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Progressive Heterosis in Autotetraploid Alfalfa: Studies Using Two Types of Inbreds

Dep. of Plant, Soil, and Insect Sciences, Univ. of Wyoming, Laramie, WY 82071
Dep. of Plant and Soil Science, Montana State Univ., Bozeman, MT 59717

W. P. Kojis and E. T. Bingham

Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706

^* Corresponding author.

Progressive heterosis is unique to autopolyploids (polysomic polyploids) where heterozygosity and heterosis are not maximal in a single-cross hybrid of inbred parents. In an autopolyploid, heterozygosity increases with additional generations of hybridization and can result in a progressive heterotic response. This study was designed to evaluate progressive heterosis for important quantitative traits (yield of forage and seed) of autotetraploid (2n =4x = 32) alfalfa (Medicago sativa L.) using inbreds produced in two different ways. In Exp. 1, partially inbred parents (F = 0.333) were produced by colchicine doubling five genotypes of cultivated alfalfa at the diploid level (CADL). sixth CADL genotype produced 2n pollen and was used as a male parent. In Exp. 2, partially inbred parents were derived by selfing three cultivars (Vernal, Saranac, and Iroquois) the S₄ generation (F = 0.518). In each experiment, populations of increasing heterozygosity were constructed by successively hybridizing the inbreds and their progenies. Single cross (SC), three-way cross (3W), and double cross (DC) populations were constructed for Exp. 1. Single cross, DC, and double-double cross (DDC) populations were constructed for Exp. 2. In both experiments, populations were evaluated for forage yield in direct-seeded replicated field trials. Populations were also evaluated for yield of seed in the greenhouse and cage (Exp. 1) and in a replicated field trial (Exp. 2). The results of both experiments demonstrated progressive heterosis for yield of forage and seed in alfalfa. More heterozygous generations nearly always outperformed their more inbred progenitors. Whenever pairwise comparisons between generations were significant (P < 0.05), the more heterozygous generation exhibited superior performance. In Exp. 1, forage yields of the progressively more heterozygous 3W and DC generations were 114% and 125%, respectively, of the SC generation. Similarly, in Exp. 2, forage yields of the DC and DDC generations were, respectively, 120% and 137% of the SC generation. The results of this and previous studies indicate that successive hybridization of diverse strains might maximize the progressive heterotic response in alfalfa.

Received for publication August 29, 1988.

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