An SSR genetic map of Sorghum bicolor (L.) Moench and its comparison to a published genetic map

Sorghum bicolor (L.) Moench is an important grain and forage crop grown worldwide. We developed a simple sequence repeat (SSR) linkage map for sorghum using 352 publicly available SSR primer pairs and a population of 277 F 2 individuals derived from a cross between the Westland A line and PI 550610. A total of 132 SSR loci appeared polymorphic in the mapping population, and 118 SSRs were mapped to 16 linkage groups. These mapped SSR loci were distributed throughout 10 chromosomes of sorghum, and spanned a distance of 997.5 cM. More important, 38 new SSR loci were added to the sorghum genetic map in this study. The mapping result also showed that chromosomes SBI-01, SBI-02, SBI-05, and SBI-06 each had 1 linkage group; the other 6 chromosomes were composed of 2 linkage groups each. Except for 5 closely linked marker flips and 1 locus (Sb6_34), the marker order of this map was collinear to a published sorghum map, and the genetic distances of common marker intervals were similar, with a difference ratio ≤ 0.05 between the 2 maps. The difference ratio is a new index developed in this study that can be used to compare the genetic distances of DNA markers between 2 maps. This SSR map carrying additional SSR markers will facilitate mapping quantitative trait loci to the sorghum genome and map-based gene cloning. Furthermore, the novel method for calculating distance between DNA markers will be a useful tool for the comparative analysis of genetic markers between linkage maps with different genetic backgrounds and the alignment of different sorghum genetic maps.