ITS Sequence Variation within and among Populations of Lomatium grayi and L. laevigatum (Umbelliferae)

Nuclear ribosomal RNA genes provide markers for retrieving phylogeny at a variety of taxonomic levels. The internal transcribed spacers (ITS), separating the coding regions, evolve relatively rapidly and may be useful for reconstructing phylogenies at the specific and generic levels. ITS-1, located between the 18S and 5.8S genes, may be especially valuable at the species level and below. However, despite the promise that this region holds for studies below the species level, few studies have addressed levels of ITS-1 variability within and among conspecific populations of plants. To assess the apportionment of ITS-1 sequence variation, a hierarchical sampling strategy was used to compare sequences from individuals within and among populations of Lomatium grayi and L. laevigatum, widespread and restricted congeners of the Umbelliferae, respectively. ITS-1 is 209 bp in length in L. grayi and 200 bp in L. Laevigatum. No sequence variation in ITS-1 was observed within populations of either species, and a single population of the more widespread L. grayi differed from the other seven populations by two base substitutions for a sequence divergence of l.0%. Sequence divergence between L. Laevigatum and the common ITS-1 sequence of L. grayi was 1.5%; that between L. Laevigatum and the variant of L. grayi was 2.5%. ITS-1 sequences in Lomatium are less variable than either allozymes or chloroplast DNA and do not appear to provide a valuable source of intraspecific markers for population-level studies. However, ITS-1 sequence comparisons seem well suited to both interspecific and intergeneric comparisons, although relationships between closely related species may not be resolved and distantly related genera may be too divergent for meaningful comparisons. Finally, because no variation was detected among conspecific individuals of the narrowly distributed L. Laevigatum and a single variant was observed in the more widespread L. grayi, sampling within species for studies of ITS-1 variation probably need not be extensive.