Methods for developing new corn inbreds

​Use of homozygous inbreds as corn hybrid parents is important to producing new hybrid seed with identical parents’ genes successful in past seasons.  Traditional method of obtaining homozygosity has been done by self-pollinating selected plants from a segregating population for 5-8 generations.  Studies from the 1950’s showed that basic heterosis with another parent could be expressed after 3 selfing generations, but the remaining selfing generations are needed to assure repeatability of minor traits.  Corn breeders have tried various methods to try to reduce the time and testing expense to this process.  
 
A quicker method of obtaining homozygous inbreds by inducing haploids to be doubled was known since 1959, but the method became more common after the year 2000.  Pollen from haploid inducer is crossed to a prospective heterozygous genetic source.  From 2-10 percent of the resulting seed will only have the genes from the female ovule although only one member of each pair of chromosomes of the female plant.  These seed can be identified visually with pigments if the inducer differs from the female plant in these pigments or perhaps by molecular methods.  Haploid chromosomes are doubled by either submersion or by injection with chemicals such as colchicine or specific herbicides.  It is estimated that, on average, to obtain 100 new inbreds by DH system, requires initial pollination of 100 - 200 plants by the inducer to obtain 1000 haploid plants to be grown in field, after doubling chemical treatment.  It is intended to obtain 100 homozygous, diploid inbreds as a result.
 
Professional Seed Research, Inc. (PSR) began experimenting with another method of quickly reaching near homozygosity by utilizing their Seedling Morphology Fingerprint (SMF™) technology used in their other services.  This utilizes the fact that because corn has only 10 pairs of chromosomes, there is are a notable percentage of near-homozygous individuals among a segregating population.  That  percentage increases if the F2 population is between related parents.  PSR selects these near-homozygous plants based upon seedling characters.  These plants are transplanted to the field and selfed.  Resulting plants range in homozygous levels equivalent to 4-8 selfing generations.  This is sufficient to evaluate heterosis with prospective parents and yet may have sufficient heterozygosity to select for simple inherited characters such as ear height, pollen production or disease resistance. PSR refers to this system as Rapid Inbreeding®.
 
The objective of all three systems is to reach a level of homozygosity in productive hybrid parents that can be repeated with each increase.  Traditional method of selfing each generation allows for selection of desirable characters in each generation but costs time.  Dihaploid breeding is quicker to reach complete homozygosity than traditional methods but requires specialized efforts and genetic backgrounds affects the success rate.  Rapid Inbreeding® (RI) offers the speed of dihaploid breeding to reach level of sufficient homozygosity for hybrid selection, works with any segregating population and allows further selection of minor characters in final inbred.  PSR Global Genetics uses RI technology to develop thousands of inbred lines each year.