Bioluminescence sheds light on the developmental origin of novel organs: An interview with Lisa Mesrop

5/7/2025

Most of us know about fireflies, but did you know that some of their distant relatives live underwater?

Lisa Mesrop, a 2023 Society for Developmental Biology Emerging Research Organism Grant awardee, studies exactly that–bioluminescent ostracod Vargula tsujii, colloquially known as “sea fireflies”. A 6th-year Ph.D. student at the University of California, Santa Barbara, Mesrop is co-supervised by Todd Oakley and Stephen Proulx in UCSB’s Ecology, Evolution, and Marine Biology department. The SDB grant supports her thesis work in establishing V. tsujii as a model system to study the developmental genetics of novelty.

What are ostracods? At first glance, ostracods look like krill pancaked within round translucent disks. And that is not far from the truth–the flat bodies of ostracods are usually protected inside a bivalve-like “shell” made of calcium carbonate.

“Most ostracods range between one to two millimeters in size, like the size of a sesame seed. They’re super small,” Mesrop said.

Previously, the Oakley lab successfully established a laboratory culture of the luminous ostracod V. tsujii, allowing anyone, for the first time, to study the complete life cycle of a bioluminescent marine animal from embryogenesis to adulthood and mating. The foundational paper that described this work opened doors for many, including Mesrop, who is interested in the evolution and development of a novel bioluminescent secretory gland located in the upper lip of V. tsujii.

“Bioluminescence has convergently evolved many times across marine and terrestrial systems, and there are a lot of adaptive functions of bioluminescence,” Mesrop said.

Within the family Cypridinidae, ostracods first evolved bioluminescence to evade predation. For example, if a fish attempts to swallow a bioluminescent ostracod, the ostracod spews out bioluminescent mucus in response to almost being eaten, triggering the fish to release the ostracod in response to the bright flash of light. However, through gene expression analyses, Mesrop and the Oakley lab recently reported that the bioluminescent upper lip co-expresses putative toxin-like genes with its novel light-emitting enzyme-substrate partner, c-luciferases and vargulin.

“We think that maybe in addition to the fish being startled by the bright flash of light, the mucus itself has toxic components that acts like an allergen, that irritates instantly the mouth of the fish causing them to spit out the ostracod,” Mesrop said. “The ancestral function of the mucus may have had toxic properties to it, but evolved bioluminescent capabilities later on.”

To collect wild-type bioluminescent ostracods for future experiments, Mesrop and the lab often make excursions to Santa Catalina Island, where the University of Southern California's Wrigley Marine Science Center (WMSC) is located. She and the lab lay out the baits—PVC pipes loaded with chicken liver and fish—near the docks of WMSC and return two hours later to collect V. tsujii that come by to scavenge.

“It's also a great time to bring out undergrads too, because they get exposed to field work, to conversations about evolution of bioluminescence, which gets them excited about science and to learn,” Mesrop said.

As her training primarily touched on evolutionary biology, Mesrop took it upon herself to stay on top of recent discoveries in developmental biology and became a member of SDB in 2021. Hearing about the ERO grant through SDB’s Twitter account, Mesrop proposed using the grant to generate a transcriptomic profile of the developmental stages of V. tsujii and establish developmental cellular lineages of luminous cell types in the novel light organ.

Lisa Mesrop with her colleagues Sofia Diaz and Christopher Bui, Masters’ students at California State University, Los Angeles, having collected ostracods from Santa Catalina Island. (Credit: Dr. Lisa Torres)

“A little part of it is to gain independence in my project, but also, the grant will really help make V. tsujii an accessible model system to study novelty,” Mesrop said.

Using the developmental stage-resolved transcriptomic data, Mesrop hopes to identify gene expression patterns that are correlated to the appearance of certain morphological structures in V. tsujii during development and validate their expression using immunostaining and microscopy.

Mesrop’s interest in developmental innovations today traces back to her general curiosity about the biodiversity around her. “I didn’t have parents who spoke English or who were from the U.S.—my mom came from a little village in Lebanon, and my father from Iraq, so I had to figure out the roadmap of science myself,” she said.

In college, she picked up foundational molecular wet-lab techniques working with sea urchins and cnidarians in Amro Hamdoun’s lab at the University of California, San Diego. There, she became interested in how novel traits evolved across different species, and what changes at the genetic and developmental level shaped these phenotypes.

“Dr. Hamdoun and my mentors in his lab pushed me to think about these things in a critical way, which was really important, and he made science fun which I think is an important aspect to the success of an individual,” she said.

“I had the opportunity to drive my own questions, learn new techniques, design projects and write. So it ended up being a wonderful experience for me,” she said. This motivated her to return to the topic of evolution and development of novel traits in her Ph.D.

Like in any research work, there are hurdles to overcome. While transcriptomes across different developmental stages were successfully assembled, V. tsujii has an enormous genome with high heterozygosity and repeats, making assembly of its genome difficult.

Secondly, to better study embryogenesis in V. tsujii, Mesrop and the lab are working to grow ostracod embryos outside their brooding chamber, a space in the rear of their shell. This would facilitate more physical and genetic manipulations in the future, such as generating transgenic ostracods.

Mesrop is grateful and happy to have received the ERO grant. “I used to constantly feel like I didn’t quite fit into the typical academic culture. I didn’t know how to do science, how to think scientifically, or how to write science. But don’t be afraid to be yourself. With the right support and by believing in yourself, you can make it—and don’t let anyone convince you otherwise,” she said.

As Mesrop heads towards the finishing line of her Ph.D., she hopes to stay in the realm of studying arthropods and evolutionary novelties and innovations in her postdoctoral position. In addition to elucidating gene co-regulatory networks that specifies and differentiates the novel bioluminescent upper lip, she also hopes to understand the genetic basis of novel sensory systems such as compound eyes in arthropods.

“The SDB grant not only helped me for my Ph.D. thesis work, but it builds the foundation for my future lab,” Mesrop said. “I think if people are a little worried to apply for the grant because they don't think they have anything for their organismal system, well that’s not true. You should apply because you have to start from somewhere.”

Mesrop believes that emerging organisms hold the keys to explain the evolution of complexity and biodiversity of life. “If we just focus on a few systems, we cannot get the full picture of how genetic, cellular, developmental and evolutionary processes across different systems give rise to the rich complexity and biodiversity on earth,” she said.

She hopes that there will be more funding like the SDB’s ERO grant to help put more organisms on the map for studying evolutionary patterns and processes that drive the development of novel traits.

Last Updated 05/07/2025