A number of years ago there was quite a lot of media attention regarding a possible case of in situ evolution of Culex pipiens form molestus in the London Underground metro system. This media buzz created speculation that this was a unique species that had risen during the over 160 year long existence of the London Underground (Byrne K & Nichols RA, 1999; Cooper Q, 2012). This observation, by Byrne and Nichols (1999), has since the subject of rather intense discussion. Is the underground dwelling Cx. molestus* merely an eco-type of it’s above ground living compatriot Cx. pipiens* or has it truly risen to become it’s own species?
Historical Debate
Culex molestus was first described in Egypt by Forskål in 1775. During the 20th century, it was mostly described as an ecotype of Cx. pipiens and was recognised as such, until Miles (1977) declared it a separate species based on observation made regarding pre-mating isolation behavior. Later articles by Harbach et al. (1984) however refuted this notion, due the Egyptian specimens of Cx. molestus being indiscernible from Swedish samples of an ecotype of Cx. pipiens. Since then, it’s status as an eco-type form of Cx. pipiens has remained as such. However, discussion concerning the species or sub-species status of Cx. molestus is still a topic of heavy debate (Harbach et al., 1984; Harbach RE, 2012).
Does the evidence support speciation?
Evidence in support of true speciation mainly consists of behavioural and ecological differences, as the morphology of these two forms has too much individual variation to consider as evidence (Harbach et al., 1984). As previously stated, Cx. molestus resides mainly resides within man-made underground habitats, whilst Cx. pipiens resides above ground. Furthermore, Cx. molestus is able to reproduce constantly due to lack of a reproductive diapause which Cx. pipiens is subject to. Cx. pipiens also has a clear preference for avian blood hosts, whereas Cx. molestus has a more diverse pallet, perhaps even a preference for mammalian and/or human hosts. The latter is likely determined by simple availability, as underground man-made structures mostly contain mammals, such as small rodents and humans. Cx. pipiens tends to mate in swarms congregating in large open spaces, named eurygamy. In contrast, Cx. molestus mates within enclosed spaces, named stenogamy. Stenogamy and the use of mammalian hosts may be adaptation to living in close association with humans (Haba Y & McBride L, 2022).
The ability to lay eggs without requiring blood from a host, termed autogeny, together with the two previously mentioned adaptations, is possibly why Cx. molestus has been able to colonise environments such as Siberia, with it’s average annual temperature of −5 °C, even though it appears this mosquito is better suited to warmer climates (Harbach et al., 1984). Differences in mating behaviour and habitat may be drivers for reproductive isolation, although genetic analysis has shown that the two forms of Cx. pipiens can often be found interbreeding.
Mono- or polyphyletic?
Another subject of debate is whether if this form or species has evolved repeatedly within different locales containing a similar environment or if it has evolved within a singular location and then spread from there through human association. According to Aardema et al. (2021), recent phylogenetic studies indicate that although local evolution of Cx. pipiens into Cx. molestus does occur, it is the exception to the trend. This means that most likely Cx. molestus arose from a single location and spread it’s wings from there, using human association to it’s benefit. Another paper also found little to no supporting evidence regarding the in situ evolution of Cx. molestus (Haba Y & McBride L, 2022).
Conclusions
All in all, there is currently no compelling evidence that sets the two discussed forms of Culex pipiens apart as separate species or even sub-species. More investigation into genomic diversion may be needed in order to provide strong evidence regarding such a thing. However, as this is a blog post, this author is curious as to what the audience thinks regarding this issue. With the currently available evidence, what do you think? Can it be said whether Cx. pipiens and Cx. molestus truly are separate species or not?
*Both Cx. molestus and Cx. pipiens are actually forms of the Culex pipiens species named Cx. pipiens form molestus and Cx. pipiens form pipiens respectively. However for clarity and ease of reading, especially whilst introducing the subject of this blog, I have decided to shorten their names to just Cx. pipiens and Cx. molestus.
References
Aardema, M. L., Olatunji, S. K., & Fonseca, D. M. (2022). The enigmatic Culex pipiens (Diptera: Culicidae) species complex: phylogenetic challenges and opportunities from a notoriously tricky mosquito group. Annals of the Entomological Society of America, 115(1), 95-104.
Byrne, K., & Nichols, R. A. (1999). Culex pipiens in London Underground tunnels: differentiation between surface and subterranean populations. Heredity, 82(1), 7-15.
Cooper, Q. (2012, May 23). Subway evolution: What lurks below. https://www.bbc.com/future/article/20120523-what-lurks-below
Haba, Y., & McBride, L. (2022). Origin and status of Culex pipiens mosquito ecotypes. Current Biology, 32(5), R237-R246.
Harbach, R. E., Harrison, B. A., & Gad, A. M. (1984). Culex (Culex) molestus Forskal (Diptera: Culicidae): neotype designation, description, variation, and taxonomic status. Proc Entomol Soc Wash, 86(3), 521-542.
Harbach, R. E. (2012). Culex pipiens: species versus species complex–taxonomic history and perspective. Journal of the American Mosquito Control Association, 28(4s), 10-23.
Miles, S. J. (1977). Laboratory evidence for mate recognition behaviour in a member of the Culex pipiens complex (Diptera: Culicidae). Australian Journal of Zoology, 25(3), 491-498.
Featured photo: © Mick Talbot, some rights reserved
Leave a Reply