In this video I will give you the basics on chondrichthyan models with a short
introduction. We will look at why cartilaginous fish are important
in evolutionary biology, which species have been used as models, how they are used
in research in various biological fields, and what genomic data are available.
Cartilaginous fish are important in evolutionary biology because they occupy
a key phylogenetic position. On this tree, we can see that vertebrates are divided into
agnaths, jawless vertebrates and gnatho- stomes, vertebrates provided with jaws.
Then, gnathostomes are comprised of chondrichthyans, which are cartilaginous
fish, and osteichthyans, which are bony vertebrates. Then, bony vertebrates are
divided into sarcopterygii and actinopterygii, and tetrapods belong
to this last group and include amphibians and amniotes.
On this tree we can see that cartilaginous fish belong to the sister group of all bony
vertebrates. They can help us to reconstruct the evolutionary history of
a biological process, for example brain asymmetries, that is well-known in bony
vertebrates. Different chondrichthyan species are used in biology. Some
examples include the small-spotted catshark, the swell shark, the spiny
dogfish, the little skate, and the elephant shark. These species help answer different
types of questions in different biological fields.
One of the fields in which these models have been used is evo-devo. Researchers in
the lab of Martin Cohn have used the little skate to study the origin of claspers, the
animals' copulatory organs. These organs correspond to a posterior extension of the
pelvic fins that carries sperm for internal fertilisation. As you can see, females do
not have these extensions. This lab investigated the molecular mechanisms
driving these posterior elongations. To do that, they implanted carrier beads loaded
with molecules that locally modify the growth of the pelvic fins.
Females were implanted with beads loaded with Sonic hedgehog and the researchers
observed an ectopic growth of the pelvic fins. Males were implanted with beads
loaded with cyclopamide, an inhibitor of the Sonic hedgehog pathway, and they
observed a truncated pelvic fin. This work has highlighted the importance of the
Sonic hedgehog pathway in the induction of pelvic fin extensions.
Another example is the little skate, which has been used as a model in experimental
biology. In a study from Hermann Haller's lab, they experimentally removed part of
the kidney and they showed that cartilaginous fish can regenerate this organ
a property that has been lost in mammals.
Sharks have also been used to analyse the evolution of gene families across
vertebrates in the field of molecular phylogeny. One notable example of this
is the evolution of Hox genes. Using the genome of the small-spotted catshark,
researchers have shown that this species lacks one of the four clusters, cluster C.
A subsequent study has revealed that in fact, the Hox C cluster was not deleted in
the elasmobranch ancestor but has flaked off during their evolution.
Sharks have also been used in trans- disciplinary studies, for example in work
that combines mathematical modelling and experimental biology. In a recent
publication, Gareth Fraser's lab has demonstrated that the pattern of dermal
denticles follows a Turing reaction-diffusion system.
This model can predict natural epidermal patterns such as
stripes or spots. In sharks, dermal denticles are organised as rows of
individual items. When they experi- mentally removed one of the denticles,
the pattern is changed and the row is deformed.
Including a gap in the initiation row, during the reaction-diffusion simulation
this time, produced the same deformation.
Genomic and transcriptomic databases are available for several species:
the white shark, the bamboo shark, cloudy catshark, whale shark, zebra bullhead shark,
little skate, ocellate spot skate, and elephant shark. There is also an ongoing genome
project for the small-spotted catshark.
In this figure you will find the reference genome papers related to each species.
So we've just had an overview on which chondrichthyan models are used and in
what kind of research contexts. Hopefully now you have a sense of their versatility
in answering a variety of research questions.
In the next part you will get an up-close and personal view of the life cycle of one
of these models: the small-spotted catshark.