Gene flow in forest trees: how far do genes really travell?

Gene flow is one of the most important factors shaping the genetic structure of populations. In recent decades, a number of studies have addressed issues of contemporary gene flow in forest trees, including pollen and seed dispersal, and gene immigration into natural and breeding populations (primarily seed orchards). Gene flow might be considered either beneficial or deleterious from the point of view of a conservation geneticist or a tree breeder. Extensive gene dispersal within local populations promotes panmixis and reduces family structuring in natural regeneration, thus reducing the potential for inbreeding. However, gene flow may reduce fitness of offspring if genes come from populations maladapted to the habitat of offspring establishment. Furthermore, substantial gene flow limits divergence among populations that might otherwise occur because of genetic drift and directional selection, and may enhance genetic diversity within populations. The robustness and discriminatory power of parentage analysis have been significantly improved in recent years due to advances in molecular marker technology and analytical techniques. However, knowledge of gene flow in forest trees is still unsatisfactory due to continued shortcomings of available markers, inherent limitations of statistical models, and the anecdotal nature of many gene flow studies, which are typically limited in scope. Another limitation is that pollen gene flow is usually estimated by sampling seeds from a number of mother trees. Restricting sampling to seeds seems adequate when evaluating gene flow in seed collections to be used for artificial reforestation. However, in naturally regenerating populations, the most important parameter is the proportion of immigrant alleles in established seedlings. If natural selection favours local genotypes, this should be reflected in the difference between potential gene flow measured at the seed stage and realized gene flow measured in established seedlings. Realized gene flow is a complex phenomenon that depends on a large number of deterministic and stochastic variables. Profound understanding of effective gene flow observed at the landscape level may require joint efforts in population genetics, ecology, advanced multivariate statistics, and spatial simulation modelling.