We have constructed a complete genetic linkage map of sorghum based on restriction fragment length polymorphisms (RFLPs) detected by maize genomic and cDNA probes (Pereira et al., Genome 1994). To facilitate comparisons with maize, some probes were selected on the basis of linkage information reported in newsletter compilations, notes, and our unpublished results (Spike and Lee, MNL 67:6-7, 1993; M. Lee, unpublished). The sorghum parents used to create the mapping population (CK60 and PI229828) were chosen to represent phenotypic extremes for panicle morphology, leaf dimensions, tillering, plant height and resistance to biotic stresses. In this note, we summarize the mapping results for RFLP loci associated with plant height in sorghum and review some evidence for orthologous regions in maize. A complete presentation will be reported elsewhere (Pereira and Lee, Theor. Appl. Genet., 1994).
Linkage between RFLP and quantitative trait loci (QTL) in sorghum was assessed with a sample of 152 F2 plants at 111 loci. Four independent QTL for plant height were identified (linkage groups A, B, E, and H). Individually, the QTL accounted for 9 to 29% of the phenotypic variation (63%, collectively) with positive additive effects of 15 to 32 centimeters. Alleles for increased plant height were derived from the tall parent (PI229828) and with one exception (QTL of linkage group H) tallness was dominant. Three of the QTL were also significantly associated with several other traits while the effect of the fourth QTL (linkage group E) was limited to plant height. Based on these observations, we have hypothesized that the plant height QTL of linkage groups A, E, and H correspond to sorghum genetic loci Dw3, Dw4, and Dw2, respectively. The Dw (dwarf) loci have been identified usually on the basis of recessive alleles with highly qualitative effects on plant height (primarily a reduction of internode length). Dwarfing alleles at the loci (Dw1-4) have been routinely manipulated in breeding programs and have been backcrossed in germplasm adaptation programs.
Placement of RFLP loci in maize and sorghum maps with common probes suggests the plant height QTL of sorghum linkage groups A, E, and H may be orthologous to plant height QTL reported for maize chromosomes 1, 6, and 9, respectively. In each case, the confidence intervals of the sorghum QTL are within those reported for maize (Beavis et al., Theor. Appl. Genet. 83:141-145, 1991 for QTL on chromosomes 1 and 9; Veldboom et al., Theor. Appl. Genet., 1994 for QTL on chromosomes 1 and 6). Also, linked or pleiotropic effects of some sorghum plant height QTL resemble those of mutant alleles at maize genetic loci in the putatively orthologous regions. On chromosome 1, two genetic loci (an1 and br1) seem to be included within the confidence intervals reported by Beavis et al. and Veldboom et al. The mutant phenotype of an1 is an andromonoecious, gibberellin-responsive dwarf with short leaves and few tassel branches. The sorghum QTL of linkage group A has significant effects on the number of primary branches per panicle. On chromosome 9, the d3 locus appears to be within the maize QTL confidence interval. The mutant phenotype of d3 is an andromonoecious, gibberellin-responsive dwarf with thickened broad leaves and a compact tassel. Likewise, the sorghum QTL of linkage group H has significant effects on leaf blade and panicle dimensions (length and width). Similar parallel effects for maize chromosome 6 and sorghum linkage group E were not apparent.
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