Now days when you drive through the countryside looking at fields of 10 and 12 foot corn, you know that if everything goes right, that corn could crowd 300 bushels per acre. It wasn’t always like that. Not long ago we were seeing average yields in the 20-40 bu/acre range. It wasn’t until the 1940’s when yields started to climb to the level they currently are. The majority of this gain can be attributed to hybridization, something we take for granted today. But what is hybridization and why does it happen? Even for a guy like me whose business was built on producing and selling hybrid corn, it still seems to have some element of black magic to it.
The simple definition of hybridization is the crossing of two unrelated lines to produce offspring that are a combination of the two parents. This is probably best illustrated by thinking about dog breeding, let’s say for instance this bichon havanais crossbreed that is a cross of two popular small breeds. Or even a Golden Retriever. Golden Retrievers have been bred by a process of inbreeding – breeding them to closely related relatives over several generations until each new generation is pretty much a copy of their parent. The Golden Retriever had lots of potential; for those who prefer a dark-coated dog, a Golden Retriever was bred with a black dog such as the Flat-coated Retriever, resulting in the black Golden Retriever. You can learn more about the black golden retriever and its history on petvblog.com. It didn’t end there though for the Golden Retriever. As they are bred more, puppies that don’t conform to the set of traits the breeders are looking for are not used for breeding. Over time you have an established breed of dog with a golden coat, good hunting nose, and a sunny disposition. They are a great breed, but they shed and are prone to bad hips. Enter the poodle – another inbred line of dogs. The poodle’s strength is non-shedding fur and intelligence, but has a history of biting. You cross the two dog lines and you have a combination of the two called the Goldendoodle – the trendy new dog hybrid. The pups selected from this hybrid have the best of the positive traits…non-shedding, golden fur, intelligent, family-oriented dog with no hip problems. In short, an improved dog (if those are the traits you are looking for). If you’re an owner of a goldendoodle, you may be interested in these how to groom a goldendoodle grooming tips.
Certain species will exhibit heterosis (commonly called hybrid vigor) in the offspring after the initial inbred crossing. Corn is a prime example of heterosis. Inbred lines that would normally yield 40-100 bu/acre have offspring that can produce 200-270 bu/acre when crossed with an unrelated inbred line. This hybrid vigor is what drives the seed industry and delivers the big yields growers bank on.
Hybrid vigor is what I consider the black magic of hybridization. To understand this we must look a little deeper into the genetic code. Every plant and animal has genes that cause production of different proteins which dictate numerous traits. All genes are not created equal. Some do a better job or cause a more positive response than others that are located in the same spot. The goal of all breeding is to accumulate as many ‘favorable’ genes in your hybrid as you can. This is complicated by the fact that each gene is normally made up of two parts (four parts for some species), with only one part carrying forward and combining with one part of the other parent in its offspring.
Using corn as our example, let me illustrate the above concepts. At one time the grain of corn was actually on the tassel just like wheat, barley, and other grasses. At some point somebody noticed that through a gene mutation the corn that produced an ‘ear’ had more yield. We will call the mutated gene that causes a corn plant to produce an ear ‘C’; the non-ear producing gene will be called ‘c’.
The initial gene would be Cc (two parts). The C (dominant gene) trumps the little c, causing the corn to produce an ear. When the plant reproduces with another plant, (see Figure 1) either the C or c can be carried into the next generation and recombined with the other parent’s gene pair (which initially would be cc). So we could have the following combinations: Cc, Cc, cc or cc. At this point, half the offspring would have ears while the others would still put their seed on the tassel. All things equal, that is how things would stay – half eared versus half non-eared corn. But then someone came along and wanted the eared version. To do this, we only breed the eared versions to other eared versions (Cc x Cc), which could produce the following combinations: CC, Cc, Cc and cc. In this case, the offspring of that cross may still produce a non-eared version 25% of the time. To have no chance of a non-eared parent, you would have to cross two CC parents (created above) to eliminate the c from the equation. Welcome to inbreeding. We just crossed corn to closely related corn to eliminate any chance of a negative gene.
Inbreeding is great for locking in desired traits so the offspring have the same traits (think dog breeds). There is a downside to inbreeding though. As you are locking in the desirable genes, you may also be locking in undesirable traits located at different locations in the DNA. Inbreds developed from one common parent and bred back to it will have an extremely high chance of carrying any bad genes with them. MM x MM will always produce an MM, not an mm. (The big M representing the bad gene in this case.) Thus, a much higher expression of hip problems in certain breeds of dog.
This is where hybridization comes in and generates hybrid vigor. Crossing the right inbred lines to one another can eliminate the weaknesses of both, making a better, higher yielding corn plant.
This is best illustrated by the following table:
|Yield Gene 1||Yield Gene 2||Yield Gene 3||Yield Gene 4|
Inbred A has positive yield expression in two of four known yield genes, while Inbred B has three of four positive yield genes. The Hybrid Offspring of these two would have positive yield results in all four yield genes since they all have a big A, B, C or D in the pair, making it dominant. If all genes produced the same amount of yield gain, the hybrid would have 25% more yield than Inbred B and 50% more than inbred A. Hybrid vigor explained!
So why is hybrid vigor a one year phenomenon in corn? Why do we have to keep going back to a seed company to get first generation hybrid seed? The answer is simple. A hybrid in a field is pollinating itself and reintroducing negative genes back into itself.
|Yield Gene 1||Yield Gene 2||Yield Gene 3||Yield Gene 4|
Let’s take our table out another year with the hybrid crossing with itself. Here the offspring will have a 25% chance of not having the positive yield gene in three out of four of the genes. 100% yield expression would now only be 81% yield on average with some offspring still at 100% and some as bad as 25%, which is worse than the original inbreds.
In theory you would not need hybridization if you could have an inbred with dominant positive genes at all sites that produced higher yield. At the moment there are roughly 250 known sites for yield in corn. Without breeding, that would translate to 62,500 possible combinations. With DNA fingerprinting, it is thought that they can get 70% of these sites populated with positive genes. But those sites may only deal with yield, not traits such as maturity, standability, disease resistance, and drought tolerance. Each site added doubles the number of combinations that need to be screened to find the “perfect” corn.
Worse is the fact that often single genes are not driving factors. Often multiple gene sets (gene clusters) can drive trait expression. The absence of any one of the genes in that cluster can neutralize the benefit. Finding the gene clusters is much harder than determining the function of a single gene.
Further complicating the process is that gene clusters effectiveness can be very environmentally dependent. Certain gene clusters are effective only when conditions are dry, while others may work under wetter conditions. This is why much of a seed company’s resources are spent testing hybrids over a variety of conditions. They are not looking for just high yield, but highly stable yields regardless of conditions.
Hybridization is not going away any time soon. Modern genetic tools are playing a roll in speeding up the process that in the past was very time consuming and somewhat guesswork. Hybridization and adding the herbicide and insect resistant genes from outside sources have gone a long way in pushing the yield ceiling higher every year. The next Earfull will further discuss this theme with how marker assisted breeding is helping this process.