Chimeras on the rise

Sea anemones can form chimeras as a result of the fusion of two or more individuals, according to a linkurl:study;http://rspb.royalsocietypublishing.org/content/early/2011/04/14/rspb.2011.0605.short published yesterday (April 20) in Proceedings of the Royal Society: B. The observation adds to a growing list of chimeric organisms, supporting the suspicion that chimerism is a fairly widespread phenomenon.

Bi-chimeric sea anemone
Image: Courtesy of Annie Mercier

"This is another example from a group of organisms that we didn't know [could be chimeric] and there are probably so many other undisclosed cases of chimerism in nature," said linkurl:Baruch Rinkevich,;http://www.ocean.org.il/eng/Researchers/page_buki.asp a marine developmental biologist at the Israel Oceanographic and Limnological Research institute who was not involved with the research. "As we look around, we find chimeras in so many groups of organisms. Maybe not to be a chimera is the exception."Chimerism has even been noted in humans: When two zygotes fuse in the womb, the different sets of DNA can coexist within a single, healthy individual. But chimerism is most frequently recognized in colonial organisms, such as sponges and corals, in which aggregations of individuals function as a single organism. Living in such close quarters, neighbors will often fuse with one another, explained marine ecologist linkurl:Nanette Chadwick;http://www.auburn.edu/academic/cosam/faculty/biology/chadwick/website/ of Auburn University who was not involved with the research. "Once they settle down, they have no option but to contact the tissue of their neighbors," she said. "It's almost like a tissue graft."

Bi-chimeric sea anemone
Image: Courtesy of Annie Mercier

linkurl:Annie Mercier;http://www.mun.ca/osc/amercier/bio.php of Memorial University Ocean Sciences Centre was studying larval size variation in the sea anemone Urticina feline when she noticed that some of these larvae had fused into chimeras. When she raised the fused embryos in isolation, she observed that many of them developed into healthy larvae, indistinguishable from non-chimeric larvae apart from being a bit larger. "If I hadn't seen it early in development, I wouldn't have guessed that these were fused together," Mercier said. "Chimerism might be more frequent than we think simply because we cannot detect it."Studies of colonial creatures have found that chimeras are larger and reach sexual maturity faster, giving them a leg up over their neighbors. But there is a "downside of fusing," said linkurl:Fadi Lakkis,;http://www.gradbiomed.pitt.edu/immunology/faculty.asp?ID=97 an immunologist at the University of Pittsburgh who was not involved in the research -- there's no guarantee that your genome will be the one passed on. "If two animals that are unrelated fuse, stem cells from one could invade the other's germ line and take over the identity of the other," he explained.

Multi-chimeric sea anemone
Image: Courtesy of Annie Mercier

This so-called germ line parasitism could be the evolutionary precursor of our own allorecognition, the ability of immune cells to distinguish self from non-self and the basis for why transplant patients' bodies often reject the new organ, said Chadwick. The risk of germline invasion among neighboring invertebrates could explain "why our acceptance/rejection system for tissues is so well developed," she said. The identification of chimeric sea anemones, which are sexually reproducing, could provide an example of that self-recognition process in a species more similar to humans, Chadwick said. "Chimerism is not just limited to fusion after larvae settle and metamorphose into a sessile phase, but occurs in a breeding process related more to what happens in humans," she said. "Allorecognition and the graft rejection system makes more sense if you understand that it was very important in our invertebrate ancestors."A. Mercier et al., "Internal brooding favours pre-metamorphic chimerism in a non-colonial cnidarian, the sea anemone Urticina felina," Proceedings of the Royal Society: B, doi: 10.1098/rspb.2011.0605, 2011.
**__Related stories:__*** linkurl:Surprises in sea anemone genome;http://www.the-scientist.com/news/display/53364/
[5th July 2007]*linkurl:One child, two fathers -- marmoset-style;http://www.the-scientist.com/news/display/53033/
[27th March 2007]*linkurl:A Question of Chimeras;http://www.the-scientist.com/article/display/15405/
[4th April 2005]

Chimeras on the rise

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