London Underground Mosquito

The London Underground mosquito is a form of mosquito in the genus Culex found in the London Underground. Biologists named the London Underground mosquito Culex pipiens f. molestus due to its voracious biting. Notably, C. p. f. molestus assaulted Londoners sleeping in the Underground during the Blitz, although similar populations were already known. This mosquito, although first discovered in the London Underground system, has been found in underground systems around the world. It may have adapted to human-made underground systems since the last century from local above-ground Culex pipiens, although more recent evidence suggests it is a southern mosquito variety related to C. pipiens that has adapted to the warm underground spaces of northern cities.

The evidence for this mosquito being a different species from C. pipiens comes from research by Kate Byrne and Richard Nichols. The species have very different behaviours, are extremely difficult to mate, and with different allele frequencies consistent with genetic drift during a founder event. More specifically, this mosquito, C. p. f. molestus, breeds all-year round, is cold intolerant, and bites rats, mice, and humans, in contrast to the above-ground species, which is cold tolerant, hibernates in the winter, and bites only birds. When the two varieties were cross-bred, the eggs were infertile, suggesting reproductive isolation.

Since that paper, more information has accumulated. The fundamental results still stand: the genetic data indicate the molestus form in the London Underground appeared to have a common ancestry, rather than the population at each station being related to the nearest above-ground population. Byrne and Nichols' working hypothesis was that adaptation to the underground environment had occurred locally in London once only – many hurdles must be overcome to become adapted to the subterranean environment, and understandably it would occur rarely. This hypothesis implies that local adaptation would be expected in different locations around Europe and beyond, as each local population evolved an offshoot that overcame the problems of living underground.

However, more recently collected genetic evidence suggests a single C. p. molestus form has spread throughout Europe and beyond, since populations over a large area share a common genetic heritage. These widely separated populations are distinguished by very minor genetic differences, which suggest the underground form developed recently; a single mtDNA difference is shared among the underground populations of ten Russian cities, and a single fixed microsatellite difference occurs in populations spanning Europe, Japan, Australia, the Middle East, and Atlantic islands. This worldwide spread might have occurred after the last glaciations or be even more recent, due to the insects hitchhiking on world trade routes; one possibility is the international second-hand tire trade. The tires retain water in which the larvae can survive, and completely removing water from an old tire can be difficult.

The reproductive isolation in northern European populations appears to be almost complete, whereas gene flow between them clearly occurs in populations to the south. Byrne and Nichols attributed this pattern to the inability of the underground form (and perhaps their hybrids) to survive above ground in harsh northern winters.

Now, the persistence of hybrids in northern climates apparently may be another evolutionary problem that can be solved, but only rarely: the Fonseca paper obtained genetic evidence of the recent colonization of America by Culex mosquitoes actually involves a strain derived from a rare successful hybridization between C. pipiens and C. p. molestus. They suggest hybridization may explain why the American form bites both birds and humans (this interpretation is controversial, see letter from Spielman et al. and the response that follows it in Science). The consequences of this more indiscriminate feeding hit the news in 1999 with the outbreak of human encephalitis in New York, caused by West Nile virus. It was the first documented introduction of this virus into the Western Hemisphere; perhaps because in the longer established populations, the Old World northern above-ground C. pipiens almost exclusively bites birds, with the human-biting ones being incarcerated below ground.