You’ve probably had someone tell you, at some point in your life, that the sense of smell is the sense most tightly linked to memory. Now, scientists have found that at least for mosquitoes, the sense of smell is also linked to the ability of their sperm to swim. The research was published in February in the journal Proceedings of the National Academy of Sciences.
Female mosquitoes use their sense of smell to find people (so they can suck their blood) and suitable sites to lay their eggs. As you might suspect, the majority of the smell-sensing machinery in mosquitoes is in their head, but scientists have found molecules called odorant receptors, which are needed to detect volatiles (ie: smelly stuff), in other body parts. Researchers Laurence Zwiebel and colleagues studied the mosquito that transmits malaria, Anopheles gambiae, to find out what these other odorant receptors were doing. Specifically, they wanted to know if they were doing anything to the sperm, because apparently that’s how they roll.
Ok, it actually isn’t all that crazy to look for smell receptors in the testes. After all, mouse testicles can taste and many studies have been published that find olfactory receptors on the sperm of mammals, including dogs and humans. But whether they exist in mosquito testes, and whether they might be important for malaria research, was a mystery.
The scientists first noted that odorant receptors have been found in the testicles (or directly on sperm) in mammals, and that a previous study found that odorant receptors overall are made at higher levels in male A. gambiae compared to females. This led them to believe that odorant receptors could be present in mosquito testes, so they went ahead and had a look. They found more than 30 odorant receptors in the testes, 9 of which were present at very high levels. They also found that the partner protein of mosquito odorant receptors, called Orco, is present in sperm, suggesting that odorants might be important for the function of sperm.
To test if Orco or any of the odorant receptors actually did anything in the testes, the researchers pulled out the testes from male mosquitoes, ripped them open, and let the sperm swim free under a microscope with a video camera. They then tested several different odorants and chemicals that specifically activate Orco and looked at how fast the sperm beat their tails. 3 different odorants plus the two chemicals that activate Orco were able to accelerate the swimming sperm. In contrast, 2 chemicals that block Orco didn’t have an affect on sperm.
The researchers then grabbed some mutant mosquitoes of another species, Aedes aegypti, that don’t have any Orco protein. When they tested the normal Aedes aegypti males, their sperm responded to the chemicals in the same way as the Anopheles gambiae males. But when the mutants were used, the ones without Orco, there was no response to the chemicals. With all these experiments combined, the researchers could be pretty sure that Orco was acting with odorant receptors in the testes to control the speed of sperm swimming.
To see exactly how they scored sperm swimming (possible scores of 1,2 or 3), check out the video below:
[Mosquito sperm video from Pitts et al. 2014]
Orco mutant mosquitoes from the second species (A. aegypti) aren’t sterile, nor did the researchers expect them to be. In fact, they seem just fine, but keep in mind that these are fat, happy mosquitoes being raised in the lab. Under more stressful conditions (eg: your undrained kiddie pool), the effect of losing Orco might be larger. We also don’t know how Anopheles sperm would react to losing the Orco protein. Unfortunately, those mutant mosquitoes don’t (yet) exist. The next important step in this research would be to see if deleting Orco or blocking the odorant-sensing machinery in Anopheles testes could suppress fertility. Anopheles females only mate once, so need to rely on the sperm from a single male to reproduce. If the sperm from that male are defective, she’s outta luck. If we could find a way to keep sperm from getting the activation signal through their odorant receptors, we could potentially decrease the spread of malaria. Obviously, that’s a lot of ifs, but I’m sure that someone’s already working to see if it will work.
In the meantime, make sure to follow the sage wisdom of this wartime poster:
Pitts RJ, Liu C, Zhou X, Malpartida JC, & Zwiebel LJ (2014). Odorant receptor-mediated sperm activation in disease vector mosquitoes. Proceedings of the National Academy of Sciences of the United States of America, 111 (7), 2566-71 PMID: 24550284