Chromosome investigations in annual Medicago species (Fabaceae) with emphasis on the origin of the polyploid Medicago rugosa and M. scutellata

Chromosome number variations have played an important role in the evolution of the genus Medicago. In addition to polyploidy there are cases of dysploidy as evidenced by the presence of two basic numbers, x=8, the most common, and x=7, limited to the annual species among which five have 2n=14. Annual forms are prevalently diploid with the exception of M. scutellata and M. rugosa which have 2n=30 and are considered the result of crosses between the 2n=16 and 2n=14 species with consequent chromosome doubling. The purpose of this study was to gain more knowledge on the karyotype structure of 2n=14 and 2n=30 species, and to acquire more cytogenetic information for shedding light on the presumed allopolyploid origin of M. scutellata and M. rugosa. To realize these objectives, fluorescence in situ hybridization (FISH) using rDNA probes and genomic in situ hybridization (GISH) were performed. All examined species had a number of rDNA sites correlated with their ploidy level except M. rugosa which displayed only one pair of 45S and 5S rDNA sites. These results were evaluated in relation to the evolutionary dynamics of the rDNA sites and the polyploid nature of the species. The GISH experiments, carried out to assess the genomic relationships between the 2n=30 and 2n=14 species did not produce any hybridization signals useful for the identification of the presumed progenitors with 14 chromosomes. On the basis of these results alternative evolutionary pathways must be considered, for example the occurrence of descending dysploidy events in already established tetraploid forms (2n=32).