Besides being a delicacy in Japan and various other parts of the world, sea urchins have a longstanding career as a model organism in biology. The first description of these spiny, spineless creatures can be found in Aristotle’s History of Animals, dating from the 4th century BC 1. From the 1800s onward, the sea urchin has been studied by zoologists and embryologists alike. It’s easy to explain this fascination when you consider its embryonic development: a young sea urchin starts it life as a bilateral symmetric embryo/larva. Within weeks, the radial adult form develops within the larva. On top of this remarkable feat, they can grow to be over 100 years old.
Even though sea urchins are capable of reaching this old age, they do so without an adaptive immune system. Jawed vertebrates (that includes us) are the only species that have such an adaptive immune system. In a nutshell, this means that when we face a pathogen which we’ve never encountered before, our immune system ‘learns’ how to recognize the germ in future encounters, so that it can react faster next time. Because sea urchins don’t have this adaptive immunity, it is interesting to see how they evolved to deal with a wide variety of pathogens (them seas are full with bacteria and viruses!).
The formidable spines of a sea urchin (Heliocidaris sp.). Photo by Richard Ling
Scientists sequenced the sea urchin’s genome in 2006. First analyses of the sea urchin quickly revealed the complexity of its immune system 2. One of the most striking observation was the sheer amount of Toll-like Receptors (TLRs). In the sea urchin’s genome, 222 of these receptors could be identified, whereas most animals only have 1 to 20! TLRs are important for sensing different pathogenic signals. They can recognize bacterial cell wall components or viral RNAs, for example. In animals, TLRs only recognize the most conserved pathogenic signals. It has been suggested that this large expansion in TLR genes increases the spectrum of pathogens that can be recognized by the spiny fellow. This expansion is not exclusive to the family of TLRs, but also includes the NLR (203) and SRCR (218) gene families.
An interesting question would be if the expanded gene families in echinoderms (sea urchins, amongst others) resemble the Deuterostomian ancestor of Echinoderms (sea urchins and starfish) and Chordates. Maybe Chordates lost this expansion when they evolved an adaptive immune system? This seems to be supported by the fact that amphioxus, which is a closer relative to vertebrates but still lacks an adaptive immune system, also shows this expansion in immunologically related gene families.
Metazoan Cladogram. Echinoderms and Chordates are very closely related. Picture by UNM labs.
Another interesting discovery is the diverse family of 185/333 genes 3, which are also implicated with pathogen resistance in echinoderms. The observed diversity in this family is partly due to the high recombination frequency of the genes, in a mechanism that is similar to VDJ-recombination in vertebrates. Few of these genes are actually expressed, suggesting that they’re primarily a source of diversity during recombination. What else is interesting about the 185/333 gene family, is that the sequences of the mRNAs are almost never identical to the genes that they were derived from! This suggests that the RNA sequences are changed after they have been transcribed, generating great differences which eventually lead to inter-individual differences. It seems that the 185/333 gene family is another strategy sea urchins employ to generate immunological diversity, allowing it to recognize and combat a wide variety of pathogenic challenges.
So, there’s more to these globular animals than you might expect from their spiny first impression. I think this is another great example that shows us we can learn much about ourselves, evolution and how immunity works by the study of seemingly bizarre corners of the animal kingdom.
1. Eleni Voultsiadou and Chariton Chintiroglou, Aristotle’s lantern in echinoderms: an ancient riddle, Cah. Biol. Mar. (2008) 49 : 299-302
2. Rast, J., Smith, L., Loza-Coll, M., Hibino, T., & Litman, G. (2006). Genomic Insights into the Immune System of the Sea Urchin Science, 314 (5801), 952-956 DOI: 10.1126/science.1134301
3. Buckley KM, Terwilliger DP, & Smith LC (2008). Sequence variations in 185/333 messages from the purple sea urchin suggest posttranscriptional modifications to increase immune diversity. Journal of immunology (Baltimore, Md. : 1950), 181 (12), 8585-94 PMID: 19050278
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Actually, jawless vertebrates like lampreys have an adaptive immune system. There have been several papers about it in the past year.
I’ve also talked to one of the authors on the 185/333 paper, and she always shrugs when I ask what it does. I didn’t actually know that it was published in JI, but its nice to find new things to read.
Cheers!
Thanks for your comment!
The 185/333 genes sure are a mysterious bunch. Their corresponding proteins have been found on the surface of phagocytes, where they dimerize and maybe facilitate cell-cell interactions..
Thanks for bringing the lampreys under my attention, I actually saw a talk by Max Cooper about the evolution of antigen receptors (by studying lampreys) the day after I wrote this post! I forgot to write about it then.. but will do so soon!