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Planet Earth

Tomorrow’s double feature: Virus traps and marmoset chimeras

The LoomBy Carl ZimmerMarch 27, 2007 7:39 AM


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Before I head for Utah, let me direct your attention to two articles of mine in tomorrow's New York Times. They don't have a whole lot in common except they are examples of cool biology... 1. Virus traps. Here's a case where ecology, evolution, and medicine all come together in an intriguing mix. You can think of any population of animals, plants, or other organisms as a leaky bucket under a running faucet. The population is boosted by sources of new individuals, and drained by sinks. Sources may include rapidly reproducing individuals, or immigrants from other populations. Sinks include the death of individuals in the population, or their failure to reproduce. Source-sink dynamics are important to ecologists, in part because the survival of a species may depend on sinks not draining its populations faster than its sources can restock them. Humans can create new sinks that tip this balance. If a forest is reduced to fragments, seeds that drift out into the surrounding farmland may be unable to grow, for example. Humans can even create so-called "ecological traps" that draw animals to their doom. Mayflies, for example, sometimes lay their eggs in parking lots because they look like rivers. The same rules that govern mayflies also govern viruses. They have their sources and sinks, too. Their sources are the new viruses that spew out of infected cells, and their sink is the immune system's attack. Some scientists are investigating whether they can add another sink: an engineered cell that lures viruses to it, but inside of which they cannot reproduce. (For more on virus traps, see this paper in the journal Ecology Letters.) 2. Marmoset chimeras. These cute little monkeys pose a serious philosophical quandary. Each individual marmoset is an amalgam of cells from two different marmosets. That's because marmosets are generally conceived as fraternal twins and then trade stem cells like mad. Even eggs and sperm can belong to siblings. It may look as if a male marmoset is fathering baby marmosets, but he may actually just be their uncle. (Here's the paper.) I called up the Harvard evolutionary biologist David Haig to talk about this finding. In 1999 Haig wrote a speculative paper (pdf) about marmoset evolution. At the time scientists had only found chimerism in marmoset blood. But Haig mused about the possibility that marmosets might actually be more mixed. Now those speculations appear to be on the mark. Haig is interested in evolutionary conflicts of interest--when natural selection favors different strategies for two intimately connected organisms. Mothers and children, for example, may not get the same evolutionary benefit from a pregnancy--which may explain disorders of pregnancy such as preeclampsia. (I wrote more about this here.) The new marmoset research has Haig musing anew. In the article I only had room to quote him on a couple points, but he had plenty of other intriguing ideas. One of those ideas had to do with the fact that marmosets are unusually prone to getting sick from viruses. This may have something to do with the fact that marmosets also produce relatively few receptors on the surface of their cells that the immune system uses to recognize them. Why would marmosets do something that was so harmful to themselves? It may have something to do with how marmosets become chimeras. The placentas of twin marmosets fuse, creating a network of vessels through which cells from one twin can travel to the other. Stem cells move back and forth thorugh this network. And perhaps immune cells do as well. Haig suggests that natural selection would favor twins that could attack their siblings in utero, sending out immune cells to wreak havoc. Reducing the receptors might be a way for a marmoset to avoid being recognized--and attacked--by the immune cells of its twin. Update 3/27: Haig link fixed.

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