Allozyme Heterosis in Radiata Pine is Poorly Explained by Overdominance

Abstract

Heterosis, the superior vegetative or reproductive vigor associated with heterozygosity, has been observed for allozymes in a wide variety of plant and animal taxa. Its causes, however, are widely debated. To gain insight into the causes of heterosis, we examined the relationship of allozyme heterozygosity and growth rate in a sample from a natural population of radiata pine. Some 244 clones derived from 30 natural stands were grown for 14 yr in an experimental plantation. Height, diameter, and crown growth rates were determined for six to nine ramets per clone; clonal genotypes were scored at 27 polymorphic allozyme loci. Levels of heterozygosity well below average were associated with depressed growth rates. The strength of heterosis within stands was positively associated with the estimated level of natural inbreeding and negatively associated with mean observed heterozygosity. Heterosis varied sporadically among loci and was unrelated to their levels of polymorphism, thus failing to support the hypothesis that the major polymorphisms are maintained by overdominant selection. Rates of growth also declined at high levels of heterozygosity, suggesting that the less frequent allozymes and/or tightly linked loci, which occur predominantly in heterozygous condition, tended to have deleterious effects.