1. ¹School of Biological Sciences, Queen Mary, University of London, London E1 4NS, UK
2. ²Balsgård-Department of Crop Science, Swedish University of Agricultural Sciences, Fjalkestadsvagen 495, S-291 94 Kristianstad, Sweden
3. ³Institute of Biophysics, Academy of Sciences of the Czech Republic, Kralovopolska 135, CS-61265 Brno, Czech Republic
4. ⁴School of Health and Bioscience, University of East London, Romford Road, London E15 4LZ, UK
5. ⁵INRA, Station d'Amélioration des Plantes Florales, La Gaudine-Frejus, 83370 Saint-Aygulf, France

Correspondence: KY Lim, School of Biological Sciences, Queen Mary, University of London, London E1 4NS, UK. E-mail: y.k.lim@qmul.ac.uk

Received 29 July 2004; Accepted 5 January 2005; Published online 16 March 2005.

Abstract

In Rosa canina (2n=5x=35), the pollen and ovular parents contribute, respectively, seven and 28 chromosomes to the zygote. At meiosis I, 14 chromosomes form seven bivalents and 21 chromosomes remain as univalents. Fluorescent in situ hybridization to mitotic and pollen mother cells (PMC) of R. canina showed that 10 chromosomes (two per genome) carry ribosomal DNA (rDNA) loci. Five chromosomes carry terminal 18S–5.8S–26S rDNA loci; three of these also carry paracentric 5S rDNA loci and were designated as marker chromosomes 1. Five chromosomes carry only 5S rDNA loci and three of these were designated as marker chromosomes 2. The remaining four of the 10 chromosomes with rDNA loci were individually identifiable by the type and relative sizes of their rDNA loci and were numbered separately. At PMC meiosis, two marker chromosomes 1 and two marker chromosomes 2 formed bivalents, whereas the others were unpaired. In a gynogenetic haploid of R. canina (n=4x=28), obtained after pollination with -irradiated pollen, chromosomes at meiosis I in PMC remained predominantly unpaired. The data indicate only one pair of truly homologous genomes in R. canina. The 21 unpaired chromosomes probably remain as univalents through multiple generations and do not recombine. The long-term evolutionary consequence for the univalents is likely to be genetic degradation through accumulated mutational change as in the mammalian Y chromosome and chromosomes of asexual species. But there is no indication that univalents carry degenerate 5S rDNA families. This may point to a recent evolution of the R. canina meiotic system.