‘London Underground mosquito’ is actually much older than previously thought

By Jacopo Prisco, CNN

(CNN) — A type of mosquito that was believed to have evolved in the London Underground has a much older origin in the Mediterranean, according to a new genetic study.

The myth originated during World War II when Londoners sheltering from German warplanes in subway stations had to endure bites from mosquitoes. The pests were so well adapted to the city’s subterranean tunnels that, in the following decades, biologists started to suggest they might have evolved down there.

The mosquito in question is commonly known as the northern house mosquito, and it exists in two forms, identical in appearance but different in behavior. One is called Culex pipiens form pipiens, which only bites birds and lives in open air environments, and the other is called Culex pipiens form molestus — from the Latin word for annoying — which bites humans and thrives belowground.

It was the latter variant, some biologists thought, that could have adapted to thrive in London’s Tube stations. “That theory was made really famous by a genetic study published in 1999, which claimed, based on what I would say was limited evidence, that the ‘London Underground mosquito’ seemed to have evolved (on site) from aboveground population,” said Yuki Haba, a postdoctoral researcher at Columbia University and first author of the new study that published Thursday in the journal Science.

In the study, Haba and his colleagues analyzed the DNA sequences of hundreds of mosquitoes from all over the world, including some historical samples that were alive during World War II, and concluded that molestus did not rapidly evolve below the surface of the English capital but has a much longer history.

“It’s a lot older than the London Tube, and it seems to have evolved around the Mediterranean region, particularly in the Middle Eastern region,” said Haba, adding that the split between the aboveground pipiens and the belowground molestus could have occurred as early as 10,000 years ago, and as late as 1,000 years ago, but most likely between 3,000 and 2,000 years ago.

“The evolutionary analysis suggests that those ancestral molestus populations were aboveground,” he added, “and they kind of gradually dispersed to other places of the world, including the London Underground.”

Migrating north

The team’s quest to test the “London Underground mosquito” hypothesis rigorously started in 2018. “We literally googled Culex pipiens and emailed every author of every paper that we found about the mosquito, telling them we needed samples to understand the origin and genetic diversity of the species,” Haba said.

Thousands of emails and a few years later, the researchers had collected dead mosquitoes — preserved in ethanol — from more than 200 sources across 50 countries. They couldn’t get any live ones from the London Underground itself, as the scientists were denied permission to collect insects directly from the Tube. They instead used historical samples collected throughout the 1900s, conserved at London’s Natural History Museum and analyzed by the Wellcome Sanger Institute, a genomics research center.

In total, the researchers analyzed 357 contemporary mosquitoes and 22 historical specimens, and used additional mosquito samples from a separate study, which beefed up the overall count to about 800.

“Our data shows that molestus is descended directly from pipiens populations that still thrive in the Mediterranean region,” said Lindy McBride, senior author of the study and an associate professor of evolutionary genomics and neuroscience at Princeton University. This finding suggests that it “evolved at Mediterranean latitudes, but probably in the Middle East, where it’s actually too arid for the bird-biting variant to exist.”

At roughly that time, people in the area were starting to create agricultural communities that used irrigation systems, providing ideal places for the mosquito to breed and enabling it to colonize these arid areas and then adapt to humans, McBride said.

The findings are corroborated by the fact that molestus was first described as a species in 1775 in Egypt by naturalist Peter Forsskål. “It had probably been there for 1,000 years, at least, before that,” McBride said. “It was then documented in southern Europe in two places, Croatia and Italy, in the 1800s and then the first records from belowground sites in Northern Europe are from about 1920.”

This sequences of detection suggest that molestus traveled north, and once it hit climates too cold to survive in the open, it found refuge underground. “They can’t survive the cold winter, so they would have been confined to southern France, Italy, Greece, Spain,” McBride said. “They couldn’t have gotten much further north than that until there were underground structures to occupy during the winter.”

There are over 3,000 species of mosquitoes, and they have by now colonized every continent except Antarctica. A mosquito was just found for the first time in Iceland, a country previously believed to be free of the insects due to its cold climate.

Mosquito opportunities

Richard Nichols, a professor of genetics at Queen Mary University of London, was not involved with the research but was one of the authors of the 1999 paper that popularized the “London Underground mosquito” hypothesis. Nichols said in an email that he thinks the new study is wonderful and convincing, and although it comes to different conclusions than his own research, “that’s how science works.”

The 1999 study, he continued, showed that the underground mosquitoes in London were genetically distinct from the aboveground populations, and had distinguishing characteristics that helped them live underground, such as the ability to go through the life cycle without a blood meal, indiscriminate biting when the opportunity arose, and an ability to mate in confined spaces and breed throughout the year.

“We interpreted these results to imply that some of the above underground population had adapted to the London (Tube) system and become reproductively isolated from them,” he said, acknowledging that the data from the new study, which is sourced from more places and has greater genetic diversity, reveals new information that was not available at the time. “In those days we could only readily survey 20 genes — not whole genomes,” he explained.

“Our results still stand, but the interpretation has changed.”

Cameron Webb, an associate professor of medical entomology at the University of Sydney and NSW Health Pathology in Australia, called it a fascinating and comprehensive study of the evolution of this globally important mosquito. “While often portrayed as having adapted specifically to the London Underground, this mosquito is actually well known to be associated with subterranean habitats around the world,” Webb, who was not involved with the work but has conducted research on molestus mosquitoes, wrote in an email. The study demonstrates the ancestral basis for the ability of this mosquito to exploit the London Underground, he added.

The “London Underground mosquito” highlights the need to better understand the biology of less-studied mosquitoes to see how they may exploit the changing urban landscape and bring with them increased pest and public health concerns, Webb concluded. “As the design of our cities adapts in response to a changing climate, we must ensure we don’t create more opportunities for mosquitoes.”

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