Backcrossing a complex black seed trait from diploid into tetraploid alfalfa avoids the complexities of tetrasomic inheritance

Black seed in alfalfa (Medicago sativa L.) may be useful as a seed marker and for pigmentation studies. The trait is controlled by three to four genes. Normal alfalfa seeds with tan coats have a dominant inhibitor of the black trait. Black seeds appear to be conditioned by a homozygous recessive allele at the inhibitor locus and a dominant allele for pigments at each of two or more additional loci. The complexity of the trait makes its manipulation inefficient at the tetraploid level. In this study, 2n eggs of diploid alfalfa plants with black seed coats were used to transfer the trait to the tetraploid level. Five black-seeded diploid plants were used as female parents and crossed with random pollen from a select group of adapted tetraploid plants. Twenty-one tetraploid hybrids were identified in the first cross. All of the hybrids had tan seed coats. Each tetraploid hybrid was back-crossed as a pollen parent to an unrelated (crisscross backcross) black-seeded diploid plant to minimize inbreeding depression. Of the thirty tetraploid backcross hybrids produced, four were black-seeded, vigorous, and fertile. The four black-seeded tetraploids were used in another cross and backcross to transfer the trait to adapted cultivars and produce desired amounts of black seed. Although the blackseeded trait has a complex inheritance pattern, it was transferred to the tetraploid level with minimum effort. Alternative procedures would have required chromosome doubling or finding the trait in extremely large segregating populations at the tetraploid level