By Erin Weeks
In the 1980s, public health officials and entomologists noticed a curious sea change in Florida.
For centuries, the yellowfever mosquito (Aedes aegypti) had been one of the deadliest and most invasive disease transmitters in the United States. Borne by stowaway mosquitoes from Africa, yellow fever surged through eighteenth- and nineteenth-century American cities, killing thousands — and even changing the course of history when the disease struck Napoleon’s army in the Americas. The landmark development of a yellow fever vaccine in 1951 helped reduce the mosquito’s status as a public-health menace in the United States. Still, researchers were surprised when yellowfever mosquito populations in Florida began disappearing in the 1980s, just as the numbers of a similar species began to swell: the Asian tiger mosquito (Aedes albopictus).
“There was obviously some kind of link there, so for the next 20 years, people were looking at why this was happening,” said Irka Bargielowski, postdoctoral researcher at the University of Florida’s Medical Entomology Lab and member of a team that is finally piecing together part of the explanation.
One theory for which researchers soon found evidence was that Asian tiger mosquito larvae were better at competing for resources than their yellowfever counterparts. But another explanation proposed for the Asian tiger take-over was a fascinating theory of mistaken identity. A few scientists wondered if the formerly isolated yellowfever mosquitoes might not recognize the difference between the Asian tiger mosquito and their own species. When yellowfever males chased or mated with Asian tiger females, the thinking went, they wasted enough precious time, energy, and sperm to harm their population over time.
Twenty years passed before some evidence of this phenomenon was confirmed in the field. Now, Bargielowski and others in her lab at the University of Florida are zeroing in on this theory of mistaken identity, which they call “satyrization.” What they’ve found offers a lesson in the lightning-paced world of insect evolution. Their latest research, published in the Annals of the Entomological Society of America, suggests that while mosquitoes may at first be confused by foreign species, they can rapidly evolve adaptations to prevent mating mix-ups.
To see what happens when the two highly invasive species first make contact, Bargielowski and her colleagues set out to recreate the conditions of 1980s Florida in the lab. It’s an event that has played out many times across the globe in recent decades, as both species’ invasive ranges have expanded, shifted, and sometimes overlapped. The yellowfever and Asian tiger mosquitoes belong to the same genus, so while they share many similarities, they cannot successfully interbreed.
That doesn’t stop mosquitoes in the lab from trying, however, according to previous research. But similar studies found low levels of interspecies breeding where these insects occurred together in the wild. Bargielowski’s team wondered why.
The research team started by collecting yellowfever and Asian tiger mosquitoes from locations where each species had been living in isolation (Key West, Florida and an Illinois-based laboratory, respectively) and a location where the two had been living in co-occurrence for decades (Vero Beach, Florida). Mosquitoes from each of these colonies were reared in the lab for 3-5 generations. Next, the scientists conducted a series of cage experiments. In each of these tests, 25 male mosquitoes from the same colony were placed in a cage with 50 same-species females and 50 females of the other species. After 24 hours, the researchers removed all the females and determined 1) whether one of the males had successfully mated with each female, and 2) whether that male had been of her own species or not.
“We were expecting to see higher levels of [mating errors] in the populations that hadn’t come in contact before,” Bargielowski said.
That’s precisely what the team’s experiments showed. The lab found similar results in both male (present study) and females mosquitoes (previous study). The mosquitoes collected from areas where they lived in isolation were more likely, when placed together in the lab, to accidentally mate with the wrong species.
“In a sense they’re naive, more likely to make mistakes,” Bargielowski said. The individuals collected from locations where both yellowfever and Asian tiger mosquitoes occurred, however, were far less likely to chase after and mate with the wrong species.
Their work suggests that at some point after coming into contact, both Asian tiger and yellowfever mosquitoes develop the means to better distinguish between their own species and a foreign species. Just how fast do these adaptations evolve? The team has not yet published data on it, but Bargielowski thinks the process occurs over just a few generations.
“Very quickly after these populations first come into contact, because it’s so costly, [the percentage of mating errors] comes right down,” she said. Her team will soon publish evidence of this phenomenon in four countries across four continents: North America, South America, Africa, and Asia.
Bargielowski would like to know if the same reproductive changes take place in other mosquito species that must distinguish potential mates of their own species from a line-up of pretenders. The actual mechanisms involved in species recognition — how the mosquitoes learn to find the right mates — also remain to be explored. While her team’s work has opened up a new field of research questions, they’ve clearly shed light on why these insects have become such successful invaders — and such an intractable problem for humans.
As for the replacement in Florida of yellowfever mosquitoes by Asian tiger mosquitoes, it turns out that while both species fall victim to mating errors, the stakes are much higher for yellowfever females. Once they’ve been mated, they cannot mate or reproduce again — so it’s tough luck if that suitor happens to be a male of the wrong species.
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