Exotic forest plantations often cover large areas and, as such, may contribute female gametes, male gametes and/or zygotes to native stands. In seed plants, these three components of exotic gene flow have not been distinguished, though they will have different genetic and demographic consequences. For example, zygotic gene flow, in which exotic mothers are pollinated by exotic fathers, may result in heterozygote deficiency. In contrast, male (in which native mothers are pollinated by exotic fathers) and female gametic (in which exotic mothers are pollinated by native fathers) gene flow will generate heterozygote excesses.
Unger et al. (2014) present an approach in which uni- and biparentally inherited markers (a set of chloroplast and nuclear microsatellites) were used to estimate contemporary exotic gene flow into two relict pine stands, Pinus pinaster and P. sylvestris. The authors built upon the Estimation of Seed and Pollen Migration Rates model (ESPM) from Robledo-Arnuncio (2012) by combining the two types of markers to estimate the three types of gene flow.
The method [of decomposing the total gene flow rate into the zygotic and gametic components] should thus be useful for plant ecologist and ecosystem managers … In [their] two-population scenarios, these double-migration events translate into the arrival of seeds born to an external mother and a local father (i.e., female gametic gene flow only), while in systems with three or more populations to which the model could be generalized, they could also involve immigration of seeds born to two external parents from different populations (i.e., female and male gametic gene flow from different populations).
The results of this study represent the first steps to genetically monitoring exotic forest plantations and the genetic and demographic consequence on native stands. Future work in native and exotic stands is necessary in order to measure levels of inbreeding depression (particularly in native stands), genetic variation at ecologically relevant traits and fitness differences between native, exotic and putative hybrids.