By Abiba Biao
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From a fruit fly infestation that swarms your kitchen to the pesky mosquitos that swarm during the summer seeking blood, insects have held a constant vendetta against humans. Well, we would all like to put down our fly swatters and bug spray. The work of John Carlson may help us tame these pests once and for all.
Carlson is the Eugene Higgins Professor of Molecular, Cellular, and Developmental Biology in the Yale Faculty of Arts and Sciences. He studies the molecules, cells and circuits which control smell, taste, and pheromone recognition in fruit flies and mosquitoes.
While in graduate school at Stanford, Carlson grew increasingly interested in olfaction, or the sense of smell, amongst animals.
“Animals are incredibly sensitive to a lot of odors and can also distinguish a vast number of different odors. Almond smells different from peppermint, which smells different from banana, right?” he said. “So how is it that the olfactory system is able to detect and distinguish among minute quantities of odors?”
It was in graduate school that he was introduced to Drosophila, a type of fruit fly, and he thought it would be a great model for studying olfaction. After earning his Ph.D. in Biochemistry in 1982, Carlson continued as a Postdoctoral Fellow at Stanford and kept thinking about olfaction.
Carlson noted that there were vanishingly few studies or labs specializing in Drosophila olfaction at the time, with most labs focusing on the visual system. Wanting to continue his quest in pursuing olfaction research in Drosophila, he located a lab in India, the only lab he could then find that was researching Drosophila olfaction. He visited that lab before returning to the US and beginning an assistant professor position at Yale in 1986. It was on his first day at Yale that he finally began working on olfaction.
Carlson has been teaching and conducting research at Yale ever since. As he enjoyed teaching introductory biology courses, he was inspired to share his love of science with non-science majors and a larger audience in 2013.
“I realized that there’s so many senators and judges and CEOs and journalists who graduate from Yale,” he said. “A lot of them go on to be leaders in the country and I was thinking how wonderful it would be if they really had a good understanding of science and how it operates.”
That pedagogy served as the foundation for his course called “Biology, the World and Us” which covers basic principles of biology as well as the process by which scientists study the natural world. While the first time Carlson taught this class he had an enrollment of 39 students, in Fall 2023 he had over 200.
“I really enjoy doing it. I like meeting the students,” Carlson said. “It feels great when you tell them something you’re excited about, and then you see them get excited about it.”
A Tsetse fly in Mozambique (Judy Gallagher/Wikimedia Commons)
Since starting at Yale, Carlson’s laboratory has made several advances in the field, including the discovery of the first insect odor receptors, the discovery of the first insect taste receptors, and the elucidation of basic principles behind odor and taste recognition.
“We’re most interested in basic principles by which insects perceive all these odors and tastants out there, but a lot of the work ends up offering potential for controlling some terribly harmful insects,” he said.
In addition to Drosophila, he’s also done work relating to tsetse flies, a type of fly native to sub-Saharan African that spreads African trypanosomiasis, also known as African sleeping sickness, to humans, and nagana, a disease amongst cattle and livestock.
In an article published last February in Science Magazine, Carlson and his lab identified a volatile pheromone emitted by the tsetse fly called methyl palmitoleate (MPO).
“It’s a small molecule that these tsetse flies use to communicate with each other,” Carlson explained.
MPO works as a lure, attracting male tsetse flies and causing them to remain stationary for some time. It also has aphrodisiac properties that induce them to mate with female flies.
Currently, the most effective method of controlling tsetse fly populations is luring flies with odors from the animals they most prefer to feed on. MPO could yield a new ingredient for formulating these traps and controlling the spread of zoonotic diseases.
Top: Ph.D. graduate student Taylor Peterson and Postdoc Lisa Baik
Bottom: Mosquito eggs and larvae growing in the water
Another part of this research involves describing the chemosensory systems of mosquitoes and how they facilitate the transmission of global diseases, including the mosquito Aedes albopictus, which transmits many diseases.
Postdoc Lisa Baik and Ph.D. graduate student Taylor Peterson carry out research on mosquitoes that they breed in the lab. The mosquitoes are kept in a separate area for safety and in environmental control chambers to give them the right humidity and temperature to facilitate their incubation and growth.
“Mosquitoes don’t always bite humans and males don’t bite at all. They just feed on nectar,” Baik explained. “Females also feed on nectar… and they don’t have to bite humans or animals until they are ready to reproduce and form eggs which is why they draw blood. Our blood has a lot more nutrition and is protein-rich unlike nectar.”
While there is pressure for results-oriented research at times, Baik and Peterson have both learned how to be open-minded and embrace the research process, no matter how bumpy the journey is.
“I think one of the things John says about science is ‘every result is an exciting result,’ and even if it’s not what you hypothesized, you still found something and are gaining knowledge,” Peterson said.
It’s the interpersonal bonds that come with student mentorship that Carlson credits as a huge driving motivator of his work, with his lab consisting of a mix of undergraduates, graduate students, and postdoctoral fellows.
“I really enjoy working with students and postdocs and getting to know them and helping them develop as independent scientists,” he said. “They start off very enthusiastic, but they don’t have much experience and then as they spend time in the lab, you can help them gain experience. And by the time they’re done, they’re much better equipped to answer scientific problems on their own.”
Not only has he seen how he has impacted students in his lab, but in his classroom as well, getting emails and chatting with students that have taken his biology courses years ago.
Carlson is continuing his train of research by taking in three new graduate students and expanding projects to different organisms, including the invasive mosquito species Aedes albopictus, which is now in Connecticut and out-competing native mosquito species. One goal is to understand how this mosquito has adapted to handle the cold.
Additionally, Carlson’s lab is now studying how animals will react to the higher carbon dioxide concentrations that accompany climate change. He hopes that Drosophila will be a good model organism, with results that can be generalizable to other animals.
“It’s a fascinating problem, how animals can detect and identify and respond to this vast number of different chemical compounds in the real world.”