Résumé : Cytological analyses show rearranged chromosomes in some highly asymmetric nuclear hybrids obtained after fusion of mesophyll protoplasts of Nicotiana plumbaginifolia (wild type) with γ-irradiated (100 krad), kanamycin-resistant mesophyll protoplasts of Petunia hybrida. Molecular, cytogenetic and in situ hybridization analyses performed on the asymmetric somatic hybrid P1, previously identified as having a clearly metacentric chromosome besides a nearly complete Nicotiana chromosome complement, are reported. Meiotic analysis and in situ hybridization experiments using ribosomal DNA as a probe showed that this metacentric chromosome represents a translocation of a chromosome fragment onto chromosome 9 of N. plumbaginifolia. Southern hybridization with an rDNA probe showed that only Nicotiana-specific rDNA was present. In situ hybridization experiments, using total genomic DNA of P. hybrida as a probe, demonstrated that the translocated fragment represented Petunia DNA. © 1992 Chapman & Hall.