Dissecting the genetic architecture of stress tolerance in crops is critical to understand and improve adaptation. In temperate climates, early planting of chilling-tolerant varieties could provide longer growing seasons and drought escape, but chilling tolerance (<15°) is generally lacking in tropical-origin crops. Here we developed a nested association mapping (NAM) population to dissect the genetic architecture of early-season chilling tolerance in the tropical-origin cereal sorghum (Sorghum bicolor [L.] Moench). The NAM resource, developed from reference line BTx623 and three chilling-tolerant Chinese lines, is comprised of 771 recombinant inbred lines genotyped by sequencing at 43,320 single nucleotide polymorphisms. We phenotyped the NAM population for emergence, seedling vigor, and agronomic traits (>75,000 data points from ∼16,000 plots) in multi-environment field trials in Kansas under natural chilling stress (sown 30–45 days early) and normal growing conditions. Joint linkage mapping with early-planted field phenotypes revealed an oligogenic architecture, with 5–10 chilling tolerance loci explaining 20–41% of variation. Surprisingly, several of the major chilling tolerance loci co-localize precisely with the classical grain tannin (Tan1 and Tan2) and dwarfing genes (Dw1 and Dw3) that were under strong directional selection in the US during the 20th century. These findings suggest that chilling sensitivity was inadvertently selected due to coinheritance with desired nontannin and dwarfing alleles. The characterization of genetic architecture with NAM reveals why past chilling tolerance breeding was stymied and provides a path for genomics-enabled breeding of chilling tolerance.

To meet anticipated markets for sweet sorghum [Sorghum bicolor (L.) Moench] production, the development of sweet sorghum hybrids is underway. As this development is in its infancy, genetic markers will be very useful to define the genetic basis of sugar yield components and reduce the time required to deliver new hybrids to market. The purpose of this research was to characterize genetic components that influence sweet sorghum productivity and agronomics. A grain x sweet sorghum mapping population was evaluated for 16 phenotypic traits across four environments. A population of 185 Fg-derived F4 (^3-4) 'inss was derived from the parents 'BTx3197' and 'Rio', which are dry-stalked grain and juicy-stalked sweet sorghums, respectively. A genetic map was constructed with 381 single nucleotide polymorphisms, and quantitative traif locus (QTL) analysis was performed. A major and previously reported QTL for soluble solids was identified on chromosome 3, but in contrast to previous studies, this QTL colocalized with other QTLs that had negative influences on desirable traits. Therefore, selection for this QTL may not be advantageous. A limited number of QTLs for percent moisture were found, and results indicated that the dry-stalk phenotype does not affect moisture content as measured in this study. Thus, breeding for moisture content will be more challenging than previously expected. The absence of dominance effects for the brlx trait indicated that brix must be high in both parents to produce high brix in hybrids.

No abstract

Colletotrichum sublineola is an aggressive fungal pathogen that causes anthracnose in sorghum [Sorghum bicolor (L.) Moench]. The obvious symptoms of anthracnose are leaf blight and stem rot. Sorghum, the fifth most widely grown cereal crop in the world, can be highly susceptible to the disease, most notably in hot and humid environments. In the southeastern United States the acreage of sorghum has been increasing steadily in recent years, spurred by growing interest in producing biofuels, bio-based products, and animal feed. Resistance to anthracnose is, therefore, of paramount importance for successful sorghum production in this region. To identify anthracnose resistance loci present in the highly resistant cultivar ‘Bk7’, a biparental mapping population of F3:4 and F4:5 sorghum lines was generated by crossing ‘Bk7’ with the susceptible inbred ‘Early Hegari-Sart’. Lines were phenotyped in three environments and in two different years following natural infection. The population was genotyped by sequencing. Following a stringent custom filtering protocol, totals of 5186 and 2759 informative SNP markers were identified in the two populations. Segregation data and association analysis identified resistance loci on chromosomes 7 and 9, with the resistance alleles derived from ‘Bk7’. Both loci contain multiple classes of defense-related genes based on sequence similarity and gene ontologies. Genetic analysis following an independent selection experiment of lines derived from a cross between ‘Bk7’ and sweet sorghum ‘Mer81-4’ narrowed the resistance locus on chromosome 9 substantially, validating this QTL. As observed in other species, sorghum appears to have regions of clustered resistance genes. Further characterization of these regions will facilitate the development of novel germplasm with resistance to anthracnose and other diseases.

Rapid environmental change can lead to population extinction or evolutionary rescue. The global staple crop sorghum ( Sorghum bicolor ) has recently been threatened by a global outbreak of an aggressive new biotype of sugarcane aphid (SCA; Melanaphis sacchari ). We characterized genomic signatures of adaptation in a Haitian breeding population that had rapidly adapted to SCA infestation, conducting evolutionary population genomics analyses on 296 Haitian lines versus 767 global accessions. Genome scans and geographic analyses suggest that SCA adaptation has been conferred by a globally rare East African allele of RMES1 , which spread to breeding programs in Africa, Asia, and the Americas. De novo genome sequencing revealed potential causative variants at RMES1 . Markers developed from the RMES1 sweep predicted resistance in eight independent commercial and public breeding programs. These findings demonstrate the value of evolutionary genomics to develop adaptive trait technology and highlight the benefits of global germplasm exchange to facilitate evolutionary rescue.