Crawling towards the evolutionary origin of novel structures

5/7/2025

Heather Bruce’s childhood attempts to raise crawfish in a kiddie pool were excellent foreshadowing for her future work maintaining millipede colonies in the lab. Bruce, formerly a postdoc at Marine Biological Laboratory and a new faculty member at University of British Columbia was the recipient of a 2023 Society for Developmental Biology Emerging Research Organisms (ERO) grant. She received the grant to kickstart her work establishing Oxidus gracilis as the first tractable millipede model to study the origins of novel structures.

Growing up in California and Arizona, Bruce “wasn't a science nerd” and initially had plans to major in history or political science. A required science class at Yavapai Community College sealed her fate as a biologist and Bruce switched majors and became a biology teaching assistant. A birthday gift from one of the course managers—a copy of Endless Forms Most Beautiful by Sean B. Carroll—steered her towards evo-devo research.

“It was so amazing that I read it in two days and at the end of that, I was like, evo-devo is…what I want to do,” she said.

As an undergraduate at the University of Arizona, Bruce worked with Lisa Nagy to figure out how the snail got its shell. This was the first step towards a more focused interest in the origin of novel structures, anatomical features that lack homologs, or counterparts, within an organism and its ancestors. As she dove into the snail shell literature, Bruce was puzzled by the prevailing co-option model that these structures seemingly “pop out of nowhere” due to the misexpression of master regulatory genes in new tissues. She highlighted the theory’s logical shortcomings. “If you just put a master regulatory gene into a weird context…it's not going to have any of its normal cofactors to…do downstream network activation. It might not even have any of the open chromatin where it can sit and do anything.”

As a Ph.D. student with Nipam Patel at Berkeley, Bruce’s work on the origin of insect wings directly challenged this notion by showing that wings likely arose from a homolog present in crustacean ancestors¹. She developed a “molecular triangulation” approach to align arthropod legs, a contentious century-old open question in the field. By comparing the expression and function of leg patterning genes in the crustacean Parhyale hawaiensis to insects, Bruce found that their leg segments could be aligned one-to-one, with the base of the crustacean leg and its side lobe corresponding to the insect body wall and its wing. This suggested that insects incorporated the leg base into the body wall, where the ancestral side lobe later evolved into a wing. Further triangulation with the other arthropod groups established a framework to decipher the origins of exites, or “sticky outy things” as she describes them, in all arthropods, many of which have been considered novel structures.

This and work on other arthropod features raised the exciting possibility that many “novel” structures were actually homologs hiding in plain sight^2,3. Bruce postulated that we have yet to recognize many cryptically persistent features, perhaps because they’re unassuming or completely different than we expect. She explained that for several decades co-option has been the dominant scientific narrative, but scientists like herself have begun to question it as they gather more molecular evidence that points to ancestral origins. “Maybe it sounds…less glamorous and mysterious…but it's comforting because it makes more sense,” she said.

As a research associate in Patel’s lab at MBL, Bruce went on to tackle another vexing question for the evo-devo community, proposed by paleontologist Stephen Jay Gould in Wonderful Life. If you could replay the tape of evolution would you get the same outcomes? Bruce boiled this down to a question of “How constrained are biological systems?” Myriapods, the arthropod group that includes millipedes and centipedes, are a living example of replaying the tape. Anatomical similarities between myriapods and insects tricked scientists into believing that the two groups were closely related, but myriapods made it onto land 100 million years before insects. Bruce strongly suspects that their nearly identical respiratory tracheae originated as ancestral gills that became internalized in separate but similar processes. She hypothesized that transcription factors guiding this process are conserved, suggesting that biological systems are perhaps more constrained in response to similar environmental pressures. She was excited to see if comparing the genes in insect and myriapod trachea with ancestral-like crustacean gills supports homology among arthropod respiratory structures, as it ties back into her work on wings. She wondered if this points to a more general evolutionary principle. “It might be easier to…modify an ancestral structure…beyond all recognition than it is to evolve a completely new structure.”

Adult millipede, Oxidus gracilis (Credit: Iustin Cret in BugGuide licensed under CC BY-ND-NC 1.0)

To explore such questions, Bruce took on the challenging task of establishing the first tractable myriapod model, with assistance from ERO funding. She decided on millipedes due to their abundance of exites for future study, but the most studied millipede species is endemic to Europe and could not survive in the lab. A myriapod expert in Germany, Thomas Wesener, directed her to Oxidus gracilis, which can be found in greenhouses all over the world. Bruce initially struggled to find animals in the wild, before discovering them at a local garden store. Once the millipedes were in the lab, figuring out their ideal environment was difficult, and the colony was hit with multiple waves of nematode and mite infestations. Despite such trials and tribulations, Bruce plans on sequencing the Oxidus trancriptome and using the species in tandem with the insect Tribolium and the crustacean Parhyale to represent three out of four arthropod groups. These three workhorses will help her determine the evolutionary origin of the trachea and all other unique exites.

Bruce advised young scientists that they can strike scientific gold in unexpected places. She explained that her Ph.D. work on insect wings initially began as a deviation from her main project and she wasn’t sure that it would result in anything fruitful. Now she feels lucky that it answered some really old evolutionary questions. She encouraged chasing down inconsistencies in your work “Keep digging…don't let go…You might find something pretty awesome and amazing.”

Bruce expressed her gratitude for the ERO grant and the SDB team. “I love the conferences, and I love everyone there, so I'm really excited to…be part of that…community and tradition and be the next generation.”

For any evo-devo enthusiasts, or those with a tolerance for creepy crawlies, Bruce will be hiring Ph.D. and master’s students for the next few years.

References

Bruce, H.S., Patel, N.H. Knockout of crustacean leg patterning genes suggests that insect wings and body walls evolved from ancient leg segments. Nat Ecol Evol 4, 1703–1712 (2020). https://doi.org/10.1038/s41559-020-03-0
Bruce, H. S., Patel, N. H. The Daphnia carapace and other novel structures evolved via the cryptic persistence of serial homologs. Current Biology, 32(17), 3792-3799 (2022). https://doi.org/10.1016/j.cub.2022.06.073
Bruce, H. S., & Patel, N. H. (2023). Cryptic Persistence of Truncated Abdominal Legs in Insects Enabled Diverse Outgrowths with Novel Functions. Preprints. https://doi.org/10.20944/preprints202212.0268.v3

Last Updated 05/07/2025