When their ranges overlap—such as in Florida, where both are invasive—the Asian tiger mosquito (Aedes albopictus, left) and the yellowfever mosquito (Aedes aegypti, right) mate but produce no offspring, in an event known as satyrization. They can evolve over the course of a few generations to learn to avoid each other, but that choosiness may cost the mosquitoes in other ways. New studies by researchers at the University of Florida and Brazil’s Oswaldo Cruz Institute shed more light on evolved resistance to satyrization and its potential costs. (Photo credits: James Gathany, CDC Public Health Image Library)

By Meredith Swett Walker

It’s a romantic comedy trope: Heroine on vacation in far flung land flirts with mismatched love interest, then hilarity ensues. And it turns out humans aren’t the only ones. Some mosquitoes also make poor romantic choices when in new environments. But the consequences for mosquitoes are more serious than hilarious.

Meredith Swett Walker

While the yellowfever mosquito (Aedes aegypti) is native to Africa and the Asian tiger mosquito (Aedes albopictus) hails from southeast Asia, the mosquito species do have a lot in common. Both are proficient vectors of diseases like dengue and Zika, and both sport striking black and white markings and like to breed in small pockets of water. They may be the two most invasive species of mosquito in the world, and when they meet up something interesting happens.

Where their ranges newly overlap, Ae. aegypti and Ae. albopictus sometimes mate with each other. But these interspecific couplings don’t result in hybridization but rather “satyrization,” and it’s not the comedic kind. Satyrization occurs when two different species mate but no offspring are produced, taking a toll on at least one of the species’ reproductive success.

Satyrization is definitely not funny for Ae. aegypti females. If they mate with an Ae. albopictus male, the Ae. aegypti females become “refractory” or unwilling to mate again—even if they have the opportunity to mate with a male of their own species. In effect, they have been sterilized. In the reverse situation, Ae. albopictus females who mate with Ae. aegypti males are still willing to mate again, so they don’t pay such a high price. This may help Ae. albopictus populations displace Ae. aegypti populations, at least temporarily.

This is what happened in Florida in the 1980s when populations of invasive Ae. aegypti began to decline soon after the subsequent invasion of Ae. albopictus. But, with their large populations and short generation times, mosquitoes can evolve relatively quickly. Researchers studying this phenomena at the University of Florida found that the mosquitoes can evolve resistance to satyrization in just a few generations; essentially, they become more choosy about their mates and less likely to mate with the wrong species.

Doomed lovers Aedes aegypti (left) and Aedes albopictus (right). Some interspecies mating leads to hybridization, but when these two mosquito species mingle, their mating produces no offspring—an effect called “satyrization.” (Image credit: Irka Bargielowski, Ph.D.)

But does this pickiness have a price? In a new study published today in the Journal of Medical Entomology, UF researchers led Irka Bargielowski, Ph.D., along with colleagues at Brazil’s Oswaldo Cruz Institute, look at the flip side of the evolution of resistance to satyrization. The paper is part of a new special collection of research on mosquito ecology and evolutionary biology in the Journal of Medical Entomology, in honor of retiring UF Distinguished Professor L. Philip Lounibos, Ph.D.

Bargielowski and colleagues sought to answer: If these mosquitoes’ interspecific mating is no longer a threat, does choosiness persist in a population? And, on an individual level, can you afford to be picky if your biological clock is running out?

In their first experiment, the researchers essentially allowed resistance to satyrization to evolve in multiple lab populations of mosquitoes. The researchers started these populations using mosquitoes collected from areas where the two species did not overlap and there was no threat of satyrization. Over six generations of exposure to equal numbers of Ae. albopictus and Ae. aegypti males, the rate of interspecific mating in Ae. aegypti went from over 50 percent to just over 10 percent.

The female Ae. aegypti evolved to be more discriminating when choosing mates. But how exactly do mosquitoes decide if a potential mate is the correct species? “Wouldn’t we all like to know?” says Bargielowski. “There is evidence suggesting wing-beat frequency plays a large role in mate recognition in Ae. aegypti and Ae. albopictus.” But the species recognition mechanism in these populations is still unclear.

Then the researchers observed what happened when Ae. aegypti females from these satyrization-resistant populations were only exposed to Ae. aegypti males. In other words, the threat of mistakenly mating with the wrong species was removed. In just eight generations, female Ae. aegypti became less choosy and the rates of interspecific mating in the populations increased rapidly, frequently almost tripling.

This study is part of a new special collection of research on mosquito ecology and evolutionary biology in the Journal of Medical Entomology, in honor of retiring UF Distinguished Professor L. Philip Lounibos, Ph.D. Read the Mosquito Ecology and Evolutionary Biology Collection.

Bargielowski and co-authors say this suggests that resistance to satyrization, or choosiness, is costly. Exactly how it costs a mosquito to be choosy is unknown, but previous research by this group shows that satyrization-resistant Ae. aegypti females are also slower to mate with males of their own species. And, for a short-lived creature like a mosquito, time may be of the essence.

How age affects choosiness was the subject of the researchers’ second experiment. Here they looked at whether young (3-4 days old) and old (15-16 days old) mosquitoes were equally choosy. They paired young and old (non-satyrization-resistant) Ae. aegypti females with young and old male Ae. albopictus. Older mosquitoes (both male and female) were more likely to engage in interspecific mating. The researchers suggest that older mosquitoes are less choosy because they are nearing the end of their lifespan and time is running out.

Understanding how resistance to satyrization evolves and how it is lost may help predict how populations of these two important vector species might interact in new areas that they invade. In addition, “the speed with which satyrization resistance has been shown to evolve does demonstrate the ability of mosquitoes to very quickly and seemingly effectively adapt their mating behavior in the face of evolutionary pressure to do so,” says Bargielowski. That’s a lesson mosquito control efforts are wise to keep in mind.

Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.