Patterns of isozyme variation as indicators of biogeographic history in Pilgerodendron uviferum (D. Don) Florín

Abstract

The effects of Pleistocene glaciations on the genetic characteristics of the most austral conifer in the world, Pilgerodendron uviferum, were analysed with specific reference to the hypothesis that the species persisted locally in ice-free areas in temperate South America. It was expected that genetic variation would decrease with latitude, given that ice fields were larger in southern Patagonia and thus refugia were probably located towards the northern distributional limit of the species as suggested by the fossil record. In addition, an increase in among-population genetic divergence was expected with increasing distance to putative glacial refugia. We examined the relationship between location and within-population variability indices of 20 Pilgerodendron populations derived from isozyme analyses. We analysed possible refugia hypotheses by the distribution of allele frequencies using multivariate discriminant analysis. The degree of genetic differentiation with geographical distance between all population pairs was investigated by Mantel tests. Results indicated that Pilgerodendron populations are highly monomorphic, probably reflecting past population bottlenecks and reduced gene flow. Southernmost populations tend to be the least genetically variable and were therefore probably more affected by glacial activity than northern ones. Populations located outside ice limits seem to have been isolated during the glacial period. The presence of centres of genetic diversity, together with the lack of a significant correlation between genetic and geographical distances and the absence of geographical patterns of allelic frequencies at most analysed alleles, may indicate that Pilgerodendron did not advance southward after the last glaciation from a unique northern refugium, but spread from several surviving populations in ice-free areas in Patagonia instead.