Environmental, Genetic, and Ecophysiological Variation of Western and Utah Juniper and Their Hybrids: a Model System for Vegetation Response to Climate Change.
Genetic variation in populations of J. osteosperma and J. occidentalis was studied in order to test morphologically-based hypotheses of interspecific gene flow, examine relationships between hybridization and genetic diversity, and better understand the role of hybridization and other population genetic factors in determining genetic structure in these species. Nineteen restriction fragment length polymorphisms in nuclear ribosomal (nrDNA) and chloroplast DNA (cpDNA) were identified that are polymorphic within populations or species. Five of these polymorphisms, three nrDNA markers and two cpDNA haplotypes, had significantly different mean frequencies among species or subspecies and were used to make inferences regarding gene flow across species' boundaries. The geographic and taxonomic distributions of these markers as well as cluster analysis of nrDNA allele presence/absence data support limited gene flow between populations of each of the two subspecies of J. occidentalis and J. osteosperma. High levels of genetic variation exist within populations of both species, with mean gene diversities for the nuclear and cytoplasmic loci examined beint 0.70 and 0.15 respectively. Differences in the extent of intrapopulation genetic variation are attributed in part to variation in population size, density, and mating system, and pronounced genetic differentiation among populations in nrDNA suggests that levels of gene flow have not been sufficient to appreciable in fluence population genetic structure at this locus. These results corroborate isozyme-based population genetic studies and paleoecological data which predict high levels of genetic diversity in relict conifer populations of the Great Basin, although we find little support for the contention that genetic diversity is a consequence of introgression.