Combining Ability Differences between Isogenic Diploid and Tetraploid Alfalfa

Abstract

A principal advantage of reducing polyploid crops for breeding at the diploid level (analytic breeding) is that inheritance is simpler and more intense selection might be applied for some traits at the diploid level. The polyploid would then be resynthesized from selected diploids. Analytic breeding has been used to effectively manipulate genes that condition several qualitative traits in autotetraploid alfalfa (Medicago sativa L.). This study was designed to compare the relative merit at the diploid and tetraploid levels of genes that contribute to important quantitative traits of alfalfa. Isogenic diploid and tetraploid clones of alfalfa were produced by colchicine doubling five clones of cultivated alfalfa at the diploid level (CADL). A sixth CADL clone produced n and 2n pollen and was used as both a diploid and tetraploid parent. The CADL clones varied in the number of diploid generations in their background from one to six generations. The six clones were mated in diallel at the diploid and tetraploid levels to evaluate combining ability. Single cross families were evaluated for forage yield and fertility at each ploidy level. Neither single cross family means nor general combining abilities were positively correlated between ploidy levels. A genotype that was the poorest parent at the diploid level was the best parent at the tetraploid level. In summary, a diallel analysis at the diploid level failed to predict performance at the tetraploid level. Selection of diploid genotypes for use at the tetraploid level would be undesirable in this set of materials.