Plant Sciences Dept., Univ. of Wyoming, PO Box 3354, Laramie WY 82071 USA groose@uwyo.edu

Autotetraploid (2n=4x=32) alfalfa (Medicago sativa L.) suffers severe inbreeding depression upon repeated self-fertilization and displays progressive heterosis with successive hybridization. Therefore, it may be said that alfalfa bears a heavy genetic load. Genetic load might depress varietal performance, especially in narrow-based synthetic cultivars, and perhaps also in modern highly-selected multiple pest resistant cultivars.

Three models have been proposed to explain breeding behavior in alfalfa according to three forms of gene action: (1) DOMINANCE where, at individual loci, favorable dominant alleles mask deleterious/lethal recessive alleles; (2) OVERDOMINANCE involving heterozygous advantage of multiple functional alleles at individual loci; and (3) PSEUDO-OVERDOMINANCE due to complementation of favorable dominant alleles at different individual loci in tight repulsion phase linkage in chromosome segments or "linkats." These three models correspond to (1) a MUTATIONAL LOAD where nonfunctional recessive alleles are exposed with selfing;

(2) a SEGREGATIONAL LOAD where positive interactions among multiple alleles are lost with selfing; and (3) another form of SEGREGATIONAL LOAD where, as complementation among linkats is lost with selfing, individual loci become homozygous for recessive alleles. MODEL (1) might be considered the simplest, but Busbice and Wilsie (1966) proposed MODEL (2) because they observed that inbreeding depression was much more severe in autotetraploid alfalfa than would be expected based on the theoretical loss of heterozygosity (and increase in F, the probability that two alleles are identical by descent) which proceeds much more slowly in the autotetraploid than in the diploid. In their model involving true overdominance, a rapid loss of diallelic interactions among multiple alleles in highly heterozygous genotypes explains severe inbreeding depression. Because it is difficult to explain the existence of numerous heterotic alleles at an individual locus, MODEL (3) was proposed as a more realistic alternative (reviewed by Bingham et al., 1994).

In this paper, using concepts of genetic load and fitness, I explicitly evaluate these three models. I show that, for one version of Busbice and Wilsie’s MODEL (2) involving exactly four different alleles at a locus, a population bears a very heavy overt genetic load (average fitness less than half of maximum fitness) but that the subsequent exposure of covert genetic load with selfing could still explain rapid inbreeding depression. Similarly, the four locus/four linkat version of MODEL (3) illustrated by Bingham and co-workers with a single favorable dominant allele per linkat bears a seemingly impossible overt genetic load. However, reformulated, with each linkat bearing a single recessive lethal allele, an intermating population bears a much smaller overt load but exposes a large covert load with selfing. Finally, I show that MODEL (1) based on simple dominance may be a much better explanation for inbreeding depression in the autotetraploid than has previously been appreciated. This is because, despite the slower theoretical approach to homozygosity in the autotetraploid than in the diploid, the autotetraploid may bear a much larger mutational load. It is well known that in a diploid where dominance is complete, q, the equilibrium frequency of an unfavorable recessive allele in an intermating population depends on a balance between mutation and selection where the frequency of homozygous recessive genotypes (q²) is equal to the ratio of the mutation rate (m ) to the selective disadvantage (s) of the recessive homozygote, i.e., q²=m /s. I show that in the autotetraploid the frequency of the recessive homozygote is the same, i.e. q⁴ = m /s. Then, for all loci where mutation rates and selective disadvantages are the same between ploidy levels, I demonstrate two important results. First, overt mutational loads are equal between ploidy levels, i.e. . Second, total (and covert) loads are much larger in the autotetraploid because, by the same equation, the frequencies of recessive alleles are much higher in the autotetraploid.

Bingham ET, Groose RW, Woodfield DR, Kidwell KK. 1994. Crop Sci. 34:823-829.

Busbice TH, Wilsie CP. 1966. Euphytica 15:52-67.

Previous Page