Species crosses in roses with different ploidy levels

Rose breeding and selection is already going on for centuries. Historically, relatively few species contributed to the modern rose cultivars. However, many wild species have interesting characteristics. Examples of valuable traits in species are a.o. yellow flower pigmentation, thornlessness, or, on the contrary, ornamental prickles, winter hardiness, drought resistance, shade tolerance, hips and disease resistance. Depending on the specific use of the rose as cut flower, garden plant, pot rose,… breeders may have interest in these specific plant characteristics. The rose breeders’ challenge is to introgress the desirable beneficial genes from wild species to tetraploid cultivars in order to accelerate the production of superior rose germplasm. Interspecific crosses between species and cultivars are hampered by several factors, including taxonomic distance and ploidy differences. The understanding of the relationships among species and cultivars is therefore a prerequisite for the effective utilization of the available genetic variability. From a cytological point of view, roses are characterized by ploidy series based on multiples of 7. The chromosome numbers found in nature range from 2n=2x to 8x or 14 to 56 chromosomes respectively. Hybrids between diploid and tetraploid roses are possible. The obtained triploid progeny frequently show reduced fertility, creating a bottleneck for further breeding. However, backcrossing a triploid F1 hybrid with a tetraploid rose cultivar fertile tetraploid F2 plantlets can be obtained. Also successful hybridization with pentaploid dogroses is possible. Besides, two different strategies can be envisaged to overcome ploidy barriers: haploidization of tetraploid cultivars and polyploidization of wild, mostly diploid genotypes. Another bottleneck is that hybrid seeds obtained from crosses between modern cultivars and wild species often show seed dormancy. Therefore germination starts only in the second year. The obtained F1 progenies also frequently show some dominant characteristics of the wild parent: no flowers are formed during the year of germination, and the growth habit is in general more wild. This makes that several backcrosses are necessary to obtain a commercial product and that breeding period is extended. However, success with interspecific crosses and ‘incompatible crosses’ can be important to achieve certain new breeding goals. Collaboration between research institutes and breeders might help to create new valuable pre-breeding material.