The invention relates generally to corn breeding and, more specifically, to an inbred corn line designated ZS0560. Corn or maize (Zea mays L.), is an agronomic crop of great commercial significance both in the United States and in many countries of the world. Corn is used as feed both for animals and humans and has a wide variety of industrial applications. At the commercial production level, the dominant form of corn is single cross hybrids or varieties. Homozygous inbred lines, although not grown as a commercial crop, are extremely important as the source material for the production of hybrid varieties. The cross-pollination of two distinct homozygous inbred lines produces a first generation hybrid variety that is heterozygous at many gene loci.
The goal of plant breeding is to combine in a single variety/hybrid various desirable agronomic traits. For field crops, these traits may include resistance to diseases and insects, reducing the time to crop maturity, greater yield, and better agronomic quality.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinated if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two homozygous lines produce a uniform population of hybrid plants that may be heterozygous for many gene loci.
Corn plants (Zea mays L.) can be bred by both self-pollination and cross-pollination techniques. Corn has separate male and female flowers on the same plant, located on the tassel and the ear, respectively. Natural pollination occurs in corn when wind blows pollen from the tassels to the silks that protrude from the tops of the incipient ears.
The development of corn hybrids requires the development of homozygous inbred lines, the crossing of these lines to make hybrids and the evaluation of the resulting hybrids. Pedigree breeding and recurrent selection breeding methods are used to develop inbred lines from breeding populations. Breeding programs combine the genetic backgrounds from two or more inbred lines or various other broad-based sources into breeding pools from which new inbred lines are developed by successive generations of inbreeding and selection of desired phenotypes. The new inbreds are crossed with other existing inbred lines and the single cross hybrids from these crosses are evaluated extensively in field performance trials to determine which of those have agronomically desirable potential.
New inbred lines are developed by plant breeders through cross pollination of breeding materials having complementary desirable characteristics which results in a breeding population genetically segregating for a number of important agronomic traits. After initial development of this breeding population, breeders perform a number of successive generations of inbreeding and selection. The objective of these inbreeding and selection generations is to identify and genetically fix a new inbred line which has improved agronomic characteristics. These improved characteristics may be per se traits such as seed yield, seed quality, or disease resistance. However, the primary objective of most breeding programs is to identify new inbred lines which produce improved hybrid plant characteristics such as grain yield and harvestability when the new lines are crossed to other existing inbred lines to produce hybrids.
The process of determining whether newly developed inbred lines provide improved characteristics in hybrids produced using the new line involves extensive field evaluation and testing of the hybrid product. Each newly developed inbred line provides potentially much different contributions to hybrid combinations than its progenitors. Each inbred corn line also is true breeding. In layman's terms this means that when an inbred corn line is planted in isolation and allowed to pollinate itself, the resulting progeny will produce corn plants essentially genetically and phenotypically indistinguishable from the parent inbred.
A single cross hybrid corn variety is the cross of two true breeding inbred lines, each of which has a genetic composition which complements the other. They hybrid progeny of the first generation cross between two parent lines is designated "F.sub.1 ". In the hybrid seed development process, the F.sub.1 hybrid seed is that which is sold to and planted by commercial growers. The preferred F.sub.1 hybrid seed source is that which produces the highest level of vigor, agronomic strength, and yield compared to other F.sub.1 hybrid alternatives.
Because the parents of an F.sub.1 hybrid are true breeding, the seed resulting from a cross between the two parents will all be genetically identical, and will thus yield a stable and predictable phenotype in the commercial grower's fields. Also, because these parents are true breeding, they can be individually reproduced continually by open pollination in an isolated environment. Thus, it follows that the F.sub.1 hybrid can be continuously reproduced from the parent lines.
A single cross hybrid is produced when two inbred lines are crossed to produce the F.sub.1 progeny. A double cross hybrid is produced from four inbred lines crossed in pairs (A.times.B and C.times.D) and then the two F.sub.1 hybrids are crossed again (A.times.B).times.(C.times.D). Much of the hybrid vigor exhibited by F.sub.1 hybrids is lost in the next generation F.sub.2. Consequently, seed from hybrid varieties is not used for planting stock.
Corn is an important and valuable field crop. Thus, a continuing goal of plant breeders is to develop high-yielding corn hybrids that are agronomically sound based on stable inbred lines. The reasons for this goal are obvious: to maximize the amount of grain produced with the inputs used and minimize susceptibility to environmental stresses. To accomplish this goal, the corn breeder must select and develop superior inbred parental lines for producing hybrids. This requires identification and selection of genetically unique individuals which in a segregating population occur as the result of a combination of genetic recombinations plus the independent assortment of specific combinations of alleles at many gene loci which results in specific genotypes. Based on the number of segregating genes, the frequency of occurrence of an individual with a specific genotype is less than 1 in 10,000. Thus even if the entire genotype of the parents has been characterized and the desired genotype is known, only a few if any individuals having the desired genotype may be found in a large segregating population. Typically, however, the genotype of neither the parents nor the desired genotype is known in any detail.
Because commercial corn production relies on the development of novel and improved inbred corn lines, considerable money and effort is devoted by commercial seed companies and plant breeders to the development of inbred lines with the combining ability to produce first generation hybrid corn with the characteristics of high yield, resistance to disease and pests, improved plant stability, uniform phenotypical characteristics to facilitate machine harvesting, and so on, all with the goal of maximizing the efficient use of land and other resources in the production of foodstuffs and raw agricultural materials.
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