Origins of the London Underground mosquito revealed

A long-held myth about a mosquito evolving in the tunnels of London’s underground railway system has been debunked.

The insect, which became known as the London Underground mosquito, is known by scientists as Culex pipiens form molestus. Evolutionary biologists have long believed that it evolved from the bird-biting pipiens form in response to rapid human development, but doubts remained.

To get to the bottom of this mosquito mystery, scientists analysed DNA sequences from hundreds of specimens, including some that we look after. This allowed them to trace the evolutionary origin of the mosquito to between 1,000 and 10,000 years ago

The study, published in the journal Science, suggests that molestus probably evolved in Ancient Egypt, living above ground alongside humans in early agricultural societies of the Middle East.

Dr Erica McAlister, our Principal Curator and an author on the study, said that studying the mosquito’s past helps scientists to better understand its future.

“As we navigate through the planetary emergency, understanding how and why species adapt to urban environments is fundamental to predicting potential ecological changes and disease risk,” Erica says. “The data held in museum collections, like ours, presents huge potential to better understand the natural world and our relationship with it.”

Debunking the myth

The so-called London Underground mosquito gained notoriety during the Second World War as it would frequently feast on Londoners who were seeking shelter from German bombs in the railway tunnels.

It was so well adapted for living underground that people assumed it must have evolved there. As the London Underground has only been around since the nineteenth century, many believed this was an example of a species that had rapidly evolved to live in a new urban environment.

But biologists have long been sceptical of this theory, and, with the help of modern DNA sequencing technology, they've now been able to tell a very different story about the origin of this species.

Dr Mara Lawniczak, co-author on the study and Senior Group Leader at the Wellcome Sanger Institute, said: “Through the wonderfully rich historic insect collections at the Natural History Museum and the expertise of team member Petra Korlević, we were able to contribute genomic data from Culex specimens collected in London through the 1900s.”

“These genomic data from old specimens helped confirm that the notorious London Underground mosquito is a form that evolved long ago, way before underground human transportation existed at all.”

Viruses in modern cities

As well as helping to debunk a popular myth, researching the evolution of the molestus form is helping scientists better understand how diseases such as the West Nile virus are spread.

The West Nile virus belongs to the same family as dengue and yellow fever, which can have similarly severe impacts on human health. It is a virus that mainly infects birds but can be spread to humans when a mosquito bites an infected bird and then a human.

Mosquitoes are more likely to transfer viruses between species when they like to bite both types of host. Researchers think that breeding between human-biting molestus and bird-biting pipiens transfers genes that create these indiscriminate biters, and might have led to the increased spread of the virus to humans over the past two decades.

This hybridisation appears to be more likely to occur in larger cities, suggesting that people who live in urban areas may be more at risk of contracting West Nile virus.

Lindy McBride is an associate professor at Princeton University and was the lead author on this latest study.

“Our work provides new insight into how this mosquito varies genetically from place to place, insight that we think will help us better understand the role this species plays in transmitting West Nile virus from birds to humans,” Lindy says.

Scientists believe that this work could open the door to further investigation into the links between urbanisation, hybridisation and transmission of this virus from birds to humans.