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Interspecific hybridization and genome duplication to form allopolyploids are major evolutionary events in angiosperms. In the parasitic genus Cuscuta (Convolvulaceae), molecular data suggested the existence of species of hybrid origin. One of them, C. veatchii, has been proposed as a hybrid between C. denticulata and C. nevadensis, both included in sect. Denticulatae. To test this hypothesis, a cytogenetic analysis was performed with CMA/DAPI staining and fluorescent in situ hybridization using 5S and 35S rDNA and genomic probes. Chromosomes of C. denticulata were small with a well-defined centromeric region, whereas C. nevadensis had larger, densely stained chromosomes, and less [CMA.sup.+] heterochromatic bands. Cuscuta veatchii had 2n = 60 chromosomes, about 30 of them similar to those of C. denticulata and the remaining to C. nevadensis. GISH analysis confirmed the presence of both subgenomes in the allotetraploid C. veatchii. However, the number of rDNA sites and the haploid karyotype length in C. veatchii were not additive. The diploid parentals had already diverged in their chromosomes structure, whereas the reduction in the number of rDNA sites more probably occurred after hybridization. As phylogenetic data suggested a recent divergence of the progenitors, these species should have a high rate of karyotype evolution. Key words: allopolyploidy, CMA/DAPI bands, homogenization of 35S rDNA repeats, interphase nuclei structure, 5S and 35S rDNA sites. L'hybridation interspecifique et la duplication genomique menant a la formation d'allopolyploides constituent des evenements importants dans l'evolution des angiospermes. Au sein du genre Cuscuta (Convolvulaceae) de plantes parasites, les donnees moleculaires suggerent l'existence d'especes d'origine hybride. Il a ete propose que l'une d'elles, le C. veatchii, resulterait de l'hybridation entre le C. denticulata et le C. nevadensis, toutes deux faisant partie des Denticulatae. Pour tester cette hypothese, une analyse cytogenetique a ete realisee au moyen de coloration CMA/DAPI et d'hybridation in situ en fluorescence a l'aide des sondes d'ADNr 5S et 35S ainsi qu'a l'aide de sondes genomiques. Les chromosomes du C. denticulata etaient petits et presentaient des regions centro-meriques bien definies, tandis que ceux du C. nevadensis etaient plus grands, intensement marques et presentaient moins de bandes heterochromatiques [CMA.sup.+].Le C. veatchii comptait 2n = 60 chromosomes, dont environ 30 etaient semblables a ceux du C. denticulata et les autres semblables a ceux du C. nevadensis. L'analyse GISH a confirme la presence des deux sous-genomes au sein de l'allotetraploide C. veatchii. Cependant, le nombre de sites d'ADNr et la longueur du caryotype haploide chez le C. veatchii n'etaient pas additifs. Les parents diploides avaient deja diverge en ce qui a trait a leur structure chromosomique, tandis que la reduction dans le nombre de sites d'ADNr est probablement survenue apres l'hybridation. Puisque les donnees phylogenetiques ont suggere une divergence recente des especes parentales, ces especes doivent presenter une evolution caryotypique rapide. [Traduit par la Redaction] Mots-cles : allopolyploidie, bandes CMA/DAPI, homogeneisation des repetitions d'ADNr 35S, structure des noyaux en interphase, sites d'ADNr 5S et 35S., Introduction Events of interspecific hybridization are common in the evolution of most genera of angiosperms. If diploid hybrids retain their fertility they will be able to generate a wide diversity [...]