Intentionally self-pollinating corn caused increasing revelation of mutations, some of which had negative effects on plant development. Corn has at least 32000 genes. As a diploid plant, having 2 copies of each chromosome, if the mutation only occurred on one DNA code in one of the pair, there would be an opportunity for the dominant form on the other member of the pair to cover up the deficiency. Self-pollination offered the chance (25%) of the new seed would be homozygous recessive and result in the expression of the mutation. What could be the effects of these mutations? Perhaps it is beneficial to our needs, like the sugary gene that gives us sweet corn, but homozygous recessives could also affect root growth, or vascular tissue size, photosynthetic rates or movement of sugars to the grain. Whereas keeping desirable ears for corn shows essentially maintained some heterosis within a variety, the open pollination nature of corn also allowed some selfing and therefore more homozygous genes for some negative characters in terms of grain production.
The move to inbred development was increasing the probability of homozygous negative recessives not being covered with dominant forms of the gene within the inbred. Not only could mutations be occurring during meiosis in the selfing generation but also those accumulated within the initial breeding population, whether from an F1 cross of two inbreds or from an established variety. Inbreds inevitably will accumulate an expression of mutations that have a negative effect on grain production performance. However, with the right combination, each hybrid parent will have a dominant form of enough negative recessive forms to overcome the major disadvantages of the other parent. Experience has shown that inbreds developed from Stiff Stalk Synthetic background have a high probability of showing hybrid vigor when crossed with inbreds developed from Lancaster Sure Crop background. However, corn breeders also are aware that the combinations for inbreds from these two backgrounds that show adequate performance to be commercially competitive are not frequent. This is apparently because of the frequency of negative recessives that become homozygous during the selfing process. Given the large number of genes in corn this does seem reasonable. Although a few inbreds may combine with a few inbreds of the other heterotic group to produce close to superior commercial hybrids, and thus be considered a general combiner, ultimately there will be one specific combination giving a truly superior hybrid, at least in the environments used for that hybrid.
One such hybrid 30-40 years ago was B73 X Mo17. Either of these inbreds would successfully combine with other inbreds but this specific combination was dominant in the Midwest with plant densities used during that period. However, this hybrid would also have stalk rot problems when stressed with higher plant densities or photosynthetic stress (cloudiness) more frequent in the Eastern corn belt. It is as if each combination of inbreds does not sufficiently cover all the pertinent negative genes, at least for the ultimate performance under all commercial environments but some specific crosses may be acceptable under some environments.
This blog issue emphasizes significance of gene dominance in explaining superior hybrid vigor. There are other genetic aspects as well, which will be discussed in future issues.
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.