It was recognized more than a century ago that crossing unrelated corn varieties often resulted in increased vigor in the progeny. It was known that self-pollinating corn plants a few generations resulted in smaller plants. Heterosis, that boost in plant size coming from the combination of genetics from two unrelated parents, allows the preferable version of a gene result in production of some metabolic product either more efficiently or in greater quantity, than the version in the other parent. It is assumed that this is usually a dominant form of the gene that overcomes the recessive form in the new hybrid.
Realizing that inbreeding would lead to depression of the plant size and grain production but that combining with the right other parent would not only restore these characters but could produce plants more productive than the original breeding stock. The large number of genes of corn, genetic diversity due to its history and cross-pollination biology, natural mutation rate, annual life cycle and human’s affecting phenotype selection, has led to huge opportunities for occurrence of detrimental versions of genes.
The commercial plant breeder’s dilemma in inbreeding must be done to obtain uniform, dependable genetics in parent stalk inevitably resulting in some negative versions of some genes and yet identify another inbred that will essentially cover up the weakness of the other hybrid parent. A traditional method of inbreeding is to grow and evaluate each selfed generation for desirable characters until the breeder is satisfied that the material is almost completely homozygous. Most of the hybrid performance can be indicated by making crosses with other parents in earlier generations but lack of complete homozygosity can lead to problems of maintaining the inbred genetics with increasing of seedstock. Waiting to test for heterosis until 6, or more, generations of selfing has led to disappointment as a homozygous inbred with excellent phenotypic characters can be identified but no other parent can be identified to cover up its few genetic deficiencies. Several seasons of breeding but no commercial product is disappointing.
Complete homozygosity can be made quicker by crossing with pollen onto silk of corn genetic source in which the female plant makes no genetic contribution to the seed. Instead this seed contains only one set of chromosomes, those of the male. But the plant being haploid is weak, However, usually a specific chemical, a low percentage of haploid embryos can be doubled, resulting in totally homozygous plants. This results in a very low percentage of plants becoming homozygous but the occurrence is genetically random and not always successful with every genetic background. It is quicker than the traditional method of several generations of selfing but must accept the lack of ability to adjust and select for minor changes.
A third way invented by PSR Inc. is to select and self plants from a large population of genetically segregating plants of those that are most homozygous. This is done by applying years of experience of evaluating corn seedlings in controlled environments. These selections are sufficiently homozygous for evaluation in hybrid performance, while continuous selfing generations are used to reach the final level of homozygosity.
The large number of genes and the diversity allowed by corn’s biology has allowed this crop to be productive in multiple environments. Continual fluctuation of crop environments will forever require new genetic combinations to meet demands for this amazing carbohydrate producer.
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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.