Quantitative chromosome map of the polyploid Saccharum spontaneum by multicolor fluorescence in situ hybridization and imaging methods

Somatic chromosomes of a wild relative of sugarcane (Saccharum spontaneum L .) anther culture-derived clone (AP 85-361, 2n=32) were identified and char acterized by computer-aided imaging technology and molecular cytological me thods. The presence of four satellite chromosomes and four nearly identical chromosome sets suggests that the clone is a tetrahaploid with the basic n umber x=8. A quantitative chromosome map, or idiogram, was developed using image analysis of the condensation pattern (CP) at the prometaphase stage o f somatic chromosomes. The 45S and 5S ribosomal RNA gene (rDNA) loci were s imultaneously visualized by multi-color fluorescence in situ hybridization (McFISH) and precisely localized to the regions of 3p3.1 and 6q1.3 on the i diogram. The simultaneous visualization of two sets of four ribosomal RNA g enes confirms tetraploidy of this clone. This conclusion is consistent with results of molecular marker mapping. The quantitative chromosome map produ ced will become the foundation for genome analyses based on chromosome iden tity and structure. Previously impossible identification of small chromosom es and untestable hypotheses about the polyploid nature of plants can now b e settled with these two approaches of quantitative karyotyping and FISH.