THE FATE OF RIBOSOMAL GENES IN 3 INTERSPECIFIC SOMATIC HYBRIDS OF MEDICAGO-SATIVA - 3 DIFFERENT OUTCOMES INCLUDING THE RAPID AMPLIFICATION OF NEW SPACER-LENGTH VARIANTS

We have characterized the genetic consequences of somatic hybridizatio n within the ribosomal DNA (rDNA) of three interspecific hybrids, each involving M. sativa as one of the parents. Restriction-fragment-lengt h-polymorphisms (RFLPs) of rDNA spacers and fluorescent-in-situ-hybrid ization (FISH) of an 18S-gene probe to mitotic chromosomes were used t o compare parental and hybrid species. The M. sativa-coerulea hybrid r etained all six parental nucleolar-organizing regions (NORs) and all p arental RFLPs representing a complete integration of rDNA. The M. sati va-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 alteration s occurred within the M. sativa-falcata hybrid where five of six paren tal NORs were retained and new rDNA RFLPs were created and amplified d ifferentially among somaclonal-variant plants. The molecular basis of the new RFLPs involved increased numbers of a 340-bp subrepeating elem ent within the rDNA intergenic spacer (IGS), suggesting that recurrent cycles of unequal recombination occurred at high frequency within the rDNA in somatic lineages.