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

Area of Science:

• Molecular Biology
• Plant Systematics
• Evolutionary Biology

Background:

• The nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) region is a key marker for plant phylogenetic studies.
• Sister genera Larix and Pseudotsuga (Pinaceae) exhibit distinct geographic distributions and morphological variations, necessitating molecular investigation.

Purpose of the Study:

• To characterize the nrDNA ITS region in Larix and Pseudotsuga species.
• To infer phylogenetic relationships within and between these genera using ITS sequence data.
• To compare molecular-derived phylogenies with existing hypotheses based on morphological traits.

Main Methods:

• Complete nrDNA ITS sequences were obtained for seven Larix species and five Pseudotsuga species.
• Sequence lengths and structural features, including subrepeats, were analyzed.
• Phylogenetic analyses were conducted using the generated ITS sequences.

Main Results:

• ITS region lengths varied between Larix (1759–1770 bp) and Pseudotsuga (1564–1571 bp).
• ITS1 was approximately three times the length of 5.8S plus ITS2, with predicted secondary structures.
• Phylogenetic analyses generally agreed with previous restriction site data, except for L. sibirica, and contradicted ovulate cone-based hypotheses for Larix.

Conclusions:

• ITS sequence data provide valuable phylogenetic information for Larix and Pseudotsuga.
• The findings challenge existing phylogenetic hypotheses for Larix, particularly regarding the origin of North American L. laricina.
• The study supports a stepping-stone model for Pseudotsuga evolution but could not resolve basal lineages for either genus.