Sharks

John

In order to counterbalance the more medical side of this forum and based on this thread, we need a thread on sharks.

Here's a fun fact I didn't know. Evolutionarily, sharks are one of the "lowest"* orders of animals to have a complex immunological response (including MHC genes). It is also interesting to note that fin development in sharks shows similarity to development of limbs in tetrapods, that some sharks undergo viviparious birth (not many non-mammalian orders can) and some sharks are not cold blooded. I wonder how strong the case is for sharks being a possible offshoot from the same evolutionary branch that gave rise to tetrapods?

Marchalonis et al. (1998) wrote:

The combinatorial immune response is restricted to jawed vertebrates with cartilaginous fishes being the lowest extant species to have the mechanism for diversification and an extensive panoply of immunoglobulins, T-cell receptors and MHC products. Here, we review the molecular events of the "big bang" or rapid evolutionary appearance of the functionally complete combinatorial immune system coincident with the appearance of ancestral jawed vertebrates, suggesting that this event was catalyzed by horizontal transfer of DNA processing systems. We analyze the nature and extent of variable and constant domain diversity among the distinct immunoglobulin sets of carcharhine sharks focusing upon the lambda-like light chains and the mu and omega heavy chains. The detection and isolation of natural antibodies from the blood of unimmunized sharks illustrates a surprising range of recognition specificities and the existence of polyspecificity suggests that the antibody-forming system of sharks offers unique opportunities for studies of immunological regulation. Although the homologies between shark and mammalian immunoglobulins are unequivocal, major differences in segmental gene organization present challenges to our understanding of basic immunological phenomena such as clonal restriction.

Bartl (1998) wrote:

Similarity in structural features would argue that sharks possess class I, class IIA and class IIB genes, coding for classical peptide-presenting molecules, as well as non-classical class I genes. Some aspects of shark major histocompatibility complex genes are similar to teleost genes and others are similar to tetrapod genes. Shark class I genes form a monophyletic group, as also seen for tetrapods, but the classical and nonclassical genes form two orthologous clades, as seen for teleosts. Teleost class I genes arose independently at least four different times with the nonclassical genes of ray-finned fishes and all the shark and lobe-finned fish class I genes forming 1 clade. The ray-finned fish classical class I genes arose separately. In phylogenetic trees of class II alpha 2 and beta 2 domains, the shark and tetrapod genes cluster more closely than the teleost genes and, unlike the teleost sequences, the class II alpha 1 domains of sharks and tetrapods lack cysteines. On the other hand, both shark and teleost genes display sequence motifs in the antigen-binding cleft that have persisted over very long time periods. The similarities may reflect common selective pressures on species in aqueous environments while differences may be due to different evolutionary rates.

*and of course there are no such things really as "lower" or "higher" orders, this is a purely human construct.

Kevster

Interesting, even more so because I'm studying immunology right now. Another interesting thing about sharks is that they aren't fish. They are instead 'elasmobranchs'. Their tail-fins aren't symmetrical (like fish), they don't have a skeleton of bone (instead they have cartilage), and they don't have scales. So, yeh, they must have evolved in a not-too-well-known pathway in history.

r3nu4l

Well sharks do actually have scales and these scales actually resemble teeth in their structure. In fact, shark teeth are actually highly modified (through evolution) scales.

Thread ends here.

AtomicHorror

Hooray for this sort of nerdism.

Kevster wrote: »

Interesting, even more so because I'm studying immunology right now.

Then it might interest you to know that whilst Sharks are the "lowest" form of animal to have the adaptive immune system from which ours and every other mammal's developed (with B cells and antibodies, T cells and TCRs and MHC), the jawless vertebrates such as lampreys and hagfish have their own entirely alternative adaptive immune system, complete with lymphocyte-like cells and proteins which work like (and even look strangely like) antibodies but are entirely unrelated. It's a really spooky example of convergent evolution. They arrived at roughly the same solution, but did so independently.

Our common ancestor with these unattractive fellows would have had an innate immune system like ours, but no adaptive system at all- much as we see all over the animal kingdom (outside of the vertebrates).

John

Do you have any good links for more info on hagfish/lamprey immunology AtomicHorror?

Kevster

Nerdism indeed... ...I want a link for that too. That sounds interesting, AtomicHorror.

AtomicHorror

Well I first heard about this stuff after seeing Max Cooper give a talk at my university. Max is a fairly legendary figure in immunology and discovered the hagfish adaptive system in work he and his team did at the University of Alabama . The main hoo-hah surrounds the variable lymphocyte receptor (VLR) which is analogous in function (but not at all in origin) to the family of proteins which include vertebrate antibodies and t cell receptors.

It's the only other example we've yet seen of an adaptive immune system besides the type we have. Whilst both groups share the innate ancestral lymphocytes, the lampreys have no T cells or B cells, just what I guess we could call "V cells".

A good (free) paper on VLR by Cooper's group:

http://www.pnas.org/content/102/26/9224.full.pdf+html

And the first paper on it in Nature (not free unfortunately but you can read the abstract):

http://www.nature.com/nature/journal/v430/n6996/abs/nature02740.html

Kevster

Thanks for the links dude. I have just spent the past few minutes skimming over that paper. It's interesting but, I suppose, not surprising that they have this unique adaptive immune system. I mean, evolution keeps pushing in every direction and something like this is a consequence of that. It was funny though because reading that paper was a mixture of stuff I learned in first year (about the kingdoms of life) and what I'm learning now (Immunity!).

Kevin