Genetic uniformity paradox
Crops such as soybean, wheat, rice and sorghum that are normally mostly self-pollinated show significantly less variety improvement in production of their harvested product than easily cross-pollinated crops like corn. The physical distance between the male and female structures of a corn plant plus the ease of aerial distribution of pollen from the extended anthers on the tassel further assures that most pollen will likely not land on the silk of the same plant, even during a day without wind. The other crops have both male and female structures within the same plant, making it much easier for selfing.
This ease of cross pollination in maize has provided the species with great diversity historically, as it became adapted to a huge range of environments world-wide. The enigma of this phenomenon is the advantage of genetic variability can also lead to disadvantages in grain production within a field. Diversity can allow selection of preferred traits such as grain type, harvest features, germination, disease resistance and mineral uptake, but each of these inherited characters require the genetic consistency from the controlled crosses of specific homozygous parents.
Obtaining the homozygous parents, in which a member of each chromosome has identical genetic code as the other member of the chromosome pair is the goal of placing pollen on the silk of the same plant for several generations or made by crossing pollen from haploid inducer onto the breeding source plant. A small percentage of the plants kernels will have only a single member of each chromosome. Chemicals applied of these haploid kernels cause the chromosomes to duplicate, forming homozygous plants. Regardless of the method of obtaining homozygosity, some of the genes have negative effects on the plants growth. Inbreds of any species carry expression of negative genes where-as crossing with other inbreds that have a dominant form of the gene can block that expression. Identifying that potential for two inbreds to express this hybrid vigor does not require complete homozygosity in the parents but reproduction of the parents for future hybrid consistency requires at least near-homozygosity for ultimate development of hybrid parents.
Inbreds used as hybrid parents often express negative characters for efficient seed production. Those used as females in a seed field, must have the capacity of dependable germinations, characters partly determined by the non-chromosomal DNA within mitochondria. Male parents must have adequate and dependable pollen production and release. Dependable extension of silks when plants are stressed, and large number of kernels are important female inbred characters. Seed producers make considerable effort to overcome the weaknesses of the inbreds and assure maximum advantage for the hybrid corn grower.
Inbreeding causes the successful, repeatable production of hybrid seed. Inbreeding also exposes the negative genes that detract from the inbred performance.
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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.