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Improvement in Two-Allele Autotetraploid Populations of Alfalfa Explained by Accumulation of Favorable Alleles

AgResearch Grasslands, New Zealand Pastoral Agric. Res. Inst., Palmerston North, New Zealand
Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706

^* Corresponding author (woodfieldd{at}agresearch.cri.nz).

Accumulation of favorable alleles in linkage blocks (linkats) is important for population improvement and increased heterosis in autotetraploid alfalfa (Medicago sativa L.). Original generation (OG) genotypes, produced by chromosome-doubling heterozygous diploid plants, are partially inbred (F = 0.33), and have a maximum of two alleles and therefore only one allelic interaction per locus. Inbreeding these OG genotypes by selfing and up to six generations of sib-mating reduces both heterozygosity (F = 0.50–0.59) and allelic interactions in these advanced generation (AG) two-allele autotetraploid populations (TAPs). Single crosses (SCs) between AG TAPs also have fewer allelic interactions than SCs between then- respective OG genotypes, and the relative performance of these SCs provides an unambiguous test of the importance of accumulating favorable alleles. The forage yield of a series of OG and AG SCs was compared from 1990 to 1992 in three replicated Wisconsin field experiments, whereas differences in fertility were determined in greenhouse studies. The mean forage yield of the AG (F₁ and F₂) SCs was 7 to 23% higher than their comparative OG SCs; however, there was no significant difference between OG and AG (S₁) SC performance. The AG SCs also had significantly higher fertility levels. The improvements in AG SC forage yield and fertility provided strong evidence that inbreeding and selection in TAPs resulted in accumulation of favorable alleles with additive to completely dominant effects.

Received for publication January 21, 1994.

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