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 n uclear probes of restriction fragment length polymorphism, was examined in western redcedar (Thuja plicata Donn. ex D. Don). Only 8 of the 29 probes p roduced polymorphisms when HindIII-digested DNA samples from 18 trees from each of five geographic regions (90 trees in total) covering the entire geo graphic range of the species were analysed. The eight polymorphic probes we re 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 betw een the regions. Species-level expected heterozygosity (or total gene diver sity, H-T) was estimated at 0.070, a low value that is quite similar to tha t at isozyme loci. Nei's genetic distances between the regions were quite s mall and D-ST, the average gene diversity between populations, was only 0.0 042, indicating a low degree of differentiation among geographic regions. N o private alleles were found, and low-frequency alleles were widespread acr oss the species range. These results suggest that the present range of west ern 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.