Cytology and Fertility of the Interspecific Hybrid <i>Trifolium ambiguum</i> ✕ <i>T. repens</i> and Backcross Populations

Interspecific hybridizations of white clover ( Trifolium repens L.), a widely grown forage legume, with kura clover ( T. ambiguum M. Bieb.) have been made to facilitate transfer of genes conferring virus resistance and longevity from kura clover to white clover. Additionally, kara clover may be improved by the seedling and aftermath vigor of white clover. The objectives of this study were to backcross both tetrapioid and octoploid interspecific hybrids of kura clover (2 n = 4 x = 32) and white clover (2 n = 4 x = 32) to both parents, and to estimate the possibility of successful introgression of genes between the two species. Cytology and pollen stainability of the parents, resulting progeny, and additional backcross plants previously obtained were examined. Eighteen plants were obtained from backcrosses of one 4 x interspecific hybrid (Hybrid 435) to white clover. Ten of the backcross progeny had 48 (6 x ) somatic chromosomes, five had 32; three others were aneuploid. The higher than expected ploidy level may have resulted from union of 2 n gametes contributed by the Hybrid 435 female parent with n gametes of white clover. Backcrosses to kura clover produced no progeny. Backcross progeny (2 n = 32) of Hybrid 435 to white clover averaged 77.9% pollen stainability. Meiotic chromosomes of plants from backcrosses of 4 x Hybrid 435 to white clover associatetl predominantly as bivalents, indicating the presence of allosyndetic pairing. The high frequencies of allosyndetic pairing that occur in first backcross progenies of 4 x interspecific hybrids to white clover, and theoretically in second backcross progeny of 8 x interspecific hybrids to kura clover, should allow recombination of genes from kura clover and white clover.