Genetic analysis of doubled haploid and unilateral cross incompatibility systems for organic maize breeding
Recently, there has been increased interest in organic agriculture due to consumer concerns over pesticide usage and genetically modified organisms (GMOs). Production of organic field corn in the U.S. consisted of 213,934 acres in 2017 and is expected to increase. A challenge facing organic corn growers is maintaining genetic purity when nearby fields are growing GMO corn. Unilateral cross incompatibility (UCI) systems provide a potential solution to this problem. There are three systems of UCI in maize that prevent pollination by pollen that does not possess the same allele as the silks. Ga1-s UCI was crossed into 9 diverse maize inbreds from three major heterotic groups and the F1 hybrids tested for exclusion of exogenous ga1 pollen. We observed significant differences in exclusion capabilities between genotypes, indicating presence of modifier loci. Additionally, to improve the efficiency of line development and trait introgression in a maize breeding program, doubled haploid (DH) lines can be derived. In a typical DH system, chemicals such as colchicine are used for genome doubling but some genotypes have an ability to naturally double their genomes without colchicine, a trait called spontaneous haploid genome doubling (SHGD). There are many mechanisms of SHGD and they are controlled by varying numbers of genes. However, there is an inbred line that was identified as having greater than 40% SHGD rate and is controlled by a single large effect QTL located on chromosome 5. Using a line with a single large effect QTL as a donor will allow introgression of the QTL into non-SHGD backgrounds with marker-assisted backcrossing. Use of SHGD can improve the genome doubling rate as compared to colchicine which results in a genome doubling rate that is between 10-30%. Ga1 and SHGD both have the potential to drastically change organic corn breeding, protecting organic corn from pollen contamination and improving its breeding efficiency.