Assessing early fitness consequences of exotic gene flow in the wild: a field study with Iberian pine relicts

Gene flow from plantations of nonlocal (genetically exotic) tree provenances into natural stands of the same species is probably a widespread phenomenon, but its effects remain largely unexamined. We investigated early fitness consequences of intraspecific exotic gene flow in the wild by assessing differences in survival among native, nonlocal, and F1 intraspecific hybrid seedlings naturally established within two native pine relicts (one of Pinus pinaster and the other of P. sylvestris) surrounded by nonlocal plantations. We obtained broad-scale temporally sequential genotypic samples of a cohort of recruits in each pine relict, from seeds before dispersal to established seedlings months after emergence, tracking temporal changes in the estimated proportion of each parental cross-type. Results show significant proportions of exotic male gametes before seed dispersal in the two pine relicts. Subsequently to seedling establishment, the frequency of exotic male gametes became nonsignificant in P. pinaster, and dropped by half in P. sylvestris. Exotic zygotic gene flow was significantly different from zero among early recruits for P. sylvestris, decreasing throughout seedling establishment. Seedling mortality resulted in small late sample sizes, and temporal differences in exotic gene flow estimates were not significant, so we could not reject the null hypothesis of invariant early viability across parental cross types in the wild.