Eco-geographically divergent diploids, Caucasian clover (Trifolium ambiguum) and western clover (T. occidentale), retain most requirements for hybridization

Abstract

Partially fertile hybrids between Trifolium ambiguum and T. occidentale were obtained for the first time. The F(1) hybrids produced seeds after open-pollination, and also produced triploid progeny in backcrosses to T. occidentale from the functioning of unreduced gametes in the hybrids. These plants were fertile and produced progeny with T. occidentale and with T. repens. Meiotic chromosome pairing in the F(1) showed six to eight bivalents per pollen mother cell, indicating pairing between the parental genomes. A chromosome-doubled form of one hybrid, produced using colchicine, showed some multivalents, indicative of interspecific chromosome pairing. The hybrid plants were robust and combined phenotypic characteristics of both species, having stolons, thick roots and a few rhizomes. Results show that despite separation by the entire breadth of Europe, the speciation process is incomplete, and these taxa have partially retained most of the genetic compatibilities needed for hybridization (possibly except for endosperm development, which was not tested). The fertile progeny populations could lead to new clover breeding strategies based on new hybrid forms.

A Eurasia-wide polyploid species complex involving 6x Trifolium ambiguum, 2x T. occidentale and 4x T. repens produces interspecific hybrids with significance for clover breeding

“…Female fertility (OP seed-set) was, however, apparently much lower in the 6x T. ambiguum x 2x T. occidentale F 1 s than in the hybrids involving 2x and 4x T. ambiguum. One 2x hybrid produced nearly one (0.8) OP seeds/head [24], while the 3x hybrid produced 1-2 OP seeds/head [25]. In contrast, the seed-sets obtained in the present study ranged from 0.00-0.14 seeds/head (Table 4).…”

“…Of six hybrids checked, five were below 10% in pollen staining. Very low pollen staining (<3%) was also observed in three F 1 hybrids between 2x T. ambiguum and 2x T.occidentale [24] and, similarly, it was approximately 9% in one 3x F 1 between 4x…”

“…Previous studies [24][25] have shown that both 2x and 4x T. ambiguum could be hybridized with 2x T. occidentale and that the F 1 s were inter-fertile with white clover. This indicated that all four taxa were derived from a common ancestor and were part of a species radiation that now covers all of Europe and parts of W Asia.…”

“…An alternative strategy could be to use the knowledge that white clover is a hybrid between ancestral forms of T. pallescens and T. occidentale [12] and that there has been successful hybridization of 2x and 4x T. ambiguum with 2x T. occidentale. This suggested that T. occidentale could be used as a genetic bridge between white clover and T. ambiguum [24][25]. In the present case, this would require the generation of not only fertile hybrids between 6x T. ambiguum and T. occidentale, but also the ability to obtain hybrids of these with a third species, T. repens.…”

Breaking through the feed barrier: options for improving forage genetics

“…In addition to direct crosses with white clover, other hybrid combinations between these related species provide a valuable source of new forage germplasm. They can also be used as genetic bridges through which new traits can be introduced into white clover (Williams et al 2011). Re-introducing genomes that have long been separated, and subsequent genetic recombination in these interspecific hybrids, are both options to increase the potential for useful variation.…”