1. ¹Laboratory of Molecular Epigenetics, Academy of Sciences of the Czech Republic, v.v.i., Institute of Biophysics, Brno, Czech Republic
2. ²Balsgård-Department of Plant Breeding and Biotechnology, Swedish University of Agricultural Science, Kristianstad, Sweden
3. ³School of Biological and Chemical Sciences, Queen Mary, University of London, UK

Correspondence: Dr A Kovarik, Laboratory of Molecular Epigenetics, Academy of Sciences of the Czech Republic, v.v.i., Institute of Biophysics, Královopolská 135, Brno CZ-612 65, Czech Republic. E-mail: kovarik@ibp.cz

Received 22 February 2008; Revised 4 June 2008; Accepted 13 June 2008; Published online 23 July 2008.

Abstract

In pentaploid dogroses, Rosa section Caninae (2n=5x=35), the pollen transmits one basic genome (x=7) derived from the seven segregating bivalents, whereas the egg transmits four basic genomes (4x=28) one set derived from the segregation of seven bivalents and three sets of univalent-forming chromosomes. Chromosomes from all five genomes carry 18-5.8-26S nuclear ribosomal DNA (rDNA) sites. This mode of sexual reproduction, known as permanent odd polyploidy, can potentially lead to the independent evolution of rDNA on bivalent- and univalent-forming chromosomes. To test this hypothesis, we analyzed rRNA gene families in pollen and somatic leaf tissue of R. canina, R. rubiginosa and R. dumalis. Six major rRNA gene families (, , ' , and ) were identified based on several highly polymorphic sites in the internal transcribed spacers (ITSs). At least two of the major rRNA gene families were found in each species indicating that rDNAs have not been homogenized across subgenomes. A comparison of ITS1 sequences from leaf and pollen showed differences: the shared rRNA gene family was more abundant among pollen clones compared to leaf clones and must constitute a major part of the rDNA loci on bivalent-forming chromosomes. The and families were underrepresented in pollen genomes and are probably located predominantly (or solely) on the univalents. The results support the hypothesis that pentaploid dogroses inherited a bivalent-forming genome from a common proto-canina ancestor, a likely donor of the rDNA family. Allopolyploidy with distantly related species is likely to have driven evolution of Rosa section Caninae.