Thursday, 22 December 2016

Researchers sequence entire genome of seahorse, investigate essential mechanisms of evolution

December 14, 2016

Without a doubt, the seahorse belongs to Darwin's "endless forms most beautiful". Its body form is one of a kind. It has neither a tail nor pelvic fin, it swims vertically, bony plates reinforce its entire body and it has no teeth, a rare feature in fish. Another peculiarity is that male seahorses are the ones to become pregnant.

The genome project, comprising six evolutionary biologists from Professor Axel Meyer's research team from Konstanz and researchers from China and Singapore, sequenced and analyzed the genome of the tiger tail seahorse. They obtained new molecular evolutionary results that are relevant for biodiversity research: the loss and duplication of genes as well as the loss of regulative elements in its genome have both contributed to the rapid evolution of the seahorse. The results will be published as the cover story in Nature on 15 December 2016.

The questions underlying genome sequencing of how diversity emerges and what its genetic basis is, can be superbly answered through the example of the seahorse because numerous unique features evolved in the seahorse within a short time. This is how the researchers around evolutionary biologist Professor Axel Meyer were able to identify the genetic basis for the disappearance of the seahorse's teeth: several genes that are present in many fish as well humans and contribute to the development of teeth, were lost in seahorses. The seahorse no longer needs teeth due to the special way in which it consumes its food. Instead of chewing its prey, it simply sucks it in with the enormous negative pressure that it can generate in its long snout. This same genetic forfeiture applies to genes that contribute to the sense of smell: seahorses hunt visually and have very good sight, using their eyes that can move independently of each other. Therefore, the olfactory sense seems to only play a minor role.

Lined seahorse (Hippocampus erectus) – a juvenile clinging to a substrate with its prehensile tail. Transcriptome of the male brood pouch at different stages of pregnancy of the lined seahorse (Hippocampus erectus) was sequenced and analysed. Credit: Qiang Lin
Particularly noteworthy is the loss of the pelvic fins. In evolutionary terms, they share the same origin as human legs. An important gene, tbx4, that is responsible for this feature, was found in nearly all vertebrates, but is missing from the seahorse's genome. In order to test the function of this gene, a functional analysis was carried out in addition to the genome analysis. For this purpose, the corresponding gene was deactivated via the CRISPER-cas method in zebrafish, a genetic model system. As a result, these fish then also lost their pelvic fins. This proved the importance of this gene in the "normal" development of the pelvic fins.





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