Comprehensive Molecular Cytogenetic Analysis of Sorghum Genome Architecture; Distribution of Euchromatin, Heterochromatin, Genes and Recombination in Comparison to Rice

¹ Texas A &M University
² USDA-Forest Service
³ Texas A & M University
⁴ USDA-ARS
⁵ Texas A&M University

^* To whom correspondence should be addressed. E-mail: jmullet{at}tamu.edu.

Submitted on July 13, 2005
Revised on August 8, 2005
Accepted on 21 August 2005

	Abstract

Cytogenetic maps of sorghum chromosomes 3-7, 9 and 10 were constructed based on the fluorescence in situ hybridization (FISH) of ~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 ~50% of the sorghum genome, ranging from ~60% of chromosome 1 (SBI-01) to ~33% of chromosome 7 (SBI-07). This portion of the sorghum genome is predicted to encode ~70% of the sorghum genes (~1 gene model/12.3kbp) assuming rice and sorghum encode a similar number of genes. Heterochromatin spans ~411Mbp of the sorghum genome, a region characterized by a ~34-fold lower rate of recombination and ~3-fold lower gene density compared to euchromatic DNA. The sorghum and rice genomes exhibit a high degree of macro-colinearity, however the sorghum genome is ~2-fold larger than the rice genome. The distal euchromatic regions of sorghum chromosomes 3-7 and 10 are ~1.8-fold larger overall and exhibit a ~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 ~3.6-fold larger in sorghum and recombination is suppressed ~15-fold compared to the colinear regions of rice chromosomes.

Key Words: cytogenetics, genome, heterochromatin, rice, sorghum

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