Cytological studies in the genus Rosa. III. Meiotic analysis of ten cultivars.

Male meiosis was studied in ten tetraploid (2n=28) garden roses. PMCs at diakinesis possessed univalents, bivalents and / or polyvalents in various frequencies. Certain combinations of chromosome associations were characteristic to each cultivar. The chiasma frequency varied from 14.77 to 18.04 per PMC and from 1.055 to 1.284 per bivalent. Various irregularities at first and second meiotic divisions resulted in 4 to 51% of PMCs with micronuclei formations which ultimately reduced the pollen fertility. Six out of ten cultivars were related and showed inheritance of various chromosome configurations. Three out of the six related cultivars carried an extra small B chromosome. This B was inherited for three generations and its presence was associated with higher univalent frequency per PMC and subsequently pollen sterility.