Seed producers attempt to make genetically pure hybrid seed by using homozygous parents. This is not easy. Inbreeding theoretically makes gene have the identical code on the other member of the chromosome pair and thus be homozygous dominant or homozygous recessive. Small mutations can occur along the way of developing these inbreds that can interfere with the process. A larger potential source of problems can come as the inbred in increased when unintended pollen invades the seed increase field.
Corn pollen viability can last only a few hours if the weather permits. Most corn pollen falls within a few feet to a few hundred feet, depending upon wind, although there is evidence of viable corn pollen found ½ mile from the source if wind and humidity is favorable. Fields for increasing parent seed inbreds are consequently isolated from other corn fields at long distances to minimize the contamination.
Hybrid production using pure inbred parents also must struggle with the same problem but with a few more dynamics. Both parents are inspected visibly looking for off type plants in the male and female rows, removing those that are obvious. Most off types will show some hybrid vigor over the inbreds and thus can be easily identified and removed before pollination. Hybrid seed is produced by preventing pollen from the designated female plants by either removing the tassels or use of male sterile female inbreds. The intent of the seed producer is to have adequate pollen from the male parent to cover all exposed female silk as they emerge. The small presence of foreign pollen in the air at the same time always causes the potential for contamination, making the timing of male parent pollen production essential. Environments greatly affect the success of this endeavor. Dry field conditions tend to delay silk elongation whereas it has little effect on pollen production. This can cause most male parent pollen to be released before silks are exposed. Wet field conditions allow more elongation of silks causing the risk of silks exposed before male inbred pollen is released. Exposure of silks when little intended pollen is present increases the probability of the wrong pollen landing the on the silk and thus fertilizing the egg cell in the ovule at the end of the silk. The first viable pollen grain to arrive ‘wins’!
Unintended genetics in production of hybrid seed production because of potential contamination is nearly impossible to avoid. If foreign pollen comes from a commercial hybrid field, all of the off types will not be identical. Genetics of that hybrid will include a strand of DNA from that hybrid’s male parent and a strand from the female parent. With meiosis occurring in the production of the hybrids pollen resulting in a minimum of 1024 new combinations of the 10 chromosomes in the pollen, contaminating pollen leads to many new genetics in new seed. Consequently, the outcrosses among intended hybrid seed vary greatly in appearance, some being taller than the canopy and some being shorter, along with a wide range of other morphological features. Usually contamination is associated with timing and production of intended male parent pollen thus the outcrosses often are more common within a seed size with the seed lot. First silks to emerge are from the base of the ear and thus the larger sizes. Poorly pollinated ears, perhaps from those in which silks emerged after most intended male pollen was exhausted and thus more likely to be pollinated by outside pollen results in more large, round seed. Best purity usually is in those from the middle of the ear of well pollinated ears (the medium flats).
Although a few contaminants in hybrids are common and have practically no effect on hybrid yield, they gain significance in special trait hybrids. That is the topic of the next blog.
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.