Genetic variation in chloroplast and nuclear ribosomal DNA in Utah juniper (Juniperus osteosperma, Cupressaceae): evidence for interspecific gene flow

Geographic patterns of genetic variation in chlorolast (cpDNA) and nuclear ribosomal (nrDNA) DNA were examined to test the hypothesis of hybridization between Juniperus osteosperma and Juniperus occidentalis in the Great Basin of western Nevada. Noncoding DNA from the trnL-trnF intergenic spacer and the trnL intron of the chloroplast genome was sequenced from seven populations of J. osteosperma and four populations of J. occidentalis sampled over a large proportion of their respective ranges. An adenine nucleotide at position 436 in the aligned sequence and within a Tru 9I restriction site was found to be present in individuals of J. osteosperma sampled from western Colorado and central Utah, but absent in sequences of J. osteosperma sampled from central and western Nevada and all sequences of J. occidentalis. Two hundred fourteen individuals from 34 populations of J. osteosperma and J. occidentalis were then screened for cpDNA haplotype by Tru 9I digestion of the trnL-trnF polymerase chain reaction (PCR) product. Two cpDNA haplotypes were evident, each consisting of restriction fragment profiles that differed solely with respect to the presence or absence of the Tru 9I site encompassing the adenine nucleotide at position 436. One of these haplotypes was monomorphic in J. occidentalis and exhibited a decreasing frequency in J. osteosperma with increasing geographic distance from J. occidentalis in west-central Nevada. Geographic patterns in nuclear ribosomal DNA (nrDNA) variation were examined by restriction fragment analysis and, although spatially more restricted, exhibited patterns of clinal variation similar to those observed in cpDNA haplotype. Genetic relationships based on DNA sequences and geographic patterns of genetic variation in chloroplast and nuclear ribosomal DNA are consistent with morphology in suggesting interspecific gene flow between J. occidentalis and J. osteosperma.