Chloroplast genome assembly of Thalassia testudinum from the Caribbean Sea, and its evolutionary analysis with related genera

Beneath the surface of the Caribbean Sea, where sunlight filters through clear blue water, lies a world built by plants. These are seagrasses, flowering plants that have returned to the ocean, forming vast underwater meadows that serve as nurseries for fish, protect coastlines from erosion, and trap carbon from the atmosphere. Among them, the turtle grass, known scientifically as Thalassia testudinum, is a dominant force, carpeting the seabed from the shallow flats to deeper waters. For scientists, understanding the genetic makeup of these plants is like reading a history book written in code; it reveals how they have adapted to their environment and how different populations are related to one another. While researchers have long studied the shape and structure of these grasses to tell them apart, this method can be misleading because the plants can change their appearance depending on where they grow. To get a clearer picture of their true identity and evolutionary history, scientists are now turning to the plants' internal instruction manuals: their genomes.

A team of researchers from the Inter American University of Puerto Rico recently turned their attention to a specific patch of turtle grass growing off the coast of Guánica, in southwestern Puerto Rico. They wanted to know if the grass in this Caribbean location was genetically the same as the turtle grass found in Florida, or if it had developed unique traits to survive the specific conditions of the island waters. To find out, they collected samples from the ocean floor, carefully cleaned them to remove any attached algae or debris, and extracted the DNA from the chloroplasts, the tiny structures inside plant cells that capture sunlight to make food. Using a modern sequencing device that reads long strands of genetic material, they pieced together the complete chloroplast genome of the Puerto Rican turtle grass. This resulted in a digital map of the plant's genetic code, measuring 179,257 base pairs in length and containing 141 genes that are essential for the plant's survival and growth.

When the scientists compared this new genetic map with those of turtle grass from Florida and a related species called Thalassia hemprichii from China, they found a story of both deep connection and subtle difference. The Puerto Rican grass was clearly a cousin to the Florida grass, sharing a very recent common ancestor, yet it was not identical. The two populations clustered together tightly in the genetic analysis, confirming they are the same species, but the Puerto Rican sample showed unique structural features. For instance, the arrangement of certain repeating sections in the DNA was different, and the Puerto Rican grass was missing a specific genetic segment called orf106 that was present in the Florida and Chinese samples. These missing or rearranged pieces are not necessarily signs of weakness; rather, they suggest that the Puerto Rican population may have undergone a specific reorganization of its genetic code, perhaps as a way to adapt to the unique temperature, salinity, and chemical fluctuations of the Caribbean Sea.

The researchers also looked closely at how the plant uses its genetic code to build proteins. They found that the Puerto Rican turtle grass, like its relatives, prefers to use certain building blocks that end in the letters A or U, a pattern that helps the plant's cellular machinery work efficiently. This preference was even more pronounced in the Puerto Rican sample, hinting that the plant has fine-tuned its internal processes to maximize energy use in its specific environment. Furthermore, the study identified small, repeating patterns in the DNA, known as simple sequence repeats, which act like genetic fingerprints. While these patterns were very similar between the Florida and Puerto Rican populations, they differed enough to show that these groups are distinct and have been evolving separately for some time.

Ultimately, this work provides a foundational genetic reference for turtle grass in the Caribbean. It confirms that the grass in Puerto Rico is indeed Thalassia testudinum, closely related to its Florida counterpart, but it also reveals that this population has carved out its own genetic identity. The findings suggest that while the core machinery for photosynthesis and growth remains highly conserved across the species, the details of the genome are flexible enough to change in response to local environmental pressures. By establishing this genetic baseline, the study offers a crucial tool for future conservation efforts, allowing scientists to track the health of seagrass meadows and understand how these vital ecosystems might respond to a changing ocean. The research underscores that even within a single, widespread species, local populations can hold unique genetic secrets that are essential for their long-term survival.