Very interesting. This is just speculation on my part --- but perhaps a reason why the fruit fly and the nematode worm only have one TNF is precisely because they are small organisms that frequently come into contact with other organisms. If they had dozens of TNFs, then it would be much easier for their apoptosis processes to be triggered by external influences, which could prove debilitating.
Doesn't explain the differential population in coral and humans; the simpler organisms have shorter lifespans, it should matter less if they're susceptible to an external influence.
More likely there are so many receptors because the type and mechanism of cell death is important for the programming of complexity in organisms and macroorganisms.
Re Quotient: The cool thing about these TNF receptors is that not all of them lead to apoptosis. In humans for a TNF receptor to move towards apoptosis it requires a "Death Domain." In corals only 11 of the 40 TNF receptors have these death domains so the other 29 TNF receptors are likely involved in a variety of other cellular process besides apoptosis
Re DaniFong: "simpler" organisms do not always have shorter lifespans. For example many clams/molluscs can live to be hundreds of years old. In fact, the oldest mollusc is estimated to be over 400 years old!
Re: Why so many TNF receptors in coral there are variety of possibilites:
In terms of the biological reasons for so many TNF-receptors in coral there are a couple possibilities
1) Genome duplication events can occur during evolution leading to expansion of gene families. This appears to be the case in the Zebra fish model organism Danio rerio, they have 33 TNF receptors which seem to have appeared through genomic duplication. I did not specefically look for genome duplication events in the coral genome, but is an interesting direction for future research.
2) Microbial-host interactions- corals harbor a diverse assemblage of bacteria, viruses, fungi, and algae which changes in response to coral health. TNF receptors may be involved in managing these complex interactions. Of the 40 TNFR in coral, only 11 have a "Death Domain". We hypothesize that these death domain TNFR are involved with directly guiding a cell towards apotosis which leaves 29 other receptors that may be involved with other cellular processes
3) Coral resilience to environmental stress- Recent work by Steve Palumni showed that TNF-receptors are involved with resilance to heat stress. They placed naturally heat resistant coral into cold water and found that expression levels of specefic TNF receptors were maintained in the heat-resistant coral but not the heat-sensitive coral. This suggests a genetic component to heat stress via TN-receptors.
If anybody has any questions about my paper, evolution, or coral in general please do not hesitate to ask me on Twitter @StevenQuistad
