Today is Thanksgiving day in the USA.  People everywhere should be thankful for the multiple of efforts with the amazing Zea mays. Those people in central Mexico several thousand years ago who selected and planted seed from the mutants in the wild plant Teosinte that held its seed instead of scattering it.  And then those of that time that chose, and propagated the seed that with larger endosperms, and thus, more starch.  We are thankful for the multiples of people that carried the seed throughout the Americas, allowing and selecting for mutants adapted to their environments and personal food desires.  
 
Native Americans had already selected multiple corn varieties adapted to their local preferences by the time Europeans ‘discovered’ the American continents.  Some carried the seed corn plant vigor expressed with the crossing of certain varieties.  Multiple people experimented with ways of developing genetic purity through inbreeding and identifying specific crosses between corn inbreds that expanded the potential grain yield for multiple uses of the grain of this species.  We are thankful for the multiples of people’s efforts during the past and that which continues today.
 
Happy Thanksgiving to everyone.

​Variety parent seed is identical to the final seed product.  Parents of hybrid seed are not identical to the hybrid.  Single cross corn hybrids are inbreds selected primarily for favorable performance when combined as a hybrid but each with homozygosity for repeatable genetics.  This inbreeding process does result in some genetic expression of negative characters in one parent that are covered up by the other when combined as hybrids.  After identifying such a combination of inbreds, selection of which parent will become the source of the seed and which will become the pollinator becomes significant to the commercial production of hybrid seed.
 
The corn kernel is a fruit.  The outer layer, the pericarp, is a structure of the female plant.  It does not include any genetics of the inbred chosen to be the pollen source.  The bulk of the corn seed within the kernel is the endosperm where storage of starch is made available as energy utilized for germination of the seed.  Cells within the endosperm include 2 copies of female plant chromosomes and one of the pollen parent.  Only the embryo has equal genetics from both parents of hybrid seed.
 
Commercially acceptable female parents of hybrids need to have reliable and consistent elongation of silks even when under some moisture stress. Silks need to be receptive to fertilization after pollenated.  High number of ovules is favored.  Pericarp structure must be inclined to withstand stress with minimal cracking. The most important character of the female parent is consistently high percentage of germination.  A major factor linking this to the female seed parent is the genetics of the mitochondria within the embryo cells.  Mitochondrial genetics originate only from the ovule.  These sources of transforming energy stored as carbohydrates into that needed for cell metabolism are full of membranes that can be damaged by rapid swelling when water infuses into dry seed.  Maintenance of the integrity of these membranes become essential to the germination process.  Tolerance of natural stresses on emergence of silks, of pathogens and stresses on pericarps and of function of mitochondria are all associated with the female parent of a corn hybrid.
 
Pollen sources for hybrid seed production do have some responsibility as well.  Most critical is reliable and timely production of live pollen grains.  Release of pollen grains from the anthers is affected by genetics, as the anther chambers must dehisce as the relative humidity drops.  Timing with the presence of receptive silks on the female parent is essential.  It is probable that part of the pressure for selecting parents that increase grain yield involves shifting the genetics for energy needed to produce pollen to that of more grain results in less pollen.
 
Commercial hybrid corn breeding programs identify which hybrid parent is best as the female or male based upon quantity and germination of the seed.  These are determined by the genetics affecting the biology within the corn seed.

​As it became apparent to some corn breeders in the early 1900’s that consistency and repeatability of genetics in corn required creating homozygous inbreds from populations frequently expressing heterosis.  The enigma was that inbreeding greatly reduced the volume of hybrid seed to be planted but the production of grain from those hybrid seed was greater than produced by indigenous varieties.  A few academic corn breeders pushed the idea of using the hybrids as parent seed to make double cross hybrids to overcome the seed volume problem.  They encouraged several farmer seed producers to adapt this concept in the 1930s.  The significance of heterosis resulting from crossing specific inbreds became obvious to many during the 30’s, stimulating investigation into the genetics and botany of corn in academia and entrepreneurship among farmer breeders.  
 
As more farmers switched to using hybrid seed, public and private corn breeders increased inbred breeding programs.  New synthetic populations were created by breeders by crossing seed from existing varieties, selecting for heterosis by crossing with opposing inbreds, recycling the best, testing again and repeating the cycle to create new improved populations from which new inbreds could be created.  Stiff stalk synthetic population created at Iowa State University became and continues to be a powerful source of new inbreds that commonly used as female parents of hybrids.  Populations derived from varieties with origin in Eastern USA and grossly identified as Lancaster often became sources of inbreds expressing heterosis with stiff stalk derived inbreds.  
 
Breeding efforts to select more productive seed parents, improved seed production methods and economics of corn grain led to the introduction of single cross hybrids in the USA in the late 1960s.  A similar pattern developed in the multiple environments on other continents as well.  Continual selection by humans from the diverse genetics selected by previous human generations has led to continual improvement of grain productivity of this species. Its biological features of separation of male and female flowers, C4 photosynthesis, easily transported seed and 30-40000 genes has served us well.

