A team of researchers from the University of Oregon and the University of Illinois at Urbana-Champaign has sequenced the genomes of two living seadragon species: the leafy seadragon (Phycodurus eques) and the weedy seadragon (Phyllopteryx taeniolatus).
The leafy seadragon (Phycodurus eques) at the Monterey Bay Aquarium. Image credit: Joseph C. Boone / CC BY-SA 3.0.
Seadragons are a remarkable lineage of teleost fishes in the family Syngnathidae, renowned for having evolved male pregnancy.
They are widely recognized and admired for their fantastical body forms and coloration.
Substantial differences exist even among the three known living species: the leafy seadragon, the weedy seadragon, and the recently described ruby seadragon (Phyllopteryx dewysea).
Their specific habitat requirements have made them flagship representatives for marine conservation and natural history interests.
“Seadragons belong to the same family as seahorses and pipefish. This group is just cool for a number of different reasons,” said first author Dr. Clay Small, a researcher at the University of Oregon.
“But seadragons are oddballs in a group of already oddball fish.”
“There’s a lot of interest in how malleable to evolution things like the head and face are,” said University of Oregon’s Dr. Susie Bassham, senior author of the study.
“And seadragons can be good case studies for that kind of question because of the extreme differences they’ve evolved fairly quickly.”
“The family that seadragons and seahorses belong to branched off about 50 million years ago, which is relatively recently by evolutionary standards.”
The researchers sequenced the genomes of two seadragon species: leafy and weedy sea dragons.
They compared those genetic sequences to pipefish and seahorses, as well as other less closely related bony fish such as zebrafish and stickleback.
Along with pipefish and seahorses, seadragons were missing a chunk of genes that guide development, a possible clue to the origins of their unique form.
Compared to their close relatives, seadragons also contained higher-than-usual amounts of repetitive DNA sequences called transposons.
In their research, the authors also used a specialized X-ray microscope to capture a high-resolution 3D image of a weedy seadragon.
They scanned the foot-long fish in sections, then stitched the images together into a complete picture.
“Nobody had ever imaged any part of a seadragon before that way, with such high resolution,” Dr. Bassham said.
At that level of detail, the team could see the fine structure of the seadragon’s bones, and also get insight into how some of the fish’s unique body structures might have evolved.
“We could see that the support structures for the leafy paddles appeared to be elaborations of spines, and then the fleshy appendages were added to the ends. It lent evidence to idea that these (ornaments) are evolutionarily derived from spines,” Dr. Bassham said.
The team’s results appear in the Proceedings of the National Academies of Sciences.
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Clayton M. Small et al. 2022. Leafy and weedy seadragon genomes connect genic and repetitive DNA features to the extravagant biology of syngnathid fishes. PNAS 119 (26): e2119602119; doi: 10.1073/pnas.2119602119
