Researchers trace the ancient origins of an urban mosquito species

New research has uncovered the ancient origins of an urban mosquito species, Culex pipiens form molestus, also known as the 'London Underground mosquito' – disproving a long-held theory of when it first evolved.

Published today (23 October) in Science, researchers from Princeton University, the Wellcome Sanger Institute, the Natural History Museum in London and their collaborators extracted and analyzed hundreds of mosquito DNA samples to trace the emergence of the urban insect species.

The result of the study revises one of the 'textbook examples' of urban evolution and adaptation, and it carries important public health implications regarding the spread of diseases.

Evolutionary biologists have long believed that the human-biting mosquito, Culex pipiens form molestus, evolved from the bird-biting form, Culex pipiens form pipiens, in subways and cellars in northern Europe over the past 200 years. It is held up as an example of a species' ability to rapidly adapt to new environments and urbanization. The evolution of the molestus form also has significant implications for how viruses are spread.

The West Nile virus is an avian virus that can spread to humans when a mosquito first bites an infected bird and then bites a human. This type of 'spillover' is most likely when mosquitoes like to bite both types of hosts. Mosquito biologists think that the transfer of genes from human-biting molestus into bird-biting pipiens through hybridization creates such indiscriminate biters and has led to increased transmission of the virus to humans over the past two decades.

In a new study led by Princeton University, researchers sought to better understand where and when hybridisation of the molestus and pipiens mosquitoes first occurred, and they did this by studying both forms of the mosquito.

The researchers worked with roughly 150 organisations from around the world to collect 12,000 samples of Culex pipiens that represent both geographic and genetic diversity. They then extracted, sequenced and analysed the DNA from 800 of these samples to look at the genetic evolution of the mosquitoes.

The team disproved the theory that Culex pipiens form molestus evolved in Europe over the last 200 years, tracing the origins of the molestus mosquito to more than 1,000 years ago in the Mediterranean or Middle East. They also found that hybridisation is much less common than previously thought but it does occur, especially in large cities, suggesting urbanisation may promote genetic mixture of the two forms. Therefore, the working hypothesis is that people in big cities could be at more risk for West Nile virus.

The researchers say that mosquito genetics and biting behavior need to be studied further. More sampling in urban and rural areas will allow further insight into the evolution of urban mosquitoes and how this relates to disease spread in humans.

Through the wonderfully rich historic insect collections at the Natural History Museum London 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."

Dr. Mara Lawniczak, co-author on the work and Senior Group Leader at the Wellcome Sanger Institute

Dr. Yuki Haba, first author and postdoctoral researcher at Colombia University, said: "Our analyses strongly suggest that molestus first evolved to bite and live alongside humans in an early agricultural society 1,000 to 10,000 years ago, most likely in Ancient Egypt. Our work opens the door to incisive investigation of the potential links between urbanisation, hybridisation, and spillover of the virus from birds to humans."

Lindy McBride, senior author and Associate Professor of Ecology and Evolutionary Biology and Neuroscience at Princeton University, said: "This enigmatic mosquito became famous during World War II in London and seemed so perfectly adapted to living underground that people thought it must have evolved there. It became a textbook example of rapid evolution in modern cities. But our analysis of DNA sequences from hundreds of mosquitoes tells a very different story. 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."