Ever since we learned to overcome the problems of tissue rejection, humans have created chimeric people by transplanting organs. And while many human chimeras owe their lives to the surgeon's skill, Buki Rinkevich explains that some chimeras are created by more natural means. Human mothers can carry stem cells donated by their offspring, while tunicate colonies can fuse and share a common network of blood vessels–if they share one of the alternative forms of a specific gene in common. Rinkevich explains that once fused, one of the partners is eventually reabsorbed by the other, but the loser isn't entirely obliterated; stem cells from the vanquished partner can circulate through the chimera's body, later parasitising either the dominant partner's germ cells or body tissue. Rinkevich was curious to know what gave the overwhelmed partner's stem cells the ability to maintain a foothold in the victor's tissue. Knowing that applying mechanical stress to a chimeric tunicate can switch the partner's dominance, Rinkevich wondered whether genetic or environmental factors, such as temperature, regulate the ability of the reabsorbed partner's stem cells to proliferate in the dominant partner's tissues (p. 3531).
Working with Irena Yankelevich, Rinkevich grew two colonies of the tunicate Botryllus schlosseri, which thrived at 20°C but could adapt to one of two different temperatures, 15 and 25°C. Dividing the colonies into subclones, Yankelevich paired subclones from the two colonies and allowed them to fuse before transferring the chimeras to environments at 15, 20 or 25°C to develop. Rinkevich admits that at the time, he had no idea which of the two colonies would turn out to be genetically dominant, but after sampling both Botryllus's blood and germ cells over a period of several weeks, it became clear that the chimeras had inherited their germ cells from only one of the two original partners: the colony adapted to 25°C. No matter what environmental conditions the chimeras experienced, they all inherited their germ cells from the same dominant colony. The environment had no effect on the inheritance pattern.
But when Yankelevich tested the chimeras' blood, it became clear that Botryllus's environment had a drastic effect on which partner dominated the resulting chimera. At 25°C, the chimeras reabsorbed the cool adapted Botryllus, leaving the 25°C adapted colony as the dominant partner, but at 15°C, Botryllus adapted to 25°C was reabsorbed and the 15°C tunicate dominated. Surprisingly, the chimeras raised at 20°C carried cells from both colonies. Rinkevich explains that the chimera's tissue is plastic, with the environmental conditions determining which partner dominates under different conditions; `[Botryllus] is able to fine-tune a plastic combination of the genetic components in its chimerical soma', says Rinkevich.
But what advantage does the absorbed partner gain from this self-sacrificing pact? Rinkevich suspects that the chimera's ability to modify its tissues to optimise survival across a wide range of environments may provide a significant selective advantage over individual tunicates. And even an absorbed partner in a chimera can contribute significantly to the next generation, thanks to the parasitic stem cells it leaves lurking in the chimera's tissues.