Genesis of bivalent pairing in hexaploid clump Verbena

Both 6x Verbena aubletia (n=15) and 2x V. tenuisecta (n=5) form bivalents during meiosis, however, their 4x F₁ hybrid (V. aubletia × V. tenuisecta) shows almost complete homoeologous pairing involving on average 19.74 out of its 20 chromosomes. In 10% cells there are 4IV+2II indicating that essentially there may be 4 homoeologous sets of 5 chromosomes each in the F₁ hybrid. Evidently, V. aubletia is segmental allo-hexaploid involving 3 homoeologous genomes (A₁A₁ A₂A₂ A₃A₃). Whether its cytologically diploid behaviour is the result of a multivalent suppressor system or due to an acute property of preferential pairing, cannot be answered with certainty. In either case intergenomal homoeologies are totally suppressed resulting in bivalent pairing, meiotic isolation of the 3 genomes and institution of normal fertility.