DEVELOPMENT OF A FERTILE GENETIC BRIDGE BETWEEN TRIFOLIUM-AMBIGUUM M.BIEB. AND TRIFOLIUM-REPENS L

Trifolium ambiguum M. Bieb and T. repens L. are taxonomically related but very difficult to cross. The rare hybrids so far reported between these two species were obtained only by embryo culture. This difficult y has been overcome in the present research by the creation of a ''fer tile bridge'' between T, ambiguum and T. repens. Characters of interes t can now be transferred from T. ambiguum to T. repens by using this ' 'fertile bridge'' without the use of sophisticated techniques. An arra y of backcross progenies was generated from crosses between a T. ambig uum x T, repens F-1 hybrid (8x H-435) and its parental species. The 8x hybrid was cross-fertile only with T. repens and resulted in 145 seed s from 1578 reciprocal crosses. Eleven of nineteen initially grown BC1 F1 plants were all hexaploid with an average pollen stainability of 41 .6%. A high frequency of multivalents at metaphase-I indicated that bo th autosyndetic and allosyndetic pairing occurred. Backcrosses of 6x B C1F1 plants to T. repens resulted in 5x BC2F1 plants with an average p ollen stainability of 59.3%. On the other hand, 6x BC1F1 x 6x T. ambig uum crosses did not produce any seed and only two pentaploid plants we re obtained from 6x BC1F1 x 4x T. ambiguum crosses. The difficulty enc ountered in generating 6x backcross progeny with 6x T. ambiguum was ov ercome by intercrossing the 6x BC1F1 plants and producing 6x BC1F2 pla nts with an average pollen stainability of 65.8%. One of these 6x BC1F 2 plants was cross-compatible as a female with 6x T. ambiguum and resu lted in CBC2 plants that were all cross-compatible with 6x T. ambiguum . The 6x BC1F1 plants are likely to be superior to 6x BC1F1 progeny, a s they have exhibited better expression of the combined rhizomatous an d stoloniferous growth habit, improved fertility, more frequent nodal rooting and heavier nodulation. Consequently, the 6x BC1F2 plants can either be used directly in the selection programme or as a ''fertile b ridge'' between the two parental species. The present work has resulte d in the development of a series of fertile hybrids by the manipulatio n of chromosome numbers, combining the agronomic characteristics of th e parent species in varying genome balances and at a range of ploidy l evels. It is concluded that the initial sterility of the primary inter specific hybrids need not be a barrier to successful inter-breeding.