A new genetic study dispels the myth of the ‘London Underground mosquito,’ pointing to an older origin of this species in the Mediterranean region and explaining its evolution beyond the borders of the British capital.
The idea that this mosquito originated specifically in London’s tunnels during World War II arose in the context of bites while sheltering in underground stations. Over time, many biologists believed that the city’s underground life and its constant humidity could have fostered the adaptation and evolution of the underground form molestus – one of the two manifestations of Culex pipiens.
The mosquito, often called the ‘northern house mosquito,’ is linked to two forms in its existence: Culex pipiens form pipiens feeds mainly on birds and lives in the open air; the other form – Culex pipiens form molestus, from the Latin molestus meaning “annoying” – loves people and reproduces well underground.
The second form was previously considered the one that could adapt to life in London’s underground stations.
“This theory was truly made famous by a genetic study published in 1999, which, in my view, was based on limited evidence, and asserted that the ‘London Underground mosquito’ allegedly evolved in situ from an above-ground population.”
– Yuki Haba
Results of the study and its context
In the work of Haba and his colleagues, genetic sequences of hundreds of mosquitoes from different corners of the world were analyzed, including samples from the World War II era. The conclusion indicates that molestus did not rapidly evolve underground in London, but has a significantly older origin.
“This predates the London Underground by a long way, and it seems to have evolved near the Mediterranean region, particularly in the Near East,” said Yuki Haba, adding that the divergence between the terrestrial pipiens and the subterranean molestus could have arisen as far back as 10,000 years ago, but most likely between 3,000 and 2,000 years ago.
“Evolutionary analysis indicates that those ancestral molestus populations were above-ground, – added he, – and they gradually spread around the world, including to the London Underground.”
Methods of study and context of the data
The team began rigorously testing the ‘London Underground mosquito’ hypothesis in 2018. “We literally searched for Culex pipiens using Google and sent letters to every author of every publication we found about this mosquito, saying that we needed samples to understand the origin and genetic diversity of the species,” said Haba.
Over the years thousands of emails and samples were collected: dead mosquitoes preserved in ethanol, from over 200 sources in 50 countries. Live specimens from the London Underground could not be obtained due to a refusal to grant permission to collect specifically in the tunnels. Instead, historical specimens preserved at the Natural History Museum in London and analyzed at the Wellcome Sanger Institute were used.
In total, 357 modern mosquitoes and 22 historical samples were analyzed, and thanks to additional samples from another study the total number rose to about 800.
“Our data indicate that molestus descends directly from pipiens populations that still thrive in the Mediterranean region.”
– Lindi McBride
“This finding suggests that molestus evolved at Mediterranean latitudes, but more likely in the Near East, where the climate is drier for the avian-feeding variant,” said McBride, adding that the gap between underground and above-ground populations occurred long before they appeared in other parts of the world.
During roughly the same period, regional people began forming agricultural communities with irrigation systems, which created conditions for the mosquito to reproduce and allowed it to colonize arid regions, and later adapt to humans.
Evidence is supported by the fact that molestus was first described as a species in 1775 in Egypt by naturalist Peter Forsskål. “Probably he was there at least a thousand years before that,” McBride noted. “Then it was documented in southern Europe in two places, in Croatia and Italy, in the 1800s, and the first mentions from underground structures in Northern Europe date to around 1920.”
These lines of inquiry hint that molestus could have travelled north and, encountering a climate that would not allow survival in open environments, sought shelter underground. McBride noted that they did not withstand cold winters, so they largely stayed in southern Europe, where underground structures for winter later appeared.
There are more than 3,000 known species of mosquitoes that now inhabit almost all continents, with the exception of Antarctica. Recently in Iceland a mosquito was found that had previously been considered unlikely due to the country’s cold.
“We interpreted these results as evidence that some above-ground populations had previously adapted to the London Underground system and became reproductively isolated from others.”
– Richard Nichols
“This is an exciting and comprehensive study of the evolution of this globally significant mosquito,” commented Cameron Webb, associate professor of medical entomology at the University of Sydney and NSW Health Pathology in Australia. “Although it is often depicted as adapted specifically to the London Underground, the mosquito is actually well known for its underground ecosystems around the world,” he added in a letter.
“The London Underground mosquito underscores the need to better understand the biology of less-studied mosquitoes to assess how they might exploit a changing urban landscape and the related health challenges for populations”
– Cameron Webb
This study underscores the need to consider global sources of mosquito evolution in the context of urbanization and climate change, as our understanding of their biology helps better assess risks to public health in modern cities.