Sexual Preferences Linked to Rose Taxonomy and Cytology

Abstract

The genus Rosa is usually subdivided into four subgenera, the largest of these is subgenus Rosa with 10 sections. Most of the genetically analysed rose species appear to be sexual and diploid (2n = 14) or tetraploid (2n = 28) although there are a few triploid (2n = 21), hexaploid (2n = 42) and octaploid species (2n = 56). The diploid species are usually self-incompatible whereas the polyploids are self-fertile. Pollen stainability is usually high in all species with even ploidy levels, i.e. 2x, 4x or 6x. Rose species are usually sexual and have a regular meiosis but there is one deviating section, Caninae, which harbours the so-called dogroses. Most of these are 5x but there are some taxa with 4x and 6x. Only seven chromosomes (derived from seven bivalents) are transmitted through the pollen grains, whereas egg cells contain 21, 28 or 35 chromosomes (derived from seven bivalents and 14, 21 or 28 univalents) depending on the ploidy level. Apomixis occurs occasionally in the dogroses and genetic selfing is probably common since these taxa are self-fertile. Interspecific hybridization takes place spontaneously among rose species at all ploidy levels and is used as a potent tool in plant breeding. Information about compatibility, breeding system, pollen viability, chromosome number and inheritance is important for optimal utilization of crosses in rose breeding.

Evolutionary implications of permanent odd polyploidy in the stable sexual, pentaploid of Rosa canina L

“…However, this seems unlikely because no trivalents were seen in the pentaploid R. canina, the frequency of chromosome pairing in the gynogenetic haploid was low and the molecular evidence (Werlemark et al, 1999;Wissemann, 1999;Nybom et al, 2004) indicates that only two of the five genomes of R. canina are closely similar. However, the formation of bivalents and trivalents in the gynogenetic haploid, albeit in low frequency, also demonstrates a limited potential for chromosome pairing between genomes of R. canina that was not expressed in the pentaploid.…”

“…If the matroclinal system of the Caninae evolved only once (Blackhurst, 1948;Werlemark, 2003;Nybom et al, 2004) and the genes that suppress homeologous pairing are located on the pairing chromosomes (Werlemark, 2003), bivalent-forming chromosomes throughout the Caninae are likely to be of common origin. This possibility is supported by the evidence of Nybom et al (2004) that the alleles shared by three pentaploid and one tetraploid species reside on the bivalent-forming chromosomes, whereas species-specific alleles reside on the univalent-forming chromosomes.…”

“…Werlemark et al (1999) showed that, in reciprocal crosses between two pentaploid species, more than half the somatic RAPD markers were transmissible only through the female parent, Wissemann (1999) detected only four distinct alleles of internally transcribed spacers of the 18S-5.8S-26S (18-26S) ribosomal DNA (rDNA) unit in a pentaploid species, and Nybom et al (2004) found only four different microsatellite alleles at each of several loci in three pentaploid species and only three different alleles at several loci in a tetraploid species. Nybom et al (2004) concluded that the bivalent chromosomes were highly similar. Darlington (1965) came to the same conclusion on the evidence of higher frequencies of chiasma per bivalent in the Caninae than other sections of Rosa.…”

Frequent silencing of rDNA loci on the univalent-forming genomes contrasts with their stable expression on the bivalent-forming genomes in polyploid dogroses (Rosa sect. Caninae)

“…Ploidy levels were determined by flow cytometry in earlier studies (Nybom et al, 2004(Nybom et al, , 2006. All of these accessions were found to be pentaploid (2n ¼ 5x ¼ 35) except for R. mollis, which was tetraploid (2n ¼ 4x ¼ 28).…”

“…However, recent molecular investigations have shed some light on the origin and structure of the individual subgenomes. Specifically, microsatellite DNA markers (Nybom et al, 2004(Nybom et al, , 2006, RAPD (Werlemark and Nybom, 2001) and nuclear ribosomal RNA genes (rDNA) (Kovarik et al, 2008a) have shown that the bivalent-forming genomes seem to be rather similar across different dogrose species, even those occurring in different subsections. In contrast, distinctly different univalent genomes occur within single genotypes, but also show levels of differentiation that are associated with taxonomic distances between genotypes.…”