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

Geographic patterns of generic 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 Basi n of western Nevada. Noncoding DNA from the trnL-trnF intergenic spacer and the trnL. intron of the chloroplast genome was sequenced from seven popula tions of J. osteosperma and four populations of J. occidentalis sampled ove r a large proportion of their respective ranges. An adenine nucleotide at p osition 436 in the aligned sequence and within a Tru 91 restriction site wa s found to be present in individuals of J. osteosperma sampled from western Colorado and central Utah, but absent in sequences of J. osteosperma sampl ed from central and western Nevada and all sequences of J, occidentalis. Tw o hundred fourteen individuals fi om 34 populations of J. osteosperma and J , occidentalis were then screened for cpDNA haplotype by Tru 91 digestion o f the trnL-trnF polymerase chain reaction (PCR) product. Two cpDNA haplotyp es were evident, each consisting of restriction fragment profiles that diff ered solely with respect to the presence or absence of the Tru 91 site enco mpassing the adenine nucleotide at position 436. One of these haplotypes wa s monomorphic in J. occidentalis and exhibited a decreasing frequency in J. osteosperma with increasing geographic distance from J. occidentalis in we st-central Nevada. Geographic patterns in nuclear ribosomal DNA (nrDNA) var iation were examined by restriction fragment analysis and, although spatial ly 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 choroplast and nuclear ribo somal DNA are consistent with morphology in suggesting interspecific gene f low between J. occidentalis and J. osteosperma.