Seahorses are some of the strangest fish in the sea. Can their genome tell us why?
Seahorses are the only vertebrates known to exhibit true male pregnancy. (Dec. 14, 2016)
The weird and mysterious seahorse just got its first genetic close-up.
A team of international researchers has sequenced the full genome of a tiger tail seahorse. It is the first seahorse genome to be sequenced.
The authors of the new work, published Wednesday in the journal Nature, say the genome could reveal clues that can help explain why these bony fish are so unique.
“As a seahorse biologist, it is terrifically exciting,” said Tony Wilson, a professor at Brooklyn College in New York who was not involved in the work. “It brings the field into the 21st century.”
Yes, seahorses are fish, but you’d be forgiven for thinking they are something else entirely. They don’t look like most fish, and they don’t act like most fish either.
Their body orientation is vertical instead of horizontal, and instead of having scales, they are covered in bony plates that protect them from predators.
They don’t have pelvic fins — in the fish world that’s akin to not having hind legs. Instead they flutter their dorsal fins as many as 30 times a second to help them move around in the water. They also have a specialized tail, kind of like a monkey’s tail, that allows them to cling to a piece of grass (or a lucky diver’s finger).
And perhaps most intriguing, seahorses are the only known vertebrates on the planet to exhibit true male pregnancy.
Seahorse mating rituals begin with an elaborate courting dance that can last up to three or four days. Eventually, the female seahorse transfers up to 1,500 eggs to the male’s brooding pouch via a special organ specifically designed for the task.
The male quickly fertilizes the eggs and provides the developing embryos safety, food and oxygen inside his growing pouch.
This male pregnancy appears to end on just as physical a note as many female pregnancies.
“When he is ready to give birth he has contractions,” Wilson said. “He is actively pushing out the embryos.”
After the birth, the baby seahorses receive no more parental care, and the mom and dad start the mating process again almost immediately.
No rest for the weary papa.
The seahorse lineage first diverged from other bony fishes over 100 million years ago in the Cretaceous period, and scientists have long wondered about the origins of the seahorses’ distinct morphology and mating behaviors. Now that they have a full genome sequence, they may be able to better understand when these traits developed as well as how they are being expressed on the genetic level, Wilson said.
The new work was led by Byrappa Venkatesh, a researcher at the Institute of Molecular and Cell Biology in Singapore, whose lab has sequenced the genomes of several other unusual fish including puffer fish, mudskippers and sunfish.
There are 47 known species of seahorse, and they are found all over the world, including off the coast of California and in the waters off New York City. However, the area with the most diversity of seahorses is in the Pacific, north of Australia.
Venkatesh said the group chose to sequence the tiger tail seahorse because it is native to Singapore and abundant there.
Although the analysis of the seahorse genome is still in its early stages, the authors were able to tease out a few intriguing hints of the genetics behind a few of the seahorses’ unique traits.
For example, they found that the seahorse genome is lacking a set of genes that primarily code for enamel. This might explain why these suction feeders have no teeth in their tube-shaped mouths.
Also missing from seahorse DNA is a gene that regulates the growth of pelvic fins. To see if this could be related to the lack of pelvic fins in these species, the authors knocked out the same set of genes in a zebrafish. Indeed, the zebrafish with the missing genes displayed no pelvic fin, although otherwise they were perfectly healthy.
The researchers also discovered that a family of genes associated with pregnancy was expanded in the seahorse genome compared with the genomes of its closest relatives, such as pipefish and seadragons. In addition, they found that genes typically involved in breaking down the membrane surrounding the eggs in female fish are expressed by male seahorses in their brood pouches.
Finally, the authors found evidence that seahorse proteins and DNA have evolved faster than those of their closest cousins, although the authors can’t say yet why that might be.
Wilson said the work represents an important step in seahorse science, but he said this first sequenced genome can’t answer scientists’ questions on its own.
“Most of what they report is a correlation; you can’t say for sure that’s what causes it,” he said.
Wilson said that the questions the paper addresses will now need to be looked at in more detail.
For example, comparing the tiger tail genome to those from other seahorse species, as well as some of the seahorses’ closest relatives, would also provide a better understanding of how the strange, idiosyncratic and almost magical seahorse came to be.
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