title: The fate of ribosomal genes in three interspecific somatic hybrids of Medicago sativa: three different outcomes including the rapid amplification of new spacer-length variants.
abstract: We have characterized the genetic consequences of somatic hybridization within the ribosomal DNA (rDNA) of three interspecific hybrids, each involving M. sativa as one of the parents. Restriction-fragment-length-polymorphisms (RFLPs) of rDNA spacers and fluorescent-in-situ-hybridization (FISH) of an 18S-gene probe to mitotic chromosomes were used to compare parental and hybrid species. The M. sativa-coerulea hybrid retained all six parental nucleolar-organizing regions (NORs) and all parental RFLPs representing a complete integration of rDNA. The M. sativa-arborea hybrid retained five of six parental NORs while losing half of the arborea-specific RFLPs, indicating that simple chromosome loss of one arborea NOR accounted for the RFLP losses. Dramatic alterations occurred within the M. sativa-falcata hybrid where five of six parental NORs were retained and new rDNA RFLPs were created and amplified differentially among somaclonal-variant plants. The molecular basis of the new RFLPs involved increased numbers of a 340-bp subrepeating element within the rDNA intergenic spacer (IGS), suggesting that recurrent cycles of unequal recombination occurred at high frequency within the rDNA in somatic lineages.
authors.affiliation: CNR Istituto di Ricerche sul Miglioramento Genetico della Fiante Foraggere, Via Madonna Alta 130, I-06128, Perugia, Italy.
authors.forename: P D
authors.initials: PD
authors.lastname: Cluster
authors.forename: O
authors.initials: O
authors.lastname: Calderini
authors.forename: F
authors.initials: F
authors.lastname: Pupilli
authors.forename: F
authors.initials: F
authors.lastname: Crea
authors.forename: F
authors.initials: F
authors.lastname: Damiani
authors.forename: S
authors.initials: S
authors.lastname: Arcioni