Breeding corn as a variety, the seed selected from open-pollinated plants displaying the traits preferred for grain production is relatively simple compared to breeding parents for hybrids. The advantages of single cross hybrids coming from uniformity of crossing 2 homozygous inbreds that have genetics resulting in heterosis in traits that provide superior performance for grain production, standability and disease resistance drive the incentive for the more difficult process of hybrid breeding.
Experience and experimentation showed that crosses of inbreds developed from different general backgrounds have a high probability of expressing heterosis. The inbreeding process, however, sorts those 30-40000 genes, in each generation of inbreeding from the beginning population with randomness. Although each inbred from a base population may share some characteristics, and many can be immediately discarded by the breeder visually with each generation of selfing, the ultimate test of acceptability comes after evaluation of performance after crossed with potential heterotic partners. Hybrid corn breeding programs devise methods to solve the conundrum of selecting preferable inbred characters while considering heterotic performance efficiently.
Traditional methods of selfing heterozygous seeds for several generations before crossing with potential hybrid partners to evaluate hybrid performance has the frequent disappointment of considerable effort over several generations because of hybrid performance. Making hybrids from potential, but not completely, homozygous inbreds has been used to more quickly and efficiently select desirable inbreds but carries the risk of genetic drift after further selfing.
Dihaploid corn breeding involves crossing heterozygous plants with pollen from a haploid inducer, resulting in up to 10% of resulting embryos only having the one set of chromosomes from the female plant and none from the male. Treatment with specific chemicals can cause up to 50% of these embryos to double this single set of chromosomes, resulting in completely homozygous two sets of chromosomes, a dihaploid. These dihaploid plants can be selfed to produce a small quantity of distinct inbreds in three generations. These dihaploids are still a random set of genes from the parent stock, the value of which must be identified in hybrid performance.
PSR's Rapid Inbreeding® (RI), a system of shortening the time to developing corn inbreds, utilizes the natural homozygosity that occurs in any genetic segregating population. PSR utilizes its skills of evaluating seedling phenotypes to pick out the individual plants that display characters most identifiable with near homozygosity. These seedlings are transplanted, grown to flowering and selfed. This seed is then crossed with potential heterotic inbreds for hybrid evaluation. These near-inbred plants are sufficiently homozygous to assure that future increases from these plants will show similar hybrid performance to that shown in the initial tests.
Breeding for hybrids is more complex than variety breeding but the final reward can be greater as well.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.