Evolution of a triplet repeat in a conifer

Area of Science:

• Molecular Evolution
• Genomics
• Paleobotany

Background:

• Tracing the evolutionary history of triplet repeats, particularly in seed plants with fossil records, is challenging.
• Microsatellites, like the CAGn repeat, provide valuable markers for evolutionary studies.

Purpose of the Study:

• To calibrate the birth and expansion of a specific CAGn triplet repeat (PtTX2133) within conifer lineages.
• To investigate the evolutionary timeline of this microsatellite in relation to paleogeographic events, such as North American vicariance.

Main Methods:

• Analysis of microsatellite PtTX2133 sequences from 18 species across two conifer genera.
• Calibration of repeat origin and expansion using fossil record data and phylogenetic comparisons.
• Comparative analysis between hard pines (Diploxylon), other pine groups (Haploxylon), and the sister genus Picea.

Main Results:

• The CAGn repeat originated in the hard-pine genome approximately 136 million years ago.
• Repeat expansion occurred 136-100 million years ago, predating a major North American vicariance event.
• Shared allelic lineages support the timing and geographic spread, with a five-unit threshold for expansion in Diploxylon pines; Haploxylon and Picea showed no such expansion.

Conclusions:

• The polymorphic triplet repeat in hard pines is a long-lived microsatellite, older than those in reptiles but younger than in insects.
• The findings provide a calibrated timescale for microsatellite evolution within a plant lineage.
• Cautionary notes are raised regarding the phylogenetic utility of this specific long-lived microsatellite.