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The genus Oenothera shows an intriguing extent of permanent translocation heterozygosity. Reciprocal translocations of chromosome aims in species or populations result in various kinds of chromosome multivalents in diakinesis. Early meiotic events conditioning such chromosome behaviour are poorly understood. We found a surprising uniformity of the leptotene-diplotene period, regardless of the chromosome configuration at diakinesis (ring of 14, 7 bivalents, mixture of bivalents and multivalents). It appears that the earliest chromosome interactions at Oenothera meiosis are untypical, since they involve pericentromeric regions. During early leptotene, proximal chromosome parts cluster and form a highly polarized Rabl configuration. Telomeres associated in pairs were seen at zygotene. The high degree of polarization of meiotic nuclei continues for an exceptionally long period, i.e., during zygotene-pachytene into the diplotene contraction stage. The Rabl-polarized meiotic architecture and clustering of pericentromeres suggest a high complexity of karyotypes, not only in structural heterozygotes but also in bivalent-forming homozygous species. Key words: translocations, meiosis, Oenothera, permanent translocation heterozygosity, multivalents. Resume : Chez le genre Oenothera, on rencontre une etendue intrigante d'heterozygotie permanente pour des translocations. Des translocations reciproques de bras chromosomiques chez des especes ou des populations produisent divers types de multivalents a la diacinese. Les evenements meiotiques precoces menant a un tel comportement chromosomique sont mal compris. Les auteurs ont observe une etonnante uniformite au cours de la periode leptotene-diplotene, sans egard a la configuration des chromosomes a la diacinese (anneau de 14, sept bivalents, melange de bivalents et de multivalents). Il semble que les toutes premieres interactions chromosomiques au cours de la meiose chez le genre Oenothera soient atypiques puisqu'elles impliquent des regions peri centromeriques. En debut de leptotene, des segments proximaux de chromosomes se rassemblent et forment une configuration Rabl tres polarisee. Des telomeres joints en paires ont ete observes au stade zygotene. L'importante polarisation des noyaux meiotiques se poursuit pendant une periode exceptionnellement longue, c.-a-d. de la phase zygotene-pachytene jusqu'au stade de contraction au diplotene. L'architecture meiotique Rabl polarisee et le regroupement des pericentromeres suggerent une grande complexite caryotypique non seulement chez les heterozygotes structurels, mais aussi chez les especes homozygotes formant des bivalents. Mots-cles : translocations, meiose, Oenothera, heterozygotie permanente pour les translocations, multivalents. [Traduit par la Redaction], Introduction For nearly a century, the genus Oenothera (evening primrose) has been a model for the study of chromosome rearrangements, behaviour, and evolution (Cleland 1972). Individual or multiple reciprocal chromosome [...]

The genus Oenothera has an outstanding scientific tradition. It has been a model for studying aspects of chromosome evolution and speciation, including the impact of plastid nuclear co-evolution. A large collection of strains analyzed during a century of experimental work and unique genetic possibilities allow the exchange of genetically definable plastids, individual or multiple chromosomes, and/or entire haploid genomes (Rennet complexes) between species. However, molecular genetic approaches for the genus are largely lacking. In this study, we describe the development of efficient PCR-based marker systems for both the nuclear genome and the plastome. They allow distinguishing individual chromosomes, Renner complexes, plastomes, and subplastomes. We demonstrate their application by monitoring interspecific exchanges of genomes, chromosome pairs, and/or plastids during crossing programs, e.g., to produce plastome--genome incompatible hybrids. Using an appropriate partial permanent translocation heterozygous hybrid, linkage group 7 of the molecular map could be assigned to chromosome 9.8 of the classical Oenothera map. Finally, we provide the first direct molecular evidence that homologous recombination and free segregation of chromosomes in permanent translocation heterozygous strains is suppressed.