The sh2-R allele of the maize shrunken-2 locus was caused by a complex chromosomal rearrangement.

KEY MESSAGE: The mutant that originally defined the shrunken - 2 locus of maize is shown here to be the product of a complex chromosomal rearrangement. The maize shrunken-2 gene (sh2) encodes the large subunit of the heterotetrameric enzyme, adenosine diphosphate glucose pyrophosphorylases and a rate-limiting enzyme in starch biosynthesis. The sh2 gene was defined approximately 72 years ago by the isolation of a loss-of-function allele conditioning a shrunken, but viable seed. In subsequent years, the realization that this allele, termed zsh2-R or sh2-Reference, causes an extremely high level of sucrose to accumulate in the developing seed led to a revolution in the sweet corn industry. Now, the vast majority of sweet corns grown throughout the world contain this mutant allele. Through initial Southern analysis followed by genomic sequencing, the work reported here shows that this allele arose through a complex set of events involving at least three breaks of chromosome 3 as well as an intra-chromosomal inversion. These findings provide an explanation for some previously reported, unexpected observations concerning rates of recombination within and between genes in this region.

The genetic architecture of maize (Zea mays L.) kernel weight determination.

Individual kernel weight is an important trait for maize yield determination. We have identified genomic regions controlling this trait by using the B73xMo17 population; however, the effect of genetic background on control of this complex trait and its physiological components is not yet known. The objective of this study was to understand how genetic background affected our previous results. Two nested stable recombinant inbred line populations (N209xMo17 and R18xMo17) were designed for this purpose. A total of 408 recombinant inbred lines were genotyped and phenotyped at two environments for kernel weight and five other traits related to kernel growth and development. All traits showed very high and significant (P < 0.001) phenotypic variability and medium-to-high heritability (0.60-0.90). When N209xMo17 and R18xMo17 were analyzed separately, a total of 23 environmentally stable quantitative trait loci (QTL) and five epistatic interactions were detected for N209xMo17. For R18xMo17, 59 environmentally stable QTL and 17 epistatic interactions were detected. A joint analysis detected 14 stable QTL regardless of the genetic background. Between 57 and 83% of detected QTL were population specific, denoting medium-to-high genetic background effects. This percentage was dependent on the trait. A meta-analysis including our previous B73xMo17 results identified five relevant genomic regions deserving further characterization. In summary, our grain filling traits were dominated by small additive QTL with several epistatic and few environmental interactions and medium-to-high genetic background effects. This study demonstrates that the number of detected QTL and additive effects for different physiologically related grain filling traits need to be understood relative to the specific germplasm.