Internal transcribed spacer region evolution in Larix and Pseudotsuga (Pinaceae)

The nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) region has been characterized in the sister genera Larix and Pseudotsuga (Pinaceae). Complete sequences were obtained for seven species of Larix from North America and Eurasia and five species of Pseudotsuga from western North America and eastern Asia. ITS region lengths ranged from 1759 to 1770 bp in Larix and from 1564 to 1571 bp in Pseudorsuga. In both genera, ITSI is three times as long as the 5.8S plus ITS2 and contains subrepeats as observed in other genera of Pinaceae. Secondary structure models predicted that the subrepeats fold into terminal stem and loop domains. ITS polymorphism detected within individuals of Larix and Pseudotsuga suggests a slow rate of concerted evolution among nrDNA loci. Except for the placement of L. sibirica, phylogenetic analyses of the ITS region agreed with previously reported restriction site analyses of Larix and Pseudorsuga. The data were not consistent with phylogenetic hypotheses for Larix based primarily upon ovulate cone characters, failing to support a derivation of the North American L. laricina from a short-bracted Eurasian lineage. The phylogenetic hypothesis did nor conflict with a stepping stone model of evolution for Pseudotsuga, but a basal lineage could not be inferred for either genus.