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Backcrossing Tetraploidy into Diploid Medicago Falcata L. Using 2n Eggs

Dep. of Agronomy, 1575 Linden Dr., Univ. of Wisconsin, Madison, WI 53706-1597

^* Corresponding author.

Chromosome manipulation may be useful in germplasm enhancement where wild and cultivated forms are at different ploidy levels. In this study, tetraploidy was backcrossed into a diploid Medicago falcata L. synthetic (2x WISFAL-1) in a series of five 2x-4x backcrosses using 2x WISFAL-1 as the recurrent parent and the 4x M. sativa L. as the nonrecurrent parent. Since nearly all 2x M. falcata plants produced a low background frequency of 2n eggs, it was necessary to pollinate only =500 2x flowers with 4x pollen to obtain one to five tetraploid hybrids per 2x plant. The new tetraploid form of M. falcata, designated 4x WISFAL-1, contains 98% M. falcata germplasm and 2% M. sativa. The broad-based synthetic 4x WISFAL-1 has all the characteristics of 2x WISFAL-1 except that it has larger vegetative and reproductive organs, and crosses directly with cultivated alfalfa. The 4x WISFAL-1 can be used for gene transfer from M. falcata to cultivated alfalfa. The yellow flower color of 4x WISFAL-1 also can be used as a codominant flower-color marker at the tetraploid level. Two narrow-based tetraploid populations were developed by backcrossing to PI 231731 and PI 258754, respectively, using 4x WISFAL-1 as the nonrecurrent parent. Thus, 4x WISFAL-1 is also useful in germplasm enhancement of individual diploid Pis by elevating them to the tetraploid level for direct crossing with cultivated M. sativa.

Received for publication November 30, 1989.