Tikalon Blog by Dev Gualtieri

Seahorse Genome

January 12, 2017

As a child, I enjoyed reading comic books, especially those having an outer space theme. In those days, comic books sold for just a dime, but there was a local convenience store that sold used comic books for two cents, buying your used books for a penny. Our modern Commissars of Continual Copyright would object to such a resale and have every child pay the full dime; or, perhaps, a dime each time the comic book was read. Yes, times have changed.

Quite unlike the silly space operas and battles with bug-eyed monsters that we see on the cable television channels, the stories in these books contained a fair amount of hard science. I remember a reference to radio astronomy in one story long before images of the Mark I telescope of the Jodrell Bank Observatory became a mainstream item after its communication with various spacecraft, including Pioneer 5.

Perhaps my interest in metallurgy sprang from my reading the Metal Men comics. What young, male scientist could resist Tina, the platinum girl? The Metal Men comics were an education in the particular properties of the metals portrayed. Lead, for example, was a "little dense," and iron was strong. They could combine to make an alloy.

Reverse image of an American Platinum Eagle coin

A different type of platinum girl. This is the reverse image of an American Platinum Eagle.

The Metal Man, Platinum, is also known as Tina. Tina is likely a contraction of patina, the chemical layer that forms on the surface of a metal.

(Via Wikimedia Commons.)

Many authors envision the future as nearly identical to today, but with spacecraft. Space Cabbie, another comic book feature, was based on the idea that there would be interplanetary taxi drivers in the 22nd century. In one story that I remember, the space cabbie thought that a passenger had forgotten a dangerous package that must be kept cool. The smudged label, deciphered as "Dangerous, Keep Cool," actually contained the address of his fare, "Dan Crouse, Khip Vool."[1]

While such content may have inspired my becoming a physical scientist, there was other content in comic books that may have inspired some to study the life sciences. That content was the ads for the Sea-Monkeys. Packets of cryptobiotic brine shrimp eggs and nutrients produced these crustaceans when added to water.

The Sea Monkey name derives from the supposed resemblance of their tails to those of monkeys. Individuals of this hybrid species of Artemia are not long-lived, so the Sea Monkeys may have discouraged as many life science careers as they encouraged.

I had always thought that Sea Monkeys were seahorses, but I find that these animals are quite different. Artemia (Sea Monkeys) are crustacean arthropods, related to shrimp, crab, and lobster, while seahorses are fish.

Artemia salina (Sea Monkeys, left) and a seahorse (right). The left image is from the National Oceanic and Atmospheric Administration, and the right image is via Wikimedia Commons.)

The seahorse, whose technical name, Hippocampus, comes from the Greek words meaning "sea-monster horse," is a strange fish, indeed. Seahorses have a segmented bony armour, a curled prehensile tail, the males carry the female's eggs in a ventral pouch, and they swim upright, although not very well. For that reason, they are mostly sedentary, anchored to an object by their curved tail.

How did a strange creature like the seahorse come to be? Genetic and homological studies indicate that seahorses are descended from pipefish, and they join the pipefish in the family, Syngnathidae. The seahorse appears to have diverged from the pipefish during the Chattian Age, also known as the Late Oligocene, 23-28 million years ago, to populate new shallow water environments.

A research team of 34 scientists from Germany, Singapore, and China have recently collaborated on a project to elucidate the genetic basis for such evolutionary oddities of the seahorse.[2-4] The results of this study are published as a cover story in Nature.[2] The researchers were from the Chinese Academy of Sciences (Guangzhou, China), the University of Konstanz (Konstanz, Germany), the Beijing Genomics Institute (Shenzhen, China), A*STAR (Singapore), Huazhong Agricultural University (Wuhan, China), Ludong University (Yantai, China), and the National University of Singapore (Singapore).[2] They sequenced and analyzed the genome of the male tiger tail seahorse (Hippocampus comes), comparing it with that of other bony fish species.

A photo of the seahorse, Hippocampus barbouri, with annotations of evolutionary adaptations.

(Seahorse photo, University of Konstanz image by Ralf Schneider.)

One of the most interesting finding was that the unique features of the seahorse evolved quite quickly.[2-3] The analyzed H. comes genome lacks a calcium-binding phosphoprotein gene, and this likely led to its loss of mineralized teeth.[2] Seahorses no longer need teeth because they don't chew prey; instead, they suck food from the sea floor through their long snouts.[3-4]

Seahorse eyes are well developed, and they're capable of moving independently of each other. This enhanced visual perception, coupled with the fact that they don't need to hunt, may have resulted in the reduced number of genes coding for olfactory sense compared with other fish.[3-4] A gene, tbx4, that codes for fins or hind legs in nearly every vertebrate is missing from the seahorse's genome.[3]

While seahorses lack these genes, they do possess six copies of a gene called Pastrisacin.[3-4] This is the gene that's associated with male pregnancy, and they apparently activate the mechanism for release of the baby seahorses from the male brood pouch.[3-4]

A Venn diagram of shared orthologous gene families in the Hippocampus comes seahorse, fugu, zebrafish and stickleback.

(Data from Figure 2a of ref. 2, drawn using Inkscape.)[2]

One confirmatory experiment done by the research team was to knock-out the tbx4 gene in zebrafish using the CRISPER-cas method. As a consequence, the zebrafish showed the same fin-loss as seahorses.[2] This proved that the tbx4 gene is essential to development of those fins.[3]

Seahorse Genome

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