Nuclear restriction fragment length polymorphism analysis of genetic diversity in western redcedar

Variation at 41 putative genetic loci, uncovered by 29 single or low-copy nuclear probes of restriction fragment length polymorphism, was examined in western redcedar (Thuja plicata Donn. ex D. Don). Only 8 of the 29 probes produced polymorphisms when HindIII-digested DNA samples from 18 trees from each of five geographic regions (90 trees in total) covering the entire geographic range of the species were analysed. The eight polymorphic probes were used against an additional sample of 160 trees (for a total of 250 trees or 50 per region) to permit higher resolving power of differentiation between the regions. Species-level expected heterozygosity (or total gene diversity, H-T) was estimated at 0.070, a low value that is quite similar to that at isozyme loci. Nei's genetic distances between the regions were quite small and D-ST, the average gene diversity between populations, was only 0.0042, indicating a low degree of differentiation among geographic regions. No private alleles were found, and low-frequency alleles were widespread across the species range. These results suggest that the present range of western redcedar is occupied by descendants of a small number of lineages from a single refugial population that survived the drastic changes in climatic conditions associated with the last ice age.