Evolutionary gallopers
Secret of male pregnancy revealed by genome.
Seahorses are enchanting but for a fish they are seriously weird.
Gone are fishy things like scales, pelvic fins and teeth. Instead their majestic bodies are covered in bony armour plates, anchored by a curling, prehensile tail, and their horsey heads end in long sucking snouts. Weirdest of all, it is the male that gets pregnant. How did this happen to a fish? Some of the answers have been revealed by an international consortium that analysed the genome of the tigertail seahorse, Hippocampus comes, as reported in Nature last December. By comparing its DNA spec sheet to that of related teleost fish, the researchers found the seahorse had lost a set of genes (known as SCPP) for making tooth enamel proteins. Also gone was the tbx4 gene required to form rear appendages like pelvic fins.
We’ve seen evolution tinker with these genes before. Birds and turtles traded teeth for beaks by losing their SCPP genes. Snakes lost their legs when their tbx4 gene was inactivated.
But when it comes to male pregnancy, it’s all about what the seahorse has gained. Males are impregnated when females deposit eggs into their pouch (in closely related stickleback fish, males merely lure females with a nicely made nest). Cells lining the male’s pouch supply nutrients, remove waste and help the embryo hatch.
Compared with other teleost fish, the sea horse has gained six copies of a gene called patristacin involved in gestation and hatching. The gene is related to one that helps embryos hatch in platyfish (where females give birth to live young).
This remarkable set of features adapted the seahorse to a life spent hiding in seagrass or coral gardens, sucking in the occasional passing crustacean and, for the males, waiting for females.
And they evolved quickly. Comparing the number of DNA changes to that of their fishy cousins since they diverged from a common ancestor 103 million years ago, seahorses hold the speed record, says co-author Axel Meyer at the University of Konstanz in Germany.