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

Interspecific hybrid plants between Melilotus altissima (yellow flowers) and either M. alba (white flowers) or M. taurica (white flowers) and between M. suaveolens (yellow flowers) and M. wolgica (white flowers) which were inviable because of heavy chlorosis, were grown successfully by in vitro culture and reached the flowering stage. Pollen fertility of hybrid plants between M. altissima and M. alba was about 46% whereas hybrids between M. altissima and M. taurica and between M. suaveolens and M. wolgica showed about 25% and 18% of pollen fertility, respectively. All F1s did not set any selfed seeds. Observing the pairing of chromosomes at metaphase I in the hybrid plants, it was revealed that the genomes of M. altissima and M. alba differ by a reciprocal translocation. This result could account for the partial sterility of pollen in the F1hybrids. On the other hand, hybrid plants between M. altissima and M. taurica and between M. suaveolens and M. wolgica showed normal pairing of chromosomes though pollen fertility of the hybrids was very low, suggesting that low pollen fertility was controlled by sterility gene(s). These results demonstrate that M. altissima and 7 other species of the subgenus Eumelilotus share the same chromosomal constitution. The only exception, M. alba, differs from the other 8 species by a segmental interchange.