Breeding potential of the backcross progenies of a hybrid between Trifolium medium × × T. pratense to T. pratense. Can. J. Plant Sci. 82: [395][396][397][398][399]. In this study, T. medium (zigzag clover, 2n = 80 and T. pratense (red clover, 2n = 28) were hybridized and backcrossed four times to T. pratense (BC 1 , BC 2 , BC 3 and BC 4 ), and the seed production and vigor of the backcross progenies were assessed under field conditions. Both female and pollen fertility of the backcross progenies increased as the backcross generations advanced. Female and pollen fertility of BC 4 were 21.3 and 65.3%, respectively. When honeybees were used as pollinators, almost all BC 4 plants produced mature seeds in the field. Most of the BC 1 , BC 2 , and BC 3 plants died by the end of the fourth year in the field, whereas 61% of the BC 4 plants survived for 4 yr. The number of chromosomes in 79% of the BC 4 plants was more than 28, which is greater than that of T. pratense.
Mots clés: Rétrocroisement, matériel génétique, hybridation interspécifique, Trifolium medium, Trifolium pratenseTrifolium pratense L. (red clover) is an important forage legume in the northern part of Japan because it is easy to cultivate due to high adaptability to acidic and humid conditions. However, its usefulness is limited due to its lack of perenniality, which is a common deficiency of T. Pratense throughout the world. Introgression of genes for longevity from perennial Trifolium species to T. pratense is a potential means for improving the persistence of T. pratense. Various hybridizations with T. pratense have been carried out with several species: T. pallidum [Palestine clover (Armstrong and Cleveland 1970)], T. diffusum (Taylor et al. 1963;Schwer and Cleveland 1972), T. hirtum [Rose clover (Schwer and Cleveland 1972)], T. sarosiense Collins et al. 1983), T. alpestre [Purple glove clover (Merker 1988;Phillips et al. 1992)], and T medium [Zigzag clover (Merker 1982);Nedbalkova et al. 1995;Sawai et al. 1990Sawai et al. , 1995. Although many hybrids have been successfully produced, there have been no reports of hybrid plants being used as germplasm in conventional breeding programs. The reason for this lack of use is that the plants have not shown sufficient levels of fertility and vigor to justify employing them as practical breeding materials. Anderson et al. (1991) produced third backcross progenies of hybrids between T. ambiguum (Kura clover) × T. repens (White clover) to T. repens, and succeeded in increasing their fertility to a sufficient level for direct use in forage production. Their study showed that backcrossing is an effective means for increasing fertility and vigor.To introduce the rhizomatous growth habit and perenniality from T. medium to T. pratense, Sawai et al. (1995) obtained first and second backcross progenies of the hybrid between T. medium × T. pratense to T. pratense. The fertility of the backcross progenies increased as their generations advanced. However, neither the fertility nor the vigor were adequate for use in a ...