1USDA-ARS, Soybean and Alfalfa Research Laboratory, Beltsville MD 20705. ²Department of Plant Breeding and Biometry, Cornell University, Ithaca, New York 14853.

High levels of inbreeding depression and evidence of heterosis led to early consideration of the development of hybrid alfalfa, however, it was concluded that self-incompatibility in alfalfa was not stable enough for the commercial production of hybrid seed and a cytoplasmic male sterility system was advocated. A major advantage to the use of a self-incompatibility system for producing hybrid alfalfa seed is that parental clones could be randomized in the field, and that F₁ seed could be harvested from all plants. Difficulties inherent in establishing a small number of genotypes for hybrid seed production may be alleviated through the field establishment of somatic embryos mass-produced in a bioreactor. Such a hybrid seed system may not be economically viable, however, a system in which a synthetic generation was produced from the F₁ progenies could have some potential. We conducted several experiments designed to assess the potential of a self-incompatibility system for enhancing hybridity and heterosis in alfalfa.

Our first experiments studied the responses of self-incompatible clones to four temperature regimes and revealed that clones with stable self-incompatibility and excellent male and female fertility could be selected. A factorial analysis of genetic variation using parents which represented a broad range of self-compatibility demonstrated that self-incompatibility was heritable (narrow-sense heritability=0.28) and that additive genetic variation was most important. In a later experiment, we evaluated the effects of self-incompatibility on forage yield in four F₁ crosses and four F₁/Syn1 pairs (Test 1), and in two partial diallel crosses within stable self-incompatible (SI) and self-compatible (SC) clones (Test 2). In Test 1, yields of some F₁ crosses and Syn1 populations were significantly greater than those of the check ‘Saranac AR.’ Except for one case, F₁ yields were significantly larger or smaller than Syn1 yields for each F₁/Syn1 pair. These differences may be due to variations in hybridity traceable to variations in stability of the self-incompatibility trait and/or to progressive heterosis in some Syn1 generations. In Test 2, some of the F₁ yields were significantly larger than those for the checks ‘KSNYPx’ and ‘Oneida VR.’ Most importantly, yields of the SI crosses were superior to those of the SC crosses, most likely due to reduced inbreeding depression in the SI diallel. RAPD and Anchored Microsatellite Priming analyses revealed no evidence of a relationship between self-incompatibility and inbreeding. We concluded that SI clones could be the products of inbreeding and/or a heritable, self-incompatibility mechanism. The importance of each factor would depend on genetic distances (GD’s) among the clones.

Environmentally stable SI clones separated by large GD’s would be a good basis for a SI-based hybrid alfalfa system or for use in other breeding schemes designed to minimize inbreeding while maximizing heterosis. Yield improvements of the F₁ crosses and Syn1 populations over checks were not sufficient to justify the added expense of implementing the proposed F₁/Syn1 seed production system. However, this system may have more potential if based on SI clones selected for maximum genetic distance and specific combining ability from more diverse populations with superior disease resistance than those studied here.