What is a hybrid cultivar?

A hybrid cultivar, by definition, is the F1 offspring of a planned cross between inbred lines, cultivars, clones, or populations. Depending on the breeding approach,

the hybrid may comprise two or more parents. A critical requirement of hybrid production isthat the parents are not identical. As will be discussed next, it is this divergence that gives hybrids their superior performance. The outstanding yields of certain modern crops, notably corn, owe their success to the exploitation of the phenomenon of heterosis, which is high when parents are divergent. Much of what we know about hybrid breeding came from the discoveries and experiences of scientists engaged in corn hybrid cultivar development. However, commercial hybrids are now available for many crops, including self-pollinating species.

Brief historical perspective

One of the earliest records on hybridization dates back to 1716 when American Cotton Mather observed the effects of cross fertilization in maize, attributing the multicolored kernels to wind-borne inter-mixture of different colored cultivars. However,it was the German T.G. Koelreuter who conducted the first systematic studies on plant hybridization in 1766. Even though previous observations had been

made to the effect that offspring of crosses tended to exhibit superior performance over the parents, it was G.H. Shull who, in 1909, first made clear scientificbased proposals for exploiting heterosis to produce uniform and high yielding cultivars. Unfortunately, the idea was at that time impractical and potentially expensive to commercially exploit. In 1918, D.F. Jones proposed a more practical and cost-effective approach to producing hybrid cultivars by the method of the double-cross. Double-cross hybrids produced significantly more economic yield than the single-cross hybrids originally proposed by Shull. Single-cross hybrid seed was then produced on weak and unproductive inbred parents, whereas doublecross seed was produced on vigorous and productive single-cross plants. The corn production industry was transformed by hybrids, starting in the 1930s.

Other notable advances in the breeding of hybrids were made by researchers, including M.I. Jenkins in 1934 who devised a method  to evaluate the effectiveness of parents in a cross. Through this screening process, breeders were able to select a few lines that were good combiners for use in a hybrid breeding.

 The next significant impact on hybrid production also came in the area of techniques of crossing. Because corn is outcrossed and monoecious, it is necessary to emasculate one of the parents as part of the breeding process. In the early years of corn hybrid breeding, emasculation was accomplished by the labor-intensive method of mechanical detasseling. The discovery and application of cytoplasmic male sterility to corn hybrid programs eliminated the need for emasculation by the late 1960s. Unfortunately, the success of CMS was derailed when the Texas cytoplasm, which was discovered in 1938 and was at that time the dominant form of male sterility used in corn breeding, succumbed to the southern leaf blight epidemic of 1970 and devastated the corn industry. It should be mentioned that mechanized detasselers are used by some major seed companies in hybrid seed production of corn today.

Realizing that the limited number of inbred lines used in hybrid programs did not embody the complete genetic potential of the source population, and the need to develop new inbred lines, scientists embarked on cyclical recombination  to generate new variability and to improve parental lines. Breeders were able to develop outstanding inbred lines to make single-cross hybrids economical enough to replace double-cross hybrids by the 1970s. By this time, corn hybrid production programs had developed a set of standard practices consisting of the following, as observed by N.W. Simmonds:

_ Maintenance and improvement of source population by open pollinated methods.

_ Isolation of new inbreds and improvement of old Ones.

_ Successive improvement of single-cross hybrids by parental improvement.

_ CMS-based seed production.

The application of hybrid methodology in breeding has socioeconomic implications. The commercial seed industry has rights to its inventions that generate royalties. More importantly, because heterosis is maximized in the F1, farmers are generally prohibited from saving seed from the current season’s crop to plant the next year’s crop. They must purchase seed from the seed suppliers each season. Unfortunately, poor producers in developing countries cannot afford annual seed purchase. Consequently, local and international  efforts continue to be largely devoted to producing propagable improved open-pollinated cultivars for developing countries.