Development of a ploidy series from a single interspecific Trifolium repens L x T-nigrescens Viv F-1 hybrid

The objective of the current research was to generate a ploidy series of backcross progenies from a single triploid (2n=3x=24) Trifolium repens x T. nigrescens F-1 hybrid (3x H-6909-5). The 3x H-6909-5 plant was highly sterile and produced no seeds from approximately 3000 reciprocal backcrosses to both parental species. Chromosome doubling by an in vitro colchicine method resulted in a marked increase in fertility. Pollen stainability was increased from 9.9% in 3x H-6909-5 to an average of 89.2% (range 87.7-90.9%) in the three chromosome-doubled 6x H-6909-5 plants. Subsequent backcrosses of 6x H-6909-5 and interbreeding of backcross derivatives resulted in an array of fertile hybrids at 4x, 5x and 7x levels and some aneuploids. The occurrence of 7x BC1F1 progeny from the T. repens x 6x H-6909-5 (4x x 6x) cross is the first unequivocal evidence of functional female 2n gametes in white clover. Meiotic pairing in F-1 and BC1F1 progeny indicated the presence of allosyndetic pairing, suggesting that genetic exchange between the two species is possible.