Leslie Vosshall and Leah Houri-Zeevi (Credit: Lori Chertoff)
The female mosquito only mates once in her lifetime, and yet she can develop many hundreds of eggs from this single event. After each blood feeding, she draws from an internal sperm storage unit to inseminate her eggs, then lays them (about 150 in each batch) and repeats this process every few days.
This feeding-and-breeding protocol isn't just a spectacularly efficient means of reproduction. It also gives nasty viruses carried by infected mosquitoes countless opportunities to infect new hosts-and that makes understanding the mechanics of the single mosquito mating a logical first step in figuring out how to intervene with mosquito reproduction and control the diseases they can spread. (Indeed, mating-based mosquito birth control is already in use-for example, releasing sterile male mosquitoes into an environment.)
But as a new study from a team led by Rockefeller University and Howard Hughes Medical Institute mosquito expert Leslie Vosshall reveals, it turns out that there's a lot about the earliest steps of mosquito reproduction that we were missing. As her team in the Laboratory of Neurogenetics and Behavior recently published in Current Biology, they've uncovered the first evidence of what happens when a female mosquito chooses to mate for the only time in her life: She very subtly moves her genitalia, and that enables genital interlocking and sperm transfer. This places her firmly in control of copulation-a discovery that is contrary to long-standing assumptions in the field.
We spoke to Vosshall and first author Leah Houri-Zeevi, a postdoctoral scientist in the lab, about their unexpected findings.
For more than half a century, scientists worked off the assumption that male mosquitoes controlled mating. How did you figure out that the females were, in fact, in charge?
Leah Houri-Zeevi: It wasn't easy; a mating between these tiny animals only lasts for 14 seconds, and the phase that includes our key finding takes only 1-2 seconds. So, there were definitely technical challenges that help explain why we were the first to figure this out, which we did by using high-speed, high-resolution imaging that we slowed down and analyzed frame by frame, as well as deep learning and transgenic mosquitoes with fluorescent sperm.
But I think there's more to it than that. There are very detailed mosquito mating studies going back to the 1950s that used flash freezing and filming under microscopes to understand this process. However, their interpretation of the observations skipped potential female control, maybe because they didn't expect the female to do any decision making.
Leslie Vosshall: It's really profound that the field assumed for so long that the female must be passive. This dogma has stood for many years despite the fact that it rests on a contradiction: that females mate only once but are also considered to have no choice about when or with whom or how many times. If they had no choice, many males would mate with a single female all the time. And it's not for lack of trying. They constantly pester females.
Sometimes you need to pick apart an accepted dogma to see if there's actually evidence to back it up. In this case, there wasn't.
That's surprising, considering that knowing how an animal reproduces is core information a biologist needs. What did you learn?
LHZ: What we found was so fascinating. The first big finding is that in both species, the female controls the mating with a subtle movement of her genitalia. There's actually a three-step process to this that begins when the male contacts the female's genitalia with his own genital tip. He has these structures called gonostyli that he inserts into the female genital tip and vibrates. Those are critical to the process. If she wants to mate, she'll elongate her genital tip, which the male can then interlock with and then transfer sperm into her.
We also saw that when previously mated females pair up with a male, no elongation happens. So this subtle female behavior really dictates whether mating will happen. We still don't understand why she only elongates the tip once, or what mechanisms change her behavior for a lifetime.
Historically, most of the efforts to study mosquitoes focused on the species that transmits malaria. But your work instead looked at two other species. Why did you choose those particular insects?
LV: Aedes aegypti is the primary mosquito we've studied in my lab for years. It's commonly known as the yellow fever mosquito, but it transmits more viruses than any other mosquito, including dengue, West Nile, and dozens more. Aedes albopictus, the Asian tiger mosquito, also spreads these viruses. They're also two of the most invasive mosquito species in the world.
LHZ: Both species are also really common in the U.S. Aedes aegypti has been here a lot longer, coming to this hemisphere in the 1600s with the Atlantic Slave Trade, but it's being outcompeted in some regions by Aedes albopictus, even though it's only been here since the 1980s. Our study shows that their mating practices likely have something to do with its success.
Interesting-how so?
LHZ: We found that while the mating process was essentially the same in both types of mosquitoes, each species had its own unique "lock-and-key" genital interaction. Only the right key opens the right lock. But the male Asian tiger mosquitoes have really large gonostyli, and we discovered that they can use them to override the lock-and-key mechanism across species, but not with their own females, who seem to be protected against this "lock picking" ability.
LV: Aedes albopictus take advantage of their own physiology to bypass female control in the other species. But they're too far apart evolutionarily to produce viable offspring, so it's a non-starter.
LHZ: And what's more is that this cross species mating still prevents the female from mating again, even though she gets no viable progeny out of it. The result is that she's effectively been sterilized. If this cycle is repeated enough times, the Aedes aegypti population will drop or even disappear, which has been observed in South Florida.
