Inter‐population Improvement Procedures for Alfalfa1

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Abstract
We compared methods of improving a synthetic cultivar with parentage that traced to two different populations. Theoretical equations that expressed changes in the mean of a twd‐population synthetic were developed for three methods of selection: (1) selection within each population followed by combination of the selected individuals into a synthetic (CS); (2) selection based on top‐crossing each population to a common tester and combining the selected individuals (TX); and (3) reciprocal recurrent selection (RRS) in which each population served as the tester parent for the other. Theoretical responses were compared for additive, dominant, duplex dominant, and overdominant genetic models in a two‐allele, single locus autotetraploid population. Two pairs of alfalfa (Medicago sativa L.) populations were compared experimentally to test the effectiveness of polycross and topcross selection for resistance to Colletotrichum trifolii Bain, an index of resistance to C. trifolii and regrowth after cutting, and an index of these two traits plus resistance to Leptosphaerulina briosiana (Poll.) Graham and Luttrell.Each of the selection methods had the same theoretical response with the additive genetic model. The efficiency of TX relative to CS and RRS varied with gene frequency combinations for all nonadditive genetic models. With complete dominance a tester parent existed for which TX selection would be at least as effective as CS or RRS, although differences were small for some gene frequency combinations. Response with TX selection was greater with complete dominance when the tester parent was inferior to the base population. Generalizations could not be made about relative effectiveness of the methods for the duplex dominance model. Negative responses to selection for a trait with overdominance could occur at some gene frequency combinations with CS and TX selection, but response was never negative with RRS.Experimentally, topeross selection was more effective than polycross selection for resistance to C. trifolii when the tester parent had less resistance than the base population, and topeross selection was less effective when the tester parent was more resistant. Comparisons could not be made for the two‐character index selection, but good agreement with theoretical expectations was observed with the three‐character index.