GENETIC DIVERSITY IN POPULATIONS OF CRONARTIUM-RIBICOLA IN PLANTATIONS AND NATURAL STANDS OF PINUS-STROBUS

Genetic diversity was studied in 22 populations of the white pine blis ter rust fungus Cronartium ribicola from natural stands and plantation s of eastern white pine, Pinus strobus. Pseudo-allelic frequencies wer e estimated at each of 7 putative RAPD loci by scoring for presence or absence of amplified fragments in dikaryotic aecidiospores. Analysis of genetic distance between all pairs of populations did not reveal an y trend with regard to geographic origin or type of white pine stand. In addition, when hierarchical population structure was analysed, tota l genetic diversity (H-s=0.214) was mostly attributable to diversity w ithin populations ((H) over bar(s)=0.199; AMOVA phi(st)=0.121, P < 0.0 1). Genetic diversity of populations relative to region of origin (eas t, centre, and west) or type of stand (natural stands vs plantations) was not significantly different from zero (P > 0.10) Nevertheless, a s ignificant proportion of genetic differentiation was found between pop ulations within region or stand type ((F) over cap(st)=0.114; phi(sc)= 0.132, P<0.001). This results indicates that some population structure exists but that it appears to be independent of region of origin or t ype of stand. At least for 2 populations from white pine plantations, it appears possible that a recent introduction of a limited number of propagules was responsible for low levels of genetic diversity. We int erpret these results as meaning that either long-distance dispersal is taking place between populations more than 1000 km apart or that thes e populations share a common recent ancestor. In addition, we suggest that C. ribicola may still be expanding its distribution by colonizing new plantations.