HETEROZYGOSITY AND DEVELOPMENTAL STABILITY UNDER INBREEDING AND CROSSBREEDING IN PINUS ATTENUATA.

The relationship between stability of annual trunk growth and heterozygosity at 24 polymorphic isozyme loci was studied in 10-year-old trees of knobcone pine (Pinus attenuata Lemm.) that were the products of contrasting systems of mating, self-, and interpopulation cross-pollination. Heterozygosity and variability of trunk growth were strongly related only when inbreds and crossbreds were compared; the crossbreds showed greater residual variability on an absolute scale, and greater responsiveness to climate on both absolute and relative scales. Within the inbreds there was no evidence of a relationship between heterozygosity and variability. Within the crossbreds, only one trait, a measure of relative trunk growth rate, showed a relationship with heterozygosity, and indicated greater variability of the more heterozygous trees. These results, and others in the literature, suggest that the relationship of heterozygosity to homeostasis for fitness components is neither simple nor monotonic; it varies between scales of measurement, genetic backgrounds, and environments.