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

August 19, 2009

In Defense of Hybrids

Filed under: Uncategorized — James @ 12:17 am

Hybrids are good. Excellent even. If anyone tells you otherwise, you send them to me.

Alright I’ll take a couple of steps back. Hybrids are always hard to explain, I’ve tied myself in nots a couple of times tying to, but maybe this time is the charm. At it’s most fundamental levels, a hybrid is nothing more than the offspring of two dissimilar parents. When Gregor Mendel discovered the fundamental principle of genetics* he wasn’t setting out to study genetics, or the basic principles of life, but instead the traits of the offspring of dissimilar pea plants. And he published his findings under the title Experiments on Plant Hybridization.

So if hybrid are just the offspring of dissimilar parents, why are they so important? The answer is that not always, but sometimes, hybrids are unusually big, strong, or tough, surpassing either of their parents. As a maize geneticist the classic picture is the one below, in an unfortunately small photo, depicting two maize inbred lines B73 and Mo17 to the left and right respectively.

Between is planted the offspring of a mating between the two lines, a hybrid. Obviously it is much taller. Less obviously, it will yield much more grain, and will better survive dry spells. Hybrid seed was first widely planted in the 1930s and 1940s and you can see for yourself what hybrid breeding (along with synthetic fertilizer, which is a more complicated story) did to the maize yield in this country:

Credit to the plant and soil sciences department of University of Nebraska-Lincoln
But while agricultural crops are my thing, other people are more into animals. The classical example from the animal kingdom (which predates intentional produced maize hybrids by thousands of years) is the mule. The mule is an interspecific hybrid** between horses and donkeys. Dating back thousands of years mules have been valued as possessing the best traits of both donkeys and horses and some that exceed either of their parents (anecdotally the last category includes increased intellegence). Thousands of years ago they pulled hittite war chariots and today mules are in use by US Marines fighting in Afghanistan. Along with oxen***, mules were vital in the early farms of the American midwest and west.

One of the few downsides of the mule is that they are sterile. The only way to get more mules is to keep breeding donkeys and horses together. Sterility is a common problem for hybrids, especially in the animal world, but even if it wasn’t, we wouldn’t have to worry about being overrun by the best stronger offspring of crosses between species. The offspring of fertile hybrids (like maize hybrids) are only half as diverse as their parents. (See my forthcoming post on principles of genetics) and loose a lot of the special vigor found in the first generation hybrids. The third generation cuts genetic diversity in half again. And so on.

The offspring of two similar individuals, for example between two siblings or a single plant pollinating itself, has the opposite effect of the vigor exhibited by the offspring of dissimilar individuals. Plants are shorter, weaker, yields go down. Animals become sickly and small. They suffer from genetic diseases, and reproductive impairments. This is called inbreeding depression. The opposite of hybrid vigor. It is the reason why people should avoid having children with their brothers, sisters, or cousins. And the reason why pedigreed dogs are often less healthy and resilient than their mutt brethren.

So now you know what hybrids are. It’s a method of breeding that’s been around for thousands of years. It produces biggest, stronger, better plants and animals (that have been of major benefit for civilization in general and in particular the agricultural technology that now feeds six and a half billion people), but ones that do not breed true. Fortunately we can also propagate the parents of our favorite hybrids and produce more by breeding those parents together. You or a farmer can even produce their own hybrid seed. Anyone with access to B73 and Mo17**** can both propagate these lines from year to year and make a, now outdated, hybrid. If you as an experimenter or self sufficient farmer want something more modern, Pioneer and some of the other seed companies patented a bunch of inbreds back in the 90’s that either are or soon will be out of patient which means anyone can order relatively modern inbred seed from ATCC.

So hopefully the next time someone tells you hybrids are unsustainable, or just part of a plot by seed companies to force farmers to buy new seed every year, or just unnatural and weird tell them why they’re wrong. This post inspired by a post at Genetic Maize.

*Mendel worked with crosses between pea plants dissimilar for a single trait. For example green and yellow peas, or wrinkled and smooth peas. His work lead among other things to the discovery of the principle of segregation: that each individual contains two versions of every heritable trait, and each of their gametes (sperm or eggs) contains one or the other of these copies. The previous thinking had been that offspring resulted from a sort of averaging on their parents. Which couldn’t explain how a child with blue eyes could be born to two brown-eyed parents (no infidelity required) or how new extremes can arise from artificial selection (look at the size difference of wild and domestic turkeys) or the change in oil composition between normal maize and the Illinois high oil line.

**An interspecific hybrid is the result of a mating across species lines unlike hybrid maize which is a hybrid between dissimilar members of the same species.

***Oxen bring up a whole different discussion on how the body is effected by the loss of hormones from the reproductive tissues

****Two important inbred lines that once formed an important line of hybrid corn and are still incredibly important in the research community. B73 was the inbred line from which the maize genome has been sequenced.

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