STANFORD, Calif. - "You can eat your relatives but not your friends," could be the off-kilter credo of a tiny marine invertebrate called a sea squirt that can physically merge with, and parasitize, its own kin. The trigger for this unseemly behavior has now been traced to a single gene, isolated by researchers at the Stanford University School of Medicine. That gene also points to a common origin with the vertebrate immune system, far back in animal evolution, potentially shedding light on the development of our own immune system.

The sea squirt with the questionable philosophy is Botryllus schlosseri, a colonial animal that looks deceptively like a small flower. Each of its apparent petals is actually a separate, though genetically identical, organism, linked to the others by a common blood vessel. Ringing the tiny petals are even tinier tentacle-like ampullae, the sensing organs that evaluate other sea squirts, determining who's related and who isn't.

If two adjacent squirts aren't related, their respective ampullae blacken and shrivel upon contact. But when the squirts are related, they begin to physically fuse together. Thus, the ampullae had to be able to sense genetic similarity among sea squirts, said Anthony De Tomaso, PhD, researcher in pathology and first author of a paper on the subject in the Nov. 24 issue of Nature. "We were looking for the genes which control how an individual can distinguish self from non-self," he said.

Fusing together benefits the filter-feeding squirts because they live in high-density areas such as marinas, where competition among sea life is fierce. Because adult squirts are sedentary, if the area around them is already occupied, they can only increase their feeding area by fusing.

The downside of fusing is that one sea squirt can parasitize the other, essentially taking over its body by means of mobile stem cells, which transplant themselves between the fused individuals through the share
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23-Nov-2005

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