GENETIC-VARIATION ASSOCIATED WITH CHRONIC WATER AND NUTRIENT STRESS IN PINYON PINE

We examined how genotypic variation at the glycerate dehydrogenase (GL Y) locus in pinyon pine might be affected by environmental stress and herbivory. We compared GLY allelic and genotypic frequencies among mat ure and juvenile trees growing in stressful cinder soil and adjacent s andy-loam soil. While no association was found with herbivory, three l ines of evidence support the hypothesis that GLY slow homozygotes (SS) are selected for under conditions of environmental stress. 1) We foun d no differences at the GLY locus between juvenile and mature trees gr owing in sandy-loam soil. 2) However, in the stressful cinder soil we found a lower frequency of SS homozygotes in juvenile trees than in ma ture trees, suggesting that selection has occurred in one generation. 3) Using biomass as a measure of fitness, SS homozygotes were superior to other GLY genotypes in cinder soil. Two lines of evidence were inc onsistent with the hypothesis of selection for SS homozygotes in stres sful environments. 1) SS homozygote frequency between habitats was not significantly different, suggesting that too few generations have pas sed for differentiation to occur, and/or insufficient selection pressu re. 2) The low frequency of SS homozygote juvenile trees growing in ci nder soil suggests that counterbalancing selection may prevent SS homo zygotes from increasing in the population.