Submitted to: Journal of Genetics and Breeding
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/1996
Publication Date: N/A
Citation: Interpretive Summary: White clover is an important perennial forage legume used in animal agriculture throughout much of the U.S., but it is susceptible to drought, low pH, and virus. On the other hand, ball clover, a less productive distantly related clover common to central Europe, is more tolerant to these conditions. Since white clover has twice the number of chromosomes (32) as does ball clover, it is difficult to obtain hybrids between the tw species in order to transfer the desirable genes into white clover. We were able to detect a few ball clover plants which produced 3.5% 2n pollen grains (male gametes with twice the normal number of 8 chromosomes). This should allow us to cross the 2n-pollen-producing plants of ball clover (2n=16) with normal white clover (n=16) through conventional crossing procedures and obtain interspecific hybrids with 32 chromosomes like white clover. In our first attempt we obtained 13 triploid hybrid plants, all with 24 chromosomes. The absence of tetraploid interspecific hybrids shoul be overcome by increasing the frequency of 2n pollen in ball clover plants. Selection has been initiated to achieve this goal. In addition, we will backcross the triploid hybrid plants to white clover to attempt to get the tetraploid interspecific hybrid.
Technical Abstract: Ball clover, Trifolium nigrescens Viv., (2n=2x=16) has been indicated as a wild relative to white clover, Trifolium repens L., (2n=4x=32) and is a potential source of germplasm for incorporating desirable traits into white clover. Twenty Italian natural populations of ball clover, collected in Sardinia, were studied and 2n pollen was detected in 17 populations. However, it was not possible to detect plants with more than 3.4% 2n pollen, a frequency lower than that found in alfalfa (Medicago sativa L.), Lotus tenuis Wald. et Kit. and red cover (Trifolium pratense L.). Furthermore, all the 13 hybrid plants obtained by controlled white clover (4x) x ball clover (2x) crosses were triploid (2n=3x=24) suggesting the "triploid block" not to be operational in this interspecific cross. The absence of tetraploid interspecific hybrids to be used as a source of germplasm for incorporating desirable traits into white clover could be overcome by increasing the frequency of 2n pollen. Therefore, a recurrent selection scheme is in progress to increase the frequency of 2n pollen and 13 plants with more than 1% 2n pollen have been selected from four ball clover populations, and intercrossed.