Genetic variation associated with chronic water and nutrient stress in pinyon pine
1994
Cobb, N.S. | Mitton, J.B. | Whitham, T.G.
We examined how genotypic variation at the glycerate dehydrogenase (GLY) locus in pinyon pine might be affected by environmental stress and herbivory. We compared GLY allelic and genotypic frequencies among mature and juvenile trees growing in stressful cinder soil and adjacent sandy-loam soil. While no association was found with herbivory, three lines of evidence support the hypothesis that GLY slow homozygotes (SS) are selected for under conditions of environmental stress. 1) We found no differences at the GLY locus between juvenile and mature trees growing in sandy-loam soil. 2) However, in the stressful cinder soil we found a lower frequency of SS homozygotes in juvenile trees than in mature 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 inconsistent with the hypothesis of selection for SS homozygotes in stressful environments. 1) SS homozygote frequency between habitats was not significantly different, suggesting that too few generations have passed for differentiation to occur, and/or insufficient selection pressure. 2) The low frequency of SS homozygote juvenile trees growing in cinder soil suggests that counterbalancing selection may prevent SS homozygotes from increasing in the population.
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