James and the Giant Corn Rotating Header Image

November 1st, 2009:

Domestication Bottlenecks

Driveway tomato garden. How much diversity do these plants contain?

Driveway tomato garden. How much diversity do these plants contain?

Crops like tomatoes, even heirloom tomatoes, aren’t found in the wild. Domestication of crops usually involves only a relative handful of individual plants. Narrowing the species down to a few hundred (or possibly even a few dozen plants) means only a limited number of copies of each gene will be carried through and many of the variant copies of the genes present in the wild population won’t be included in that number. Keeping the population small for multiple generation reduces variability even more as by chance some rare version of genes in one generation won’t be passed to any of the offspring in the next.

Genetic bottlenecks happen in the animal world as well. Skin grafts between unrelated Cheetahs aren’t rejected because the animals are so genetically similar their immune system can’t distinguish the grafted skin as being different from its own skin. Even less fortunate are the tasmanian devils who have so little genetic diversity that they are being decimated by a transmissible cancer. After fighting with an infected devil, which has tumors on its face and neck, tiny bits of the cancer will get into an uninfected devil’s wounds, and since the immune system can’t distinguish the foreign cancer cells from the devil’s own cells, the cancer cells reproduce unchecked, the trait that makes normal cancers, produced by mutated versions of our own cells, so deadly. And the solution mentioned in the article, to save the species by protecting 200 individuals, while better than letting them all die, will sacrifice even more genetic variability by subjecting the already inbred devils to a new population (and genetic) bottleneck. (more…)

Disease Resistance and Diversity

Matt has a great new post up on The Scientist Gardener called The Myth of Monocultures. He gives talks about the PBS special on Michael Pollen’s Botany of Desire (which I still need to watch myself), addresses some misconceptions about diversity and monoculture and talks about the best way to ensure crops aren’t wiped out by pathogens that can evolved to overcome a single resistance gene:

Theoretically, it would be useful to maintain crop populations with diverse resistance genes. The industrial application of this (multilines) invovles breeding many different versions of a favorite crop variety that are identical except for their resistance genes. As appealing as this idea is, it hasn’t really worked out in the real world. The alternate approach (pyramiding) seems to be more effective. Here, many different resistance genes are combined into a single crop variety. Pests and pathogens may be able to overcome a single gene at a time, but it’s usually almost impossible to simultaneously overcome several.

Think of it like the story of the three little pigs. [spoilers ahead ;)] (more…)