Re: Can a hydra smell other hydras or fishes?

Dear Joelle:

I believe your question has a couple of answers. First, the clown fish do seem to have some native immunity to the nematocyst poison, although clown fish can also be killed by anemones (some of the references I cited last time refer to this). The protection afforded by the mucus passed from the anemone to the clown fish has been defined as "chemical," and I have an interesting idea about what it might be.

*WARNING: THIS IS SPECULATION**

I am going to jump here from marine biology/zoology to immunology, one of my other favorite subjects. One of the most fascinating things about our immune system is its ability to discriminate dangerous from nondangerous foreign proteins as well as from self. Let me explain what I mean: When you eat a meal, all of those foreign proteins (and fats, and carbohydrates) get broken up and absorbed into your bloodstream. It would be bad if you had an immune response to all of those molecules that comprise your food - you would always be running a fever and feeling ill. On the other hand, if a bacterium enters the bloodstream, you want your immune system to fight it immediately. Likewise, you don't want your body to attack your stomach because it is making different proteins than your finger, and vice versa.

When the immune system fails to respond to the body's own proteins or food antigens, we call this tolerance. When tolerance breaks down, diseases like allergies and autoimmunity (lupus, multiple sclerosis, type I diabetes, etc) occur. This tolerance is generally enforced via a group of molecules known as the major histocompatibility complex, or MHC for short. The major histocompatibility complex got its name from transplantation surgery: histo= tissue and compatability=compatibility; in other words, if you could transplant a tissue from one individual to another, those individuals were considered histocompatible. Later it was found that these MHC molecules present protein pieces/antigens to the cells of the immune system. MHC molecules are very polymorphic (highly variable from individual to individual), some of the most polymorphic genes known. This makes it very difficult to find two individuals who are histocompatible (think of finding matches for people for bone marrow transplants - the HLA typing they do for bone marrow transplants is checking for the same MHC molecules in the two individuals).

Ok, but what do MHC molecules have to do with sea anemones? Nothing, yet, but some recent research by Irving L. Weissman's group at Stanford may yield a clue. As well as researching "mainstream" immunology, Weissman also studies the evolution of the immune system - how did we end up with this foreign recognition system in the first place? Sharks are the most primitive organisms that have been proven to have MHC molecules as we know them, but Weissman has gone further. He has found that the colonial ascidian, Botryllus schlosseri possesses a sort of histocompatibility locus. He has found that if you place two genetically related colonies of Botryllus next to one another, they will fuse into one colony, while genetically unrelated colonies will reject one another, much as a foreign tissue is rejected in a human.

Now, sea anemones, which are coelenterates and more primitive even than the ascidiacea, may have an even more primitive self/nonself recognition system, either a chemical or a MHC-like system contained within the mucus, which allows the anemone to recognize friend or food - and the clown fish tricks the anemone by coating itself in "friend" disguise, protecting itself from being eaten.

I hope this helps!
Ingrid
MadScientist

References:
De Tomaso AW, Saito Y, Ishizuka KJ, Palmeri KJ, Weissman IL. Mapping the genome of a model protochordate. I. A low resolution genetic map encompassing the fusion/histocompatibility (Fu/HC) locus of Botryllus schlosseri. Genetics 149: 277-287, May 1998.

Stoner DS, Rinkevich B, Weissman IL. Heritable germ and somatic cell lineage competitions in chimeric colonial protochordates. Proceedings of the National Academy of Sciences 96: 9148-9153, August 1999.

Laird DJ, De Tomaso AW, Cooper MD, Weissman IL. 50 million years of chordate evolution: seeking the origins of adaptive immunity (commentary). Proceedings of the National Academy of Sciences 97: 6924-6926, June 2000.

Weissman IL. Stem cells: Units of development, units of regeneration, and units of evolution (review). Cell 100: 157-168, January 2000.

Fagan MB, Weissman IL. Linkage analysis of HSP70 genes and historecognition locus in Botryllus schlosseri