A hybrid plant results from a cross of two genetically different plants. The two parents of a single-cross hybrid, which is also known as a F1 hybrid, are inbreds. Each seed produced from crossing two inbreds has an array (collection) of alleles from each parent. Those two arrays will be different if the inbreds are genetically different, but each seed contains the same female array and the same male array. Thus, all plants of the same single-cross hybrid are genetically identical. At every locus where the two inbred parents possess different alleles, the single-cross hybrid is heterozygous.
Plants of a single-cross hybrid are more vigorous than the parental inbred plants. In Figures 2a and 2b, the single-cross hybrid plant and ear are shown with the plants and ears of the parental inbreds. Clearly, the hybrid plant is taller and the hybrid ear is larger. The increase in vigor of a hybrid over its two parents is known as hybrid vigor.
Breeders often measure the degree of hybrid vigor of a trait with the following formula:
where Hyb = the value of the trait in the hybrid and
MP = the average (mid-parent) value of the trait in the two parents. For For example, in Figure 2a the height of the single-cross hybrid is 3.0 m (this equals Hyb), the average height of the inbreds is 2.0 m (this equals MP), and the value of hybrid vigor is 50%. Hybrid vigor calculated in this way is called mid-parent hybrid vigor. Another type is high-parent hybrid vigor. This is the superiority, expressed as a percentage, of the hybrid over the parent with the better or higher value of the trait. Corn breeders will be successful in increasing hybrid performance if the hybrid vigor of a new hybrid compared to an older hybrid is increased and the two sets of parents have equal performance and/or if hybrid vigor is unchanged but the mid-parent value of the parents of the newer hybrid is superior to that of the parents of the older hybrid.
The genetic basis of hybrid vigor is not completely understood. However, experience has shown that a hybrid produced by crossing two inbreds that are closely related usually will exhibit less hybrid vigor than a hybrid produced by crossing inbreds that are more distantly related.
If a single-cross hybrid is allowed to open-pollinate (pollen is dispersed freely), each of the plants grown from the resulting seed will be genetically unique. To understand why this is so, first consider a single locus. All plants of a single-cross hybrid are genetically identical, so at a single heterozygous locus any cross- or self-pollination occurring with open-pollination can be represented as:
One-half of the egg cells produced by each plant carries the A1 allele and one-half carry the A2 allele. The same is true of the pollen cells. The egg and pollen cells then combine at random during pollination.
The A1A2 and A2A1 genotypes are functionally identical, so three unique genotypes can be produced at a single heterozygous locus when open-pollination occurs.
With two heterozygous loci, A and B, the situation can be illustrated as follows:
In the two-locus case, nine unique genotypes are produced. This occurrence of multiple genotypes among progeny arising from the self- or cross-pollination of parents that all have the same heterozygous genotype at one or more loci is known as genetic segregation.
This segregation occurs at hundreds or even thousands of gene loci. The number of unique genotypes resulting from genetic segregation at n loci is given by 3n. Thus, if n=1 (i.e., one locus), then the number of unique genotypes is three, and if n=2 the number of unique genotypes is nine. But, if n=20, the number of unique genotypes balloons to 3,486,784,401 (=320) Any commercial single-cross hybrid of corn is likely heterozygous at many more than 20 loci. That is why open-pollination of such a single cross results in progeny that are all genetically unique.
The progeny produced from self-pollination of a F1 single-cross hybrid are known as F2 plants. On average, F2 plants will have vigor that is approximately half-way between the single-cross parental plants and the average of the two inbred grandparents; that is, half of the hybrid vigor is lost. This is illustrated in Figure 3. The F2 ears on the bottom row vary in size, but on average are larger than the ears from their inbred grandparents and smaller than the ear from their single-cross parent. That is why farmers have an incentive to purchase new single-cross hybrid seed each year.
When single-cross hybrid seed is commercially produced, one inbred is the male parent and the other the female parent. Either the female parent must be male-sterile (pollen is not produced or is not functional) or the tassel on each female plant must be removed (this is called detasseling) prior to any pollen production (Figure 4). In either case, all the seed produced on the female parent will be single-cross hybrid seed.
Developing an inbred from a single-cross hybrid requires approximately seven generations of repeated self-pollinations (Figure 1). Each year in the United States, commercial seed companies produce hundreds of new inbreds and test in field trials many thousands of new single-cross hybrids obtained by crossing these inbreds. Compared to existing commercial hybrids, the vast majority of these new hybrids will be poorer or no better in performance. Only the hybrids that have superior performance in these trials are produced in mass quantities and sold as commercial hybrids to farmers.
Considerable time and inputs are required to develop, select, and produce single-cross hybrids. Achieving a high level of cost efficiency of these processes typically requires large-scale operations.