
AsianScientist (Dec. 15, 2016) – Seahorses may swim slowly, but they evolve faster than other fishes, according to a study published on the cover of Nature.
With their vertical posture, prehensile tails and chameleon-like eyes, seahorses immediately strike us as highly unusual fish. Beyond their strange looks, seahorses are unique in that males undergo pregnancy, incubating the growing embryos in a structure called a brood pouch and giving birth to fully formed, tiny seahorses.
By sequencing and analyzing the genome of the tiger tail seahorse (Hippocampus comes), an international team of researchers led by Professor Byrappa Venkatesh of Singapore’s Agency for Science, Technology and Research’s (A*STAR) Institute of Molecular and Cell Biology (IMCB) has uncovered the secrets of the seahorse’s genetic innovations.
Focusing on protein and nucleotide sequences that have a one-to-one ortholog in other bony fishes, the researchers constructed a phylogenetic tree and estimated the rate of substitution, a measure of the speed of evolution.
“We found that seahorses have the highest evolutionary rate of all teleosts. This is exactly the opposite of what we previously found in elephant sharks, which are the slowest evolving fish,” Venkatesh told Asian Scientist Magazine.
The finding that slow-moving seahorses have such a high rate of evolution came as something of a surprise. However, the authors suggest that the high evolutionary rate could explain why seahorses look so different from other fishes.
“This fast evolution could have helped seahorses acquire new genetic material that formed the basis of their many morphological innovations,” Venkatesh explained.
The researchers also linked the loss of the tbx4 gene in seahorses with their lack of pelvic fins, structures that are equivalent to legs in humans. While tbx4 seems to only affect pelvic fin formation in fishes, the gene has acquired additional functions in mammals, Venkatesh said, and could be involved in human genetic diseases related to leg development, such as small patella syndrome.

Turning their attention to male pregnancy, the researchers sequenced the transcriptome of the more lab-friendly H. erectus seahorse at different stages of brood pouch development, collecting tissue at the pre-pregnancy stage, early pregnancy and late pregnancy. This RNA sequencing method helped them to identify five genes, named patristacins, that are highly expressed in the male brood pouch. One of these genes has previously been linked to male pregnancy in closely related pipefishes.
Using the seahorse genome as a template, Venkatesh and his team plan to study the population genetics of seahorses in Singapore. The group, which has recently sequenced the genomes of the mudskipper, spotted gar, arowana and ocean sunfish, also plans to continue sequencing the genomes of other interesting fish, such as the bichir.
“Fishes are the largest group of vertebrates, with over 29,000 species. The bichir, an unusual ancient fish that has lungs like land animals, represents the basal lineage of all fishes. Its genome will give us an insight into the ancestors of all bony vertebrates,” Venkatesh added.
The article can be found at: Lin et al. (2016) The Seahorse Genome and the Evolution of its Specialized Morphology.
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Copyright: Asian Scientist Magazine; Photo: IMCB/A*STAR.
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