Dogroses represent an exceptional system for studying the effects of genome doubling and hybridization: their asymmetrical meiosis enables recombination in bi-parentally inherited chromosomes but prevents it in maternally inherited ones. We employed fluorescent in situ hybridization, genome skimming, amplicon sequencing of genomic and cDNA as well as conventional cloning of nuclear ribosomal DNA in two phylogenetically distinct pentaploid (2n = 5x = 35) species, Rosa canina and Rosa inodora, and their naturally occurring reciprocal hybrids, Rosa dumalis (5x) and Rosa agrestis (5x, 6x). Both progenitor species differed in composition, meiotic behaviour and expression of rDNA loci: R. canina (five 18S and 5-8 5S loci) was dominated by the Canina ribotypes, but R. inodora (four 18S loci and 7-8 5S loci) by the Rubiginosa ribotype. The co-localized 5S/18S loci occurred on either bivalent-forming (R. canina) or univalent-forming (R. inodora) chromosomes. Ribosomal DNA loci were additively inherited; however, the Canina ribotypes were dominantly expressed, even in genotypes with relatively low copy number of these genes. Moreover, we observed rDNA homogenization towards the paternally transmitted Canina ribotype in 6x R. agrestis. The here-observed variation in arrangement and composition of rDNA types between R. canina and R. inodora suggests the involvement of different genomes in bivalent formation. This results supports the hypothesis that the asymmetrical meiosis arose at least twice by independent ancient hybridization events.

Department of Botany Senckenberg Museum of Natural History Görlitz Am Museum 1 D 02826 Görlitz Germany

Department of Ecology and Environment Protection University of Applied Sciences Zittau Görlitz Theodor Körner Allee 16 D 02763 Zittau Germany

Department of Molecular Epigenetics Institute of Biophysics Academy of Sciences of the Czech Republic v v i Královopolská 135 612 65 Brno Czech Republic

Leibniz Institute on Aging Fritz Lipmann Institute Beutenbergstr 11 D 07745 Jena Germany

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