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

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.