Scientists utilize a few standard research organisms, such as fruit flies and mice, to investigate the evolutionary development (evo-devo) of animal lineages. However, expanding the range of research organisms can significantly enhance our understanding of life and contribute to advancements in biomedicine and ecological conservation.
Researchers at the Marine Biological Laboratory (MBL) in Woods Hole and the Stazione Zoologica Anton Dohrn (SZS) in Naples, Italy, have expanded the evo-devo toolbox by designating Holothuria tubulosa, a species of sea cucumber, as an experimental research organism.
The sea cucumber, plentiful in the Mediterranean Sea and the eastern Atlantic Ocean, belongs to the echinoderm group, which includes sea urchins, sea stars, and sand dollars. Certain echinoderm species have served as valuable developmental models for over a century due to their affordability, high reproductive rate, transparent larvae, and their recent suitability for genetic research.
“Echinoderms are the closest invertebrates to humans genetically, which means we have most of our genes in common. If we understand how those genes function in an echinoderm, then we also know how they function in humans,” said Margherita Perillo, a research scientist at MBL who led the study.
“Sea cucumbers also have attributes and special skills – such as being deposit feeders, which cleans the ocean floor, and the ability to completely regenerate their whole body – that could be useful in conservation and biomedicine,” she said.
The first crucial step in establishing H. tubulosa as a research organism involved the development of a protocol to efficiently produce embryonic cultures in a lab setting. Existing methods were found to be complicated, inefficient, or both, such as mimicking the animal’s natural breeding cycle and inducing the release of all the animal’s organs by evisceration, a behavior that sea cucumbers normally exhibit when threatened.
To address this issue, the team, led by Rossella Annunziata (SZS) and Perillo, introduced a noninvasive technique to repeatedly harvest a small number of gametes (sperm and eggs) over an extended period of time. This microsurgery technique involves a small incision near a sea cucumber’s reproductive organs, allowing for the retrieval of testes or ovaries. Importantly, the incision heals quickly, enabling researchers to harvest every few days from the same animal.
Given that eggs retrieved using this technique have not reached maturation and cannot be fertilized, the researchers then expose the harvested ovaries to a synthetic peptide – Thioredoxin-2 peptide, known to work in another species – to make them receptive to sperm. Subsequently, the fertilized egg is cultured, reaching the metamorphosis stage in about eight weeks.
“Our protocol removes a major bottleneck that has kept H. tubulosa from being used as a research organism and opens the door for more scientists to use it,” Perillo said.
The research team used advanced microscopy and immunohistochemistry to track the development of the larvae, focusing on its distinct structures. This detailed analysis will provide a crucial framework for future studies aiming to investigate developmental processes in H. tubulosa through genetic manipulations.
Furthermore, they demonstrated how echinoderm larvae can be utilized to explore the evolution of anatomical structures in closely related organisms. Specifically, they employed serotonin immunostaining to illustrate variations in the location of serotonin neurons among different echinoderm types. The mechanisms behind this diversification remain an intriguing open question in evolutionary development biology.
“The sea cucumber is a fascinating animal, and the better we understand it, the more valuable it is as a research organism,” said Perillo. “My plan now is to develop genetic tools to help further characterize it as an emerging comparison model in evo-devo. At the same time, this collaborative work laid the foundation for establishing a new sea cucumber species here at the MBL.”
Journal reference:
- Margherita Perillo, Tanya Alessandro, Alfonso Toscano, Rossella Annunziata. Larval development of Holothuria tubulosa, a new tractable system for evo-devo. Frontiers in Evolutionary Developmental Biology, 2024; DOI: 10.3389/fevo.2024.1409174