​As it became clear that corn hybrids could create large boosts in yield when two unrelated inbreds were crossed, it became clear that the biggest gain came when the inbreds were derived from distinct heterotic groups.  Lancaster Sure Crop was created in Lancaster county Pennsylvania.  It featured long, flinty ears with disease resistance consistent with the humid eastern USA environment.  Meanwhile, the Reid dent corn, developed from the accidental crosses of New England Flint with Southern dent in central Illinois spread throughout the corn belt with many sub-varieties selected for differing moisture and soil stresses.  These two heterotic groups became major sources of inbreeding in the 1920 and 30’s as hybrid development advantages were clear.
 
In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn.  This became known as Iowa Stiff Stalk Synthetic population (SS).  New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds.  These cycles were continually resulting in more populations at Iowa State University.  Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations.  Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA.  Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.
 
The cause of heterosis lack some clarity but most of the evidence implies that negative, recessive genes in one inbred are overcome by the dominant version of the gene in the other inbred.  Given that a corn plant has at least 30,000 genes, and that any selfing, whether intentional to make inbreds or coincidental in open pollinated varieties, probability of homozygous recessive genes is high. Despite some randomness in matching dominant genes for the recessives of the other parent, inbreds derived from heterotic populations increase the probability of covering up the negative recessive genes.
 

​Separation of the male and female flowers of corn and ease of distribution of corn seed from its origin in central Mexico to the various environments of the earth increased the genetic variability in the species.  Selection by humans for those plants best fit for their individual use also reduced genetic variability of some other traits.  Some favorable corn traits are expressed best when a specific gene pair includes at least one dominant form of a gene.  But if the plant is heterozygous (one dominant and one recessive) for that gene, one quarter of the progeny created by pollination of the female flowers by pollen from the same plant will be homozygous recessive for that gene. That gene will not not be expressed in the plant growing from that seed.  
 
Corn has a large number of genes and rarely does homozygosity of a detrimental recessive gene have an overwhelming affect on the plant, but accumulation of these events does detract from maximum growth and expression of favorable traits by corn.  Regardless, the saving of seed in the various environments resulted in accumulation some homozygous recessive genes unique to that variety.  
 
This is the enigma of selection of corn for uniformity that allows for uniformity of characters needed to get efficient and maximum harvest.  Self pollinating preferred plants also increases the probability of eliminating the expression of favorable traits in the next generation.  Selfing increases uniformity but detracts from many physiological processes involved in the ultimate goal of maximum grain yield. The boost in yield resulting from crossing plants from two of the distinct corn populations that were developed in different environments over many years apparently occurred because each had different negative recessive genes covered up by the dominant gene of the other population. The selfing process to obtain uniformity increased probability of negative traits but crossing with certain inbreds from another genetic family could overcome the negative traits of each. 
 
Realization of this advantage of hybridization led to major advances in corn culture. Selfing of corn for favorable traits in today’s corn culture and hybridization with selected inbreds continues to lead to improved corn hybrids.

​Separation of male and female flowers in corn allows for spreading of pollen among corn plants and, consequently, some mixing genes. However, much considerable amount of pollen falls on the silk of the same pollen producing plant, resulting in increasing homozygosity among the genes on seed saved from from each generation. Although those saving seed from desirable plants, the next season would have some of the desirable characters recognized by the seed savers but would also show the affect of homozygosity of some genes that would detract from maximum performance. Thus, this practice of annually saving seed in the various areas tended to select for characteristics favored but also resulted in eventual selection of less less obvious genetics inhibiting maximum expression of the capacity of grain yield in this species.

Some of the seed savers witnessed the extra vigor when some of these isolated populations were mixed, showing the advantages of intentionally crossing between them. James L. Reid’s father, Robert Reid used a seed variety called Gordon Hopkins adapted to their Ohio area in the mid 1800s. He and his son James L. Reid migrated to Illinois in the late 1800s and brought along some of the corn seed. Robert Reid started intentionally crossing and selecting seed of desirable plants in Ohio, and later with his son in Illinois, developing a popular variety known as Reid yellow dent corn. It included some New England Flint and Southern Dent corn genetics as well. This variety was sold and distributed to many areas of the USA but was especially adapted to the environments of the Eastern half of the USA.

The detrimental affect of inbreeding and the advantage of crossing between between varieties became clear to students of corn breeding in the early 1900’s. Inbreeding allowed the reliable reproduction of some traits but with the depression of others.

In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn. This became known as Iowa Stiff Stalk Synthetic population (SS). New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds. These cycles were continually resulting in more populations at Iowa State University. Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations. Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA. Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.

The advantage of crossing of inbreds derived from different populations was becoming understood but would take some time and effort to economically produce hybrid seed.