Development of a fertile genetic bridge between Trifolium ambiguum M. Bieb. and T. repens L.

Abstract

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 difficulty has been overcome in the present research by the creation of a “fertile bridge” between T. ambiguum and T. repens. Characters of interest can now be transferred from T. ambiguum to T. repens by using this “fertile bridge” without the use of sophisticated techniques. An array of backcross progenies was generated from crosses between a T. ambiguum×T. repens F1 hybrid (8x H-435) and its parental species. The 8x hybrid was cross-fertile only with T. repens and resulted in 145 seeds from 1578 reciprocal crosses. Eleven of nineteen initially grown BC1F1 plants were all hexaploid with an average pollen stainability of 41.6%. A high frequency of multivalents at metaphase-I indicated that both autosyndetic and allosyndetic pairing occurred. Backcrosses of 6x BC1F1 plants to T. repens resulted in 5x BC2F1 plants with an average pollen stainability of 59.3%. On the other hand, 6x BC1F1×6x T. ambiguum crosses did not produce any seed and only two pentaploid plants were obtained from 6x BC1F1×4x T. ambiguum crosses. The difficulty encountered in generating 6x backcross progeny with 6x T. ambiguum was overcome by intercrossing the 6x BC1F1 plants and producing 6x BC1F2 plants with an average pollen stainability of 65.8%. One of these 6x BC1F2 plants was cross-compatible as a female with 6x T. ambiguum and resulted in CBC2 plants that were all cross-compatible with 6x T. ambiguum. The 6x BC1F2 plants are likely to be superior to 6x BC1F1 progeny, as they have exhibited better expression of the combined rhizomatous and 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 bridge” between the two parental species. The present work has resulted in the development of a series of fertile hybrids by the manipulation of chromosome numbers, combining the agronomic characteristics of the parent species in varying genome balances and at a range of ploidy levels. It is concluded that the initial sterility of the primary interspecific hybrids need not be a barrier to successful inter-breeding.