The fate of ribosomal RNA genes in spontaneous polyploid dogrose hybrids (Rosa L. sect. Caninae (DC.) Ser.) exhibiting non-symmetrical meiosis.

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 insitu 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 Rosainodora, 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. Significance Statement The asymmetrical meiosis in dogroses transfers in addition to pairing chromosomes several non-recombining genomes exclusively via egg cells to the offspring and is presumed to be a unique product of the allopolyploid origin of these plants. Cytogenetic and genomic analyses of two distantly related dogrose species and their natural hybrids revealed that pairing chromosomes profoundly differ in rDNA composition, copy number, and expression supporting a multiple origin of the meiosis system.