Grass Lab

By Kelsey Calhoun

The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka” but “That’s funny…”
—Isaac Asimov (1920–1992)

The process of science is rarely predictable: there are some 180s, some hard left turns, and quite a few long and winding roads. Graduate student Drew Friedmann can attest to this fact: a year and a half ago he was pursuing a completely different research topic and getting nowhere. But it was at the end of some long and frustrating months that he uttered some of the most exciting words you can hear from a scientist: “That’s funny,” or more specifically in this case, “Zebrafish don’t see with their tails.”

Primary motor neurons fluorescing in the young zebrafish, home to the unexpected VALopA Photo cred: D. Friedmann & Isacoff Lab

Primary motor neurons fluorescing in the young zebrafish, home to the unexpected VALopA. Photo credit: D. Friedmann & Isacoff Lab

Friedmann, a 2015 Grass Fellow at the MBL, had originally set out to study what controls the movement of zebrafish. These two-inch-long fish are widely studied, partly because they are transparent when young, making it easy to track their development. Ehud Isacoff’s lab at University of California-Berkeley, where Friedmann is a graduate student, has mapped the flow of calcium—a proxy for neuronal activity—in the nerve cells of developing zebrafish as they move.

Taking this further, Friedmann hoped to focus on how these young fish manage to move by looking at their gap junctions, the direct connections between cells which help them talk to one other. But there are over 30 different types of building blocks, called connexins, that make up these gap junctions, and few clues as to which ones help control movement as the zebrafish develop. Friedmann spent a long, frustrating year tackling this question with different tools and methods, without getting many interesting results.


One day, he tried genetics. Analyzing only the neurons controlling movement in zebrafish tails—the most motile part of the fish—yielded a long list of active genes. One gene on the list, VALopA, caught Friedmann’s eye, because it codes for an opsin, a light-sensitive protein usually only found in eyes. “I went, what is that doing here?” Friedmann remembered. “There are no eyes in the sample!”

The “that’s funny” moment seemed odd enough to merit a little digging. “I’ll just flash some lights and see what happens,” Friedmann thought. The Isacoff lab uses a plethora of microscopes to track the flow of calcium around zebrafish, including a bright green laser. “I was expecting to flash the laser and see a calcium event,” Friedmann explained. But no such luck—light was not stimulating the neurons, embedded with light-sensitive proteins, to fire. Frustrated, Friedmann tried for a while longer, and finally noticed something else odd. If these neurons really didn’t respond to light at all, there should have been a random calcium spike or two right after a light flash, but there wasn’t. There was never a spike after a light flash; instead, the light was actually inhibiting the neurons from firing.

Motor neurons innervating the zebrafish tail Photo credit: D. Friedmann & Isacoff Lab

Motor neurons innervating the zebrafish tail
Photo credit: D. Friedmann & Isacoff Lab

This presents a whole new unexpected puzzle: Why are light-sensitive, movement-controlling neurons inhibited by light? “This opens up two, maybe three big questions,” Friedmann says. “One is how, one is why, and one is how common is this?” Zebrafish always lay their eggs at sunrise, so their development may be affected by light and movement, making it evolutionarily advantageous for the two to be linked. Friedmann’s goal this summer at MBL is to figure out what other cells and systems these light-sensitive neurons are connected to, and trace the full circuit involved in light response. He’s supported at the MBL by a Grass Foundation fellowship, which are given to early-career scientists to carry out independent, investigator-designed projects. The Grass Lab at MBL provides space, cutting-edge equipment, supplies, and housing, so young scientists can spend a summer dedicated to experimentation.

The ability to detect light evolved before eyes, Friedmann explains, and when eyes did evolve, there was no reason to get rid of the old way of sensing light, especially for transparent creatures like zebrafish. These light-sensitive neurons may be heavily involved in healthy zebrafish development and behavior, paving Friedmann’s winding road with all sorts of interesting questions.

A closeup of an Aedes aegypti mosquito biting its host. Photo credit: Alex Wild,

The yellow fever mosquito,  Aedes aegypti, biting its prey. Photo credit: Alex Wild,

By Laurel Hamers

It’s a question asked by many a summer stargazer: How do mosquitoes home in on their human prey, turning a relaxing evening into an itchy disaster?

Meg Younger, an MBL Grass Fellow and a postdoctoral scientist at Rockefeller University, is trying to find out by looking at mosquitoes’ neural responses to different combinations of odors.

Behavioral studies have identified several cues that are mildly attractive to mosquitoes: carbon dioxide, heat, and lactic acid, a component of sweat. Presented alone, none of these cues is particularly powerful; when paired together, however, their effects multiply.

“What we don’t know yet is how these stimuli that are ignored or only mildly attractive are transformed into very attractive stimuli in the brain when presented simultaneously,” Younger says.

Younger is using electrophysiology and calcium imaging to monitor olfactory neurons, looking for differences in brain activity when mosquitoes are presented with certain stimuli alone or in different combinations.

She is carrying out her research in the yellow fever mosquito, Aedes aegypti, which is found in tropical and subtropical areas and is the also the major vector for dengue fever and chikungunya.

“The more we know about how mosquitoes process different stimuli to find humans, the more potential we have to come up with creative ways to stop them from biting people and spreading diseases,” Younger says.