HISTORICAL SEPARATION AND PRESENT GENE FLOW THROUGH A ZONE OF SECONDARY CONTACT IN PONDEROSA PINE

I examined the effects of historical division and secondary contact between eastern and western varieties of ponderosa pine (Pinus ponderosa Laws Pinaceae) on extant patterns of genetic variation. Fossil and biogeographic evidence both indicate that the current point of contact between these two varieties represents secondary contact following historical separation during the Wisconsin glaciation. Current gene flow was assessed by observing the degree of introgression of paternally inherited cpDNA and maternally inherited mtDNA polymorphisms. Both seeds and pollen are wind dispersed in ponderosa pine. Introgression was primarily from west to east, the direction of the prevailing wind, for both organelles, but introgression of cpDNA far exceeded that of mtDNA. Thus pollen is the main agent of contemporary gene flow between the two varieties. Neither seeds nor pollen showed enough introgression since secondary contact to have homogenized the two gene pools. However, allozyme differentiation was minimal. This calls into question assumptions of selective neutrality for at least some of the markers. Theory predicts that nuclear markers will show a high locus-to-locus variance of F_ST following historical separation. This prediction is confirmed by the allozyme data for ponderosa pine, and may provide a useful means of identifying historical separations from allele frequency data.