New mitochondrial genome organization hybrids of Medicago sativa includingthe of substoichiometric mitochondrial DNA in three interspecific somatic parent-specific amplification units

Three somatic hybrid plants produced by protoplast fusion between Medicago sativa and each of the three species Medicago coerulea, Medicago falcata an d Medicago arborea have been analysed for the composition of their mitochon drial DNA. Restriction fragment length polymorphism (RFLP) analysis of mito chondrial genes in somatic hybrids and their parental lines showed various degrees of rearrangement. The M. sativa+M. coerulea hybrid retained all of the M. coerulea-specific bands but lost all the major M. sativa-specific ba nds. The M. sativa+M. falcata hybrid showed only M. sativa-specific bands t ogether with non-parental bands, and the M. sativa+M. arborea hybrid showed a partial incorporation of bands from both parents together with non-paren tal bands. The three different outcomes were attributed mainly to differenc es in the genetic distance between the parents of each hybrid. Analysis of the sexual progeny of the M. sativa+M. coerulea hybrid showed that a residu al mitochondrial DNA subunit of M. sativa was retained in the hybrid cytopl asm. This subunit was amplified and inherited in a mutually exclusive, alle lic-like fashion with its M. coerulea homologous counterpart in the sexual progeny of the hybrid. Possible mechanisms for the partitioning of mitochon drial DNA in the generative lineage of the somatic hybrids are discussed in relation to the creation of new nucleuscytoplasm assortments otherwise imp ossible to obtain by a sexual cross in Medicago.