2018 ASHS Annual Conference

Roses (x=7) can have a range of ploidy levels from diploid to decaploid, though most garden rose cultivars are tetraploid, triploid, or diploid. For the purposes of genetic studies, it is useful to distinguish between ploidy levels; however, there are not consistent, reliable morphological differences between roses of different ploidy levels, making direct ploidy determination necessary. In our program, we have found root squashes to be most useful for ploidy determination in roses, as flow cytometry results can be challenging for roses and pollen assessment is not always reliable. Using this technique, we have determined or confirmed the ploidy of 58 cultivars and species, and have tentatively determined the ploidy of 12 more. Among those definitely identified, we found 16 diploids, 7 triploids, and 34 tetraploids. Some of these results were surprising based on pedigree; for example, ‘Himmelsauge’, allegedly the progeny of diploid Rosa setigera and diploid Rosa rugosa, was shown to be either triploid or tetraploid. Furthermore, as roses can be successfully bred between ploidy levels, we also determined the ploidy of seedlings from crosses between the triploid cultivar ‘Home Run’ and various diploids and tetraploids. These interploidy crosses were found to produce both triploids and tetraploids, confirming that triploids likely produce mostly diploid pollen. Interestingly, a diploid by triploid cross was found to produce one tetraploid. This information will be used to inform breeding efforts as well as identify diploids for a potential genome-wide association study (GWAS).