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Corn Breeding: Lessons From the Past

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Inbreeding, Hybrid Vigor, and Hybrid Corn

At about the same time the corn shows were peaking in popularity, in the northeast United States two researchers were experimenting with controlled pollinations in corn. Their findings would soon cause a remarkable change in the corn culture of North America. Both Edward East at the Connecticut Experiment Station and George Shull at the Carnegie Experiment Station on Long Island had begun to self-pollinate corn.

Under self-pollination, the silks of an ear are pollinated by pollen from the same plant. Typically, little self-pollination occurs in a field of corn. Most silks of a given plant are pollinated by pollen from surrounding plants. This is known as cross-pollination (Figure 8).

Fig. 8: Cross pollination in corn


When a plant from an open-pollinated variety is self-pollinated, all the progeny resemble that plant, although they all differ from each other and from the parent plant to some extent. If one of the progeny plants is self-pollinated, the new progeny again differ from each other and from the parent plant, but the degree of the difference is not as great as occurred after the first self-pollination. If this process is repeated about seven times, then a plant known as an inbred is produced. An inbred is a pure-breeding strain of corn. This means that if an inbred is self-pollinated, all of the progeny will be genetically identical to each other and to the inbred parent.

Fig. 9: Inbreeding in a variety of corn. (University of Nebraska-Lincoln, 2004)


This process of repeated self-pollinations is known as inbreeding. Inbreeding corn results in loss of vigor (Figure 9).

n figure 9, the plant at the far left is non-inbred, the plant second from left was produced by one generation of self-pollination, and the two plants on the right were produced by two generations of self-pollination. Inbred plants developed from open-pollinated varieties of corn are not as vigorous or high yielding as the non-inbred plants of the open-pollinated varieties. But what East and Shull observed was that when the self-pollinated plants were cross-pollinated to produce hybrid progeny, these plants sometimes were even more vigorous than the plants from which the inbreds had been developed. This phenomenon is called hybrid vigor (Figures 10a and 10b).





 


Fig. 10a: Inbred plant B73 (left), inbred plant Mo17 (middle), and hybrid plant B73 x Mo17 (right). (University of Nebraska-Lincoln, 2004)

Fig. 10b: B73 ear (left), B73 x Mo17 hybrid ear (middle), and Mo17 ear (right) (University of Nebraska-Lincoln, 2004)

A hybrid developed by crossing two inbreds is known as a single-cross hybrid. As is true for an inbred, all plants of a single-cross hybrid are genetically identical to each other. The productivity of a hybrid depends on the relationship between the two inbreds. A hybrid produced by crossing two inbreds developed from different but equally productive open-pollinated varieties usually will produce a more vigorous hybrid than one produced by crossing two inbreds from the same open-pollinated variety.

When a single-cross hybrid is allowed to open-pollinate (as happens in a farmer’s field), approximately half the hybrid vigor is lost. This is the basis of the hybrid corn seed industry. The crop produced from open-pollinated seed harvested from a single-cross hybrid will not be as productive as the original single cross.

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