Comprehensive molecular cytogenetic analysis of sorghum genome architecture: Distribution of euchromatin, heterochromatin, genes and recombination in comparison to rice

Cytogenetic maps of sorghum chromosomes 3-7, 9, and 10 were constructed on the basis of the fluorescence in situ hybridization (FISH) of similar to 18-30 BAC probes mapped across each of these chromosomes. Distal regions of euchromatin and pericentromeric regions of heterochromatin were delimited for all 10 sorghum chromosomes and their DNA content quantified. Euchromatic DNA spans similar to 50% of the sorghum genome, ranging from similar to 60% of chromosome 1 (SBI-01) to similar to 33% of chromosome 7 (SBI-07). This portion of the sorghum genome is predicted to encode similar to 70% of the sorghum genes (similar to 1 gene model/12.3 kbp), assuming that rice and sorghum en code a similar number of genes. Heterochromatin spans similar to 411 Mbp of the sorghum genome, a region characterized by a similar to 34-fold lower rate of recombination and similar to 3-fold lower gene density compared to euchromatic DNA. The sorghum and rice genomes exhibit a high degree of macrocolinearity; however, the sorghum genome is similar to 2-fold larger than the rice genome. The distal euchromatic regions of sorghum chromosomes 3-7 and 10 are similar to 1.8-fold larger overall and exhibit an similar to 1.5-fold lower average rate of recombination than the colinear regions of the homeologous rice chromosomes. By contrast, the pericentromeric heterochromatic regions of these chromosomes are on average similar to 3.6-fold larger in sorghum and recombination is suppressed similar to 15-fold compared to the colinear regions of rice chromosomes.