as well as some closely related species.
The results suggest that, rather than arising in London tube tunnels, the molestus mosquito probably evolved in the Middle East thousands of years ago. There are three main lines of evidence for this.
First, the molestus form is genetically closer to pipiens populations from the Mediterranean basin than it is to pipiens populations in northern Europe. They are like close cousins to these Mediterranean pipiens mosquitoes, says McBride, suggesting that one arose from the other.
Also, molestus mosquitoes from the eastern Mediterranean region are genetically more variable than molestus in underground habitats from northern Europe. “This suggests that they have been present in the eastern Mediterranean for a significantly longer period of time,” says McBride.
Finally, the pipiens form doesn’t exist in the Middle East. This makes it much easier to imagine how the ancestors of the molestus mosquitoes could have colonised the region and evolved to bite humans in isolation, without interbreeding with the bird-biting pipiens insects, says McBride.
Based on the team’s analysis of genetic mutations, it is almost certain that the molestus mosquitoes are far more ancient than previously thought, says McBride. “Our calculations tell us that it must have been at least 1000 years ago and more likely 2000 to 10,000 years ago [that they evolved]. This lines up perfectly with the development of agriculture in the Middle East.”
“Instead of evolving from scratch in urban underground spaces, molestus was already primed for city life thanks to much older adaptations,” she says. “Once established in cities, it has likely evolved further.”
The crowded environments of cities may lead to new hybrids between the bird-biting and human-biting forms, which would have public health implications, says McBride. “Even if hybridisation is rare, these mosquitoes may show intermediate behaviour and increased ability to transmit West Nile [virus] from birds to humans.”
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