Review of Partial Hybrids between Herbaceous Medicago sativa and Woody Medicago arborea and Their Potential Role in Alfalfa Improvement

Medicago sativa (2n = 4x = 32) and M. arborea (2n = 4x = 32) were thought to be reproductively isolated until hybrids (Alborea) were produced by sexual reproduction for the first time in 2003 in Wisconsin. The hybrids were asymmetric, at or near 2n = 4x = 32, and with a predominance of the alfalfa genome. Only M. sativa seed parents with reproductive abnormalities, including unreduced eggs, have produced hybrids; where M. arborea has been used as the seed parent, no hybrids have resulted. Pedigree selection within derivatives of the two original M. sativa seed parents (MB and M8) has been successful in increasing the frequency of hybrids produced. While Alborea individuals more closely resemble M. sativa, a number of M. arborea-specific traits have been observed across different hybrid individuals. These include single-coil flat pods, large seeds, yellow flowers, indeterminate growth, a minimal crown, lodging, frost resistance, and anthracnose resistance. These M. arborea traits have the potential to restructure alfalfa to increase its versatility and utilisation. There is emerging evidence from North and South America and Australia that some Alborea selections have the capacity to complement adapted alfalfa cultivars for yield. Work is continuing to introgress M. arborea traits of value into alfalfa.