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 Pina ceae) on extant patterns of genetic variation. Fossil and biogeographic evi dence both indicate that the current point of contact between these two var ieties represents secondary contact following historical separation during the Wisconsin glaciation. Current gene flow was assessed by observing the d egree of introgression of paternally inherited cpDNA and maternally inherit ed mtDNA polymorphisms. Both seeds and pollen are wind dispersed in pondero sa pine. Introgression was primarily from west to east, the direction of th e prevailing wind, for both organelles, but introgression of cpDNA far exce eded that of mtDNA. Thus pollen is the main agent of contemporary gene flow between the two varieties. Neither seeds nor pollen showed enough introgre ssion since secondary contact to have homogenized the two gene pools. Howev er, allozyme differentiation was minimal. This calls into question assumpti ons of selective neutrality for at least some of the markers. Theory predic ts that nuclear markers will show a high locus-to-locus variance of F-ST fo llowing historical separation. This prediction is confirmed by the allozyme data for ponderosa pine, and may provide a useful means of identifying his torical separations from allele frequency data.