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
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