Résumé
Recently, we physically mapped eight 18S-5.6S-25S (18S-25S) rRNA sites in S. of ficinarum clone BNS 3066 (2n = 80) and eight in S. spontaneum clone SES 14 (2n = 64). We interpreted the results as an indication that the basic chromosome numbers for S. officinarum and S. spontaneum were 10 and 8, respectively (D'Hont et al. 1996). We report here the results of the physical mapping of a second rDNA locus, the SS rRNA genes, simultaneously with the 18S-25S rRNA genes in S. officinarum and different cytotypes of S. spontaneum and S. robustum. These results allowed us to determine the basic chromosome number of these three species. The implications of these results for the phylogeny of this genus and for current cultivar genetic maps is discussed. The 18S-25S rRNA genes, or simultaneously the 18S-25S and 5S rRNA genes, were located on chromosomes of different S. spontaneum cytotypes (Table 1; Fig. lb). In the various clones studied, the 18S-25S and the SS rRNA genes were localized at an interstitial position on different sets of chromosomes. The cytotypes with 2n = 64 (SES 14 and SES 106B) displayed 8 sites for both types of gene, whereas the cytotypes with 2n = 80 (NG 51-2 and Mol 5801) displayed 10 sites. This was interpreted, for both types of genes, as the presence of one locus, and of 8 and 10 copies for the cytotypes with 2n = 64 and 2n = 80, respectively. The clones Mandalay (2n = 96) and Glagah (2n = 112) displayed 12 and 14 5S rDNA sites, respectively. This was interpreted, for the SS rRNA genes, as the presence of one locus, and of 12 and 14 copies for the cytotypes with 2n = 96 and 2n = 112, respectively. However, only 10 18S-25S rDNA sites were detected in the clone Glagah (2n = 112), while 12 18S-25S rDNA sites were observed in the clone Mandalay (2n = 96), five of which were tenuous and barely detectable. In the various clones studied, minor as well as major 18S-25S rDNA sites were detected, probably reflecting a reduction in the number of repeats; in the clones with very high chromosome numbers, this phenomenon may have been accentuated to the point where sites are very difficult to detect or have even been completely deleted. Except for the clone Glagah, the total number of chromosomes of the different cytotypes is proportional to the number of rDNA sites, which is in agreement with a basic chromosome number of x = 8 for S. spontaneum. The 18S-25S rDNA locus was genetically mapped to an interstitial position in two S. spontaneum cosegregation groups of this linkage group VIII. Saccharum spontaneum chromosomes bearing an rDNA site can thus be assigned to our linkage group VIII (Grivet et al.1996; D'Hont et al.1996) and, based on common probes, to the linkage group V of the SES 208 map (da Silva et al.1995). The 18S-25S rDNA locus has not yet been mapped in S. officinarum cosegregation groups. Owing to the general colinearity between the two genomes, and the physical location of the 18-26S sites in the two species, we may expect this locus to be localized terminally on one of the S. officinarum linkage groups homoeologous to S. spontaneum linkage group VIII. However, rDNA loci do not always match the general colinearity observed between closely related Gramineae species (Dubkovsky and Dvorak 1995). Likewise, although we have physically detected a unique 18S-25S rDNA locus on sorghum (data not shown), and our current comparative mapping studies show a large synteny and colinearity between sorghum and sugarcane (Dufour et al. 1996, 1997), we have genetically mapped the 18S-25S rDNA locus on our sorghum linkage group C, which is not homoeologous to our sugarcane group VIII. The 5S and 18S-25S rRNA genes have allowed us to physically mark two sets of chromosomes in the Saccharum genus. The development of additional physical markers, such as BACs (bacterial artificial chromosomes; Hanson et al. 1995), offer exciting possibilities for investigating the genome structure of sugarcane, especially when combined with the extensive molecular genetic information now accumulating for the genus.