Abstract
BackgroundSorghum is a tropical C4 cereal that recently adapted to temperate latitudes and mechanized grain harvest through selection for dwarfism and photoperiod-insensitivity. Quantitative trait loci for these traits have been introgressed from a dwarf temperate donor into hundreds of diverse sorghum landraces to yield the Sorghum Conversion lines. Here, we report the first comprehensive genomic analysis of the molecular changes underlying this adaptation.ResultsWe apply genotyping-by-sequencing to 1,160 Sorghum Conversion lines and their exotic progenitors, and map donor introgressions in each Sorghum Conversion line. Many Sorghum Conversion lines carry unexpected haplotypes not found in either presumed parent. Genome-wide mapping of introgression frequencies reveals three genomic regions necessary for temperate adaptation across all Sorghum Conversion lines, containing the Dw1, Dw2, and Dw3 loci on chromosomes 9, 6, and 7 respectively. Association mapping of plant height and flowering time in Sorghum Conversion lines detects significant associations in the Dw1 but not the Dw2 or Dw3 regions. Subpopulation-specific introgression mapping suggests that chromosome 6 contains at least four loci required for temperate adaptation in different sorghum genetic backgrounds. The Dw1 region fractionates into separate quantitative trait loci for plant height and flowering time.ConclusionsGenerating Sorghum Conversion lines has been accompanied by substantial unintended gene flow. Sorghum adaptation to temperate-zone grain production involves a small number of genomic regions, each containing multiple linked loci for plant height and flowering time. Further characterization of these loci will accelerate the adaptation of sorghum and related grasses to new production systems for food and fuel.
Highlights
Sorghum is a tropical C4 cereal that recently adapted to temperate latitudes and mechanized grain harvest through selection for dwarfism and photoperiod-insensitivity
We found that combining two separate double digests nearly doubled the number of single nucleotide polymorphism (SNP) called per sample (Additional File 2)
We used three complementary approaches - introgression mapping, association mapping, and population differentiation (Fst) - to characterize the genetic architecture of adaptation to temperate-zone grain production in sorghum
Summary
Sorghum is a tropical C4 cereal that recently adapted to temperate latitudes and mechanized grain harvest through selection for dwarfism and photoperiod-insensitivity. Cereals have been selected by humans for thousands of years, first during their domestication from wild grasses and subsequently for increased yield, uniformity, and adaptation to new environments and management practices [1,2,3]. Similar phenotypic changes occurred during the creation of dwarf grain sorghum suitable for mechanized harvest. Temperate adaptation for grain production in sorghum requires photoperiod-insensitivity, for early maturity, and dwarfism, both of which involve at least four major loci [10]. Of the major dwarfing loci (Dw1-Dw4), Dw3 has been identified as PGP1/PGP19, an auxin transporter orthologous to maize brachytic2 [13]. Dw2 and Dw1 are uncloned, with the former closely-linked to Ma1 [14] and the latter mapping to chromosome 9 [15,16]
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