Corn mutation implications

​We have benefited greatly from the mutations that occurred over the thousands of years in multiple environments where people selected desirable characteristics in corn.  With isolation and self-pollination, populations became distinct with certain genes ‘fixed’ with homozygous dominant or recessive versions of some genes. Mutations and cross pollination in the varieties maintained variability, presenting advantages but also disadvantages as the genetic variability presented problems in dependable replication of the favorable genetics. Selfing these plants in attempt to gain complete homozygosity became the goal, with the objective of getting repeatability in inbred and hybrid characters for each generation.  Consequently, development of inbreds involved 7-10 generations of selfing, with complete homozygosity as the goal.  The limit is the low rate of mutations occurred at each generation, and even with most mutations not affecting the performance of the line it did allow for gradual ‘genetic drift’.  This was witnessed when a few popular public inbreds such as B73 and A632 were maintained by many separate public and private breeders and then compared for minor characters.  The basic heterosis between these stiff stalk lines with Lancaster inbreds remained but minor character differences were revealed.  Although some of this experience could be attributed to outside pollen contamination, it is assumed the accumulation of mutations over the years of isolation attributed to the differences among these established inbreds.
 
Significance of variability in parent seed presents an enigma for hybrid corn seed producers. Genetic drift in parent seed as seed is increased from a single ear to the large number needed to produce only 10000 bags of hybrid seed.  By the time of hybrid seed production, it is unlikely that any seed is precisely genetically like the original hybrid between two newly developed inbreds. These changes probably have no measureable effect on heterosis or performance of the commercial hybrid.  The seed industry does struggle to differ between inconsequential genetic differences from the original hybrid genotype and those that affect performance.
 
Dihaploid breeding produces perfectly homozygous inbreds with the first generation of doubling the haploid plant’s chromosomes.  However, further increasing the seed for hybrid testing and eventual hybrid production allows for slight mutations. 
 
Single cross hybrids produced by the crossing of two inbreds are rarely completely genetically pure due to minor mutations that have occurred during the increasing of the parent seed, but the performance difference due to the genetics from the original experimental hybrid is rarely measureable.  Mutations are a blessing and an enigma to all involved in hybrid corn seed development, production and use.