Mystery of the seadragon solved

Date:
August 18, 2021
Source:
University of Konstanz
Summary:
The genome of the seadragon, a very unusual fish, has been decoded.

Seadragons (Phyllopetryx taeniolatus) live off the coast in western
and southern Australia. Evolutionary biologists have now found the
genetic basis for some external characteristics of the seadragon,
like its lack of teeth and its distinct leaf-like appendages. The
team also localized the sex-determination gene in the seadragon
genome.



FULL STORY ========================================================================== Seadragons (Phyllopetryx taeniolatus) live off the coast in western
and southern Australia. An international team involving evolutionary
biologist Axel Meyer from the University of Konstanz has now found the
genetic basis for some external characteristics of the seadragon, like
its lack of teeth and its distinct leaf-like appendages. The team also localized the sex-determination gene in the seadragon genome. The study
will be published in Science Advances on 18 August 2021.


========================================================================== Experts in camouflage Seadragons belong to the family Syngnathidae along
with seahorses and pipefishes. Their name refers to the dragon-like
shape of their bodies and the spectacular colouring of their special
leaf-like skin appendages. They are considered experts in camouflage
because of their ability to mimic the appearance of seaweed. Like the
other members of the Syngnathidae family, seadragons exhibit special adaptations and behaviours: They have a tube-shaped, toothless mouths,
they lack the ventral and pelvic fins and scales typical for fish,
but they instead have a bony shell covering their entire bodies.

Dried seahorses are used in traditional Chinese medicine, which has led
to many species being highly endangered. They do not swim horizontally,
but instead glide slowly through coral reefs and shallow coastal
waters, almost vertically -- like a horse -- with their heads bent
downwards. Their prehensile tail can be used to hold on to things. Like
other seahorse species, the seadragon males are responsible for protecting
the bright pink eggs attached to the outside of their bodies until
they hatch.

While sequencing the genome and examining the genetic basis for other
external characteristics of seadragons, researchers from five teams
from China, Singapore, Japan and Germany focussed their efforts on sex-determination, toothlessness and newly-evolved skin appendages
in seadragons. The research team, led by Professor Qiang Lin from the
Chinese Academy of Sciences in Guangzhou, showed that a series of genes
is responsible for this evolutionary development that usually control
fin development. The leaf-like skin appendages of the seadragons are
thus highly altered fin rays.

Like seahorses, seadragons are also toothless. They use their long snouts
to suck in their food, small crustaceans, and swallow them whole. The
genome analysis shows that, in the seahorse relatives, too, several genes
are missing that contribute to the development of teeth in other fishes
as well as in humans. The research team tested the hypothesis about
the corresponding scpp5 gene by switching off this gene in zebra fish,
a well-researched model organism with pharyngeal teeth. As expected,
the mutated fish displayed reduced teeth.

The function of the missing gene responsible for tooth loss was thus
proven in molecular-biological CRISPR-Cas experiments.

Males care for fertilized eggs It is also typical that male members of
the seahorse family care for fertilized eggs until they hatch. Seahorse
males have developed brood pouches, but males of the older seadragon
species still carry the sticky eggs visibly on their tails. The females
lay the eggs onto this special location on the males' bodies, and they
are then carried around by the male seadragons and thus protected from predators. It is generally more common that male fish care for fertilized
eggs rather than females, although this particular form has only evolved
in members of the seahorse family.

In this context, the researchers looked for sex-determination mechanisms
that were previously unknown in seadragons. On the whole, it is difficult
to localize sex-determination in fish, since, for the most part, they
do not have special sex chromosomes, like the X and Y chromosomes in
mammals. The team found the molecular basis for sex-determination in
seadragons to be in the Mullerian hormone, as had previously also been documented for seahorses.

The sequencing of the seadragon genome was completed as part of an
additional project in the context of a long-term collaboration between
the research teams in Guangzhou and Konstanz. The first publication --
on first the seahorse genome -- was published by both teams in 2016
in Nature, with a second one with 21 new seahorse genomes appeared
this year in Nature Communications. Axel Meyer: "Our research into
the genome seeks to derive the corresponding phenotype or "essence" of
these animals. We are striving to understand what an animal looks like
based on its sequence of genomes and our understanding of the genes'
function. Seadragons are fish that do not look like typical fish at all.

They are a particularly fascinating and beautiful species." ========================================================================== Story Source: Materials provided by University_of_Konstanz. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Meng Qu, Yali Liu, Yanhong Zhang, Shiming Wan, Vydianathan Ravi,
Geng
Qin, Han Jiang, Xin Wang, Huixian Zhang, Bo Zhang, Zexia Gao,
Ann Huysseune, Zhixin Zhang, Hao Zhang, Zelin Chen, Haiyan Yu,
Yongli Wu, Lu Tang, Chunyan Li, Jia Zhong, Liming Ma, Fengling
Wang, Hongkun Zheng, Jianping Yin, Paul Eckhard Witten, Axel Meyer,
Byrappa Venkatesh, Qiang Lin. Seadragon genome analysis provides
insights into its phenotype and sex determination locus. Science
Advances, 2021; 7 (34): eabg5196 DOI: 10.1126/sciadv.abg5196 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210818153730.htm

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