Among the group of exotic marine organisms, Sea dragons stand out. Relatives of seahorses and pipe fish, sea dragons have a long, narrow nose that they use as a straw to suck meals from microscopic crustaceans. Instead of scales, this fish is covered with bony armor, and its spine is twisted. Like their seahorse cousins, male sea dragons carry the female’s fertilized eggs in pouches on their tails.
They belong to two groups of species – leafy and common. ‘Leaks’ have elaborate branching appendages that make them nearly indistinguishable from floating algae in their South Australian habitat. “Regular” sea dragons are more streamlined, but also more colourful, with purple stripes and yellow polka dots.
Bill Crisco of the University of Oregon studies sea dragon genetics to answer a basic question: He and his colleagues want to know how these fish turned out to be what they are.
“We’re really fascinated with, ‘How could an organism like this exist?'” Crisco said. What has changed in the genome? “
A study published in June in the Proceedings of the National Academy of Sciences attempted to answer these questions. The researchers sequenced the genomes of leafy and common sea dragons and compared them with those of other fish.
The sea dragon’s strange appearance made the team believe that something unusual might have happened to the fibroblast growth factor genes, “which is really important for the evolution of things like teeth, which they don’t have, or the shape of faces.” or growth of appendages, to name a few,” said Susie Basham, a researcher at Crisco Lab and one of the paper’s authors.
But when they focused on the animals’ genomes, they were surprised to see sea dragons lack many of these key developmental genes.
“I didn’t really believe it at first,” said Clay Small, another author on the paper also at the University of Oregon.
Although sea dragons lack these growth genes, their genomes were filled with repeating pieces of code called transposons. This type of genome-wide repetitive code was once called “junk DNA” because scientists weren’t sure what it did. But transposons, or “jumping genes,” are actually able to cut themselves from one point in the genetic code and paste themselves at another, preventing other genes from shaping the characteristics of an organism.
Researchers can’t say for sure whether the transposon genes are responsible for the absence of the growth factor genes. There is evidence of replication of the genetic code near points where genes are missing, Small said, which may indicate the activity of the transposons. But scientists will need more genomes from the entire fish family tree to confirm a cause-and-effect relationship.
In addition to studying sea dragon DNA, the team performed tomograms of sea dragons at the highest resolution they were aware of. X-ray images gave them information about the fish’s limbs, which the researchers now suspect are modified spinous augments.
Axel Meyer, a sea dragon researcher at the University of Constance in Germany, said the study increases understanding of extreme body schemas. “This is a creature that symbolizes the abundance of evolution. It’s like the madness of evolution,” said Mayer, who was not involved in the study. “The fun of being an evolutionary biologist is that you can study these crazy animals and try to understand them genetically.”
Crisco says sea dragon DNA can provide insights into how to preserve them, which is critical because they are rare in the wild and difficult to keep in captivity. The cost of one fish can reach $10,000, and it has been proven almost impossible to breed.
“Believe me, there are a lot of people I work with who don’t want to take care of them because it takes so much work,” said Leslie Matsushige, who has treated sea dragons at Birch Aquarium at the Scripps Institution of Oceanography in Scripps. San Diego for more than 20 years. Fish are sensitive to temperature and light fluctuations, requiring gradual changes to mimic sunrise and sunset. Matsushige even tried to reconcile the tank’s nightlight with the phases of the moon.
Despite the difficulties, Matsushige said it’s important that aquariums like Birch and those in Tennessee — which have provided sea dragon tissue for DNA study — find ways to help the fish reproduce. As climate change threatens their habitats, sea dragons’ survival may depend on breeding programs in zoos and aquariums, so it is important to “know as much as possible”.
Translated by Luis Roberto M. Gonsalves
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