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

Abstract

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.

Keywords: Pinus pinaster; Pinus sylvestris; early fitness; gene flow; introgression; plantations; pollen dispersal; seed dispersal.

Figure 1

Maps of Pinus pinaster study site in Fuencaliente (Spain) showing: (A) the location of Fuencaliente (marked with a dot) and the distribution of native populations of the species within the Iberian Peninsula, (B) a digital elevation model of the study area and the distribution of exotic plantations (in dark grey) and the native stand (in light grey, and framed), and (C) the distribution of sampled seed–trees and sequential recruit samples (R1, R2, and R3) within the boundaries of the native stand (in light gray).

Figure 2

Maps of Pinus sylvestris study site in Sierra Nevada National Park (Spain) showing: (A) the location of Sierra Nevada (marked with a dot) and the distribution of native populations of the species within the Iberian Peninsula, and (B) a digital elevation model of the study area and the distribution of exotic plantations (in dark grey) and native stands (in light grey). Sampling plots within the native stand are indicated by circles (seed and two sequential seedling samples available) and triangles (seed and one sequential seedling sample available).