A type of mosquito thought to have evolved on the London Underground has origins older than the Mediterranean, according to new genetic research.
This myth began during World War II, when Londoners evacuating from German fighter jets in a subway station had to endure mosquito bites. The pests were so well adapted to the city's underground tunnels that in subsequent decades biologists began to suggest that they may have evolved there.
The mosquito in question is commonly known as the northern mosquito, and there are two forms that look the same but behave differently. One is called the Culex mosquito, which only bites birds and lives in outdoor environments, and the other is called the Culex mosquito. Molestas comes from the Latin word meaning "nuisance" and they bite humans and breed underground.
Some biologists thought it was the latter variant that had adapted to thrive in London Underground stations. "This theory was made very popular by a genetic study published in 1999, which argued, based on what could be described as limited evidence, that the 'London Underground Mosquito' likely evolved (in situ) from above-ground populations," said Yuki Haba, a postdoctoral researcher at Columbia University and lead author of the new study. published Published in Science on Thursday.
In the study, Haba and his colleagues analyzed the DNA sequences of hundreds of mosquitoes around the world, including some historical samples that lived during World War II, and concluded that molesters did not evolve rapidly beneath the British capital, but have a much longer history.
"It's much older than the London Underground and appears to have evolved in the Mediterranean region, particularly the Middle East," Haba said, adding that the split between the above-ground pipiens and the below-ground Molestas could have occurred as early as 10,000 years ago or as late as 1,000 years ago, but was probably between 3,000 and 2,000 years ago.
"Evolutionary analysis suggests that these ancestral molestation groups existed on land and gradually dispersed to other parts of the world, such as the London Underground," he added.
The research team's quest to test the "London Underground Mosquito" hypothesis began strictly in 2018. "We literally searched for Culex mosquito and emailed all the authors of every paper we found about this mosquito, telling them we needed samples to understand the origins and genetic diversity of this species," Haba said.
Thousands of emails and several years later, the researchers collected dead mosquitoes preserved in ethanol from more than 200 sources in 50 countries. The scientists were unable to obtain live insects from the London Underground itself, as they were denied permission to collect insects directly from the tube. Instead, they used historical samples collected throughout the 1900s, kept at London's Natural History Museum, and analyzed by the Wellcome Sanger Institute, a genomics research center.
In total, the researchers analyzed 357 modern mosquitoes and 22 historical specimens, and used additional mosquito samples from another study, increasing the overall number to about 800.
"Our data show that Molestus is a direct descendant of pipiene populations that still thrive in the Mediterranean region," said Lindy McBride, senior author of the study and associate professor of evolutionary genomics and neuroscience at Princeton University. This finding suggests that it "evolved in Mediterranean latitudes, but probably in the Middle East, where it is actually too dry for bird-biting variants to exist."
Around that time, McBride said, people in the area began creating farming communities that used irrigation systems, providing ideal breeding grounds for mosquitoes, allowing them to colonize these arid areas and adapt to humans.
This finding is supported by the fact that molestation occurred first. explained The species was discovered in Egypt in 1775 by naturalist Peter Forskar. "It was probably there for at least 1,000 years before that," McBride said. "Then it was recorded in two places in southern Europe in the 1800s, Croatia and Italy, and then the first records from underground sites in Northern Europe date from around 1920."
This series of detections suggests that Molestus migrated north and took refuge underground when it reached climates too cold to survive outdoors. "They wouldn't be able to survive the cold winters, so they would have been trapped in southern France, Italy, Greece and Spain," McBride said. "They wouldn't be able to go any further north until they had underground structures that they could occupy during the winter."
There are more than 3,000 species of mosquitoes and they are now established on every continent except Antarctica. Mosquitoes have been discovered for the first time in Iceland, which was previously thought to be free of mosquitoes due to its cold climate.
Richard Nicholls, a professor of genetics at Queen Mary University of London, was not involved in the study but was one of the authors of the 1999 paper that popularized the "London Underground Mosquito" hypothesis. Nichols said in an email that the new study is impressive and convincing, and while it reaches different conclusions than his own, "that's how science works."
He went on to say that a 1999 study showed that London's underground mosquitoes are genetically different from above-ground mosquitoes and have traits that help them live underground, including the ability to complete their lives without ingesting blood, the ability to bite indiscriminately if given the opportunity, and the ability to mate and reproduce year-round in confined spaces.
"We interpreted these results to imply that some of the above-ground subterranean populations have adapted to the London (underground) system and are reproductively isolated from London," he said, acknowledging that the new study's data, collected from more locations and with greater genetic diversity, revealed new information not available at the time. "At that time, we could only easily study 20 genes; we could not study the entire genome," he explained.
"Our results are still valid, but the interpretation has changed."
Cameron Webb, associate professor of medical entomology at the University of Sydney and NSW Health Pathology in Australia, said this was a fascinating and comprehensive study of the evolution of this globally important mosquito. "Although often portrayed as being particularly adapted to the London Underground, this mosquito is actually well known to be associated with subterranean habitats around the world," Webb, who was not involved in the study but has conducted research on the molester mosquito, wrote in an email. The study shows an ancestral basis for the mosquito's ability to use the London Underground, he added.
Mr Webb concluded that the 'London Underground Mosquito' highlights the need to better understand the ecology of little-studied mosquitoes to understand how they exploit changes in the urban landscape and become a growing pest and public health concern. "As the design of our cities adapts as the climate changes, we need to avoid creating more opportunities for mosquitoes to enter."
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