James and the Giant Corn Genetics: Studying the Source Code of Nature

It’s a common leap when people start talking about Bananas, and how their lack of genetic diversity meant it was impossible to breed resistance to the blight that destroyed them (see more here), to then jump to condemnation of our modern, monoculture based agriculture as a fragile edifice waiting to be toppled over by the first strong breeze (disease, climate change, oil running out, whatever the disaster of the day happens to be).

Obviously I disagree.

Every Gros Michael banana was genetically identical to every other. The bananas were propagated vegetatively (a natural form of cloning) because they were sterile, producing no seeds, and because it meant the same traits that made being love a Gros Michael banana were found in EVERY Gros Michael banana they ate. After fungus wiped them out, banana producers switched to the Cavendish banana but that just swapped one identical banana genome, spread across millions of plants, for another, which is why banana cultivation is threatened again by a fungus for which no Cavendish has any natural resistance.

A monoculture of bananas left them vulnerable to being wiped out, so surely the vast monoculture of corn spread across the heart of our nation is even more at risk? No. For three reasons. Corn isn’t sterile, corn is hugely diverse, and corn has crop breeders.

The fact that corn plants can actually mate with each other is what enables the seconds two points. Plants that mostly or only fertilize themselves(like soybeans), and plants that don’t reproduce sexually at all (like bananas or seedless oranges) have a harder time maintaining genetic diversity than species like corn that, if left to their own devices will breed wildly. Every kernal on a corn cob might have been fertilized by a different plant. Corn’s reproductive system also makes things much easier on breeders who can either control who mates with who using paper bags, or mate a single plant with itself again and again to produce the highly inbred lines used in the production of hybrid seed.

What’s this about diversity? Every reader of Michael Pollen’s work knows that every corn plant in a field is genetically identical as much clones of each other as bananas.* What doesn’t seem to get mentioned (outside of maize genetics conferences) is how big the genetic differences are between the corn in one field, and the corn in the next field over. The genetic differences between two unrelated lines of corn are greater than between your average human, and your average chimpanzee. The remarkable thing about corn isn’t how much the same it is, it’s that it still manages to be one species with that much genetic variation crammed into it!

And finally breeders. Bananas are sterile. Plant breeders can’t work with plants that don’t breed, only genetic engineers can. Corn is a breeder’s dream, with separate male and female flowers that make it easy to control which pollen fertilizes which ear, and big ears that mean a single cross can easily produce two hundred or more individual kernals. And there’s so much diversity to select from. It’s selecting from diversity that brings up the issue of pyramiding that I was addressing in my last post. If a breeder discovers three different genes which have versions that protect against a disease he can either breed each of them into a third of the lines of corn he develops, (more diversity), or put all of them into all the corn (pyramiding). It turns out pyramiding provides more protection when a breeder has identified resistant genes even though it’s marginally less diverse.

*The mechanism is more complex. Two very different inbred lines are breed together, and the hybrid offspring (what perform amazingly well compared to their open pollinated relatives) have all inherited the same set of chromosomes from their mother and a different (but also identical from plant to plant) set of chromosomes from their father. And it’s not even that cut and dried since some fraction of the seeds in any bag of hybrid seed are actually maternal inbreeds, seeds pollinated by the plants intended by the breeder to be the mother, not the father.