Originally published as Genetics Published Articles Ahead of Print on October 16, 2004.

Genetics, Vol. 169, 955-965, February 2005, Copyright © 2005
doi:10.1534/genetics.104.026765

Molecular Cytogenetic Maps of Sorghum Linkage Groups 2 and 8

Jeong-Soon Kim^*,, Patricia E. Klein^*, Robert R. Klein, H. James Price, John E. Mullet^* and David M. Stelly^*,,1

^* Institute for Plant Genomics and Biotechnology, Texas A&M University, College Station, Texas 77843
USDA-ARS, Southern Plains Agricultural Research Center, College Station, Texas 77845
Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas 77843

¹ Corresponding author: Department of Soil and Crop Sciences, Texas A&M University, 370 Olsen Blvd., College Station, TX 77843-2474.
E-mail: stelly{at}tamu.edu

To integrate genetic, physical, and cytological perspectives of the Sorghum bicolor genome, we selected 40 landed bacterial artificial chromosome (BAC) clones that contain different linkage map markers, 21 from linkage group 2 (LG-02) and 19 from linkage group 8 (LG-08). Multi-BAC probe cocktails were constructed for each chromosome from the landed BACs, which were also preevaluated for FISH signal quality, relative position, and collective chromosome coverage. Comparison to the corresponding linkage map revealed full concordance of locus order between cytological and prior segregation analyses. The pericentromeric heterochromatin constituted a large quasi-uniform block in each bivalent and was especially large in the bivalent corresponding to LG-08. Centromere positions in LG-02 and LG-08 were progressively delimited using FISH to identify landed BACs for which the FISH signals visibly flanked the centromere. Alignment of linkage and cytological maps revealed that pericentromeric heterochromatin of these sorghum chromosomes is largely devoid of recombination, which is mostly relegated to the more distal regions, which are largely euchromatic. This suggests that the sorghum genome is thus even more amenable to physical mapping of genes and positional cloning than the C-value alone might suggest. As a prelude to positional cloning of the fertility restorer, Rf1, FISH of BAC clones flanking the Rf1 locus was used to delimit the chromosomal position of the gene. FISH of BACs that contain the most proximal linkage markers enabled localization of Rf1 to a 0.4-Mbp euchromatic region of LG-08. Cytogenetic analyses of Rf1 and other trait loci will aid in assessing the feasibility of positional cloning and help formulate strategies required for cloning this and other agriculturally critical genes.

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