Population genetics, molecular markers and the study of dispersal in plants

1 Long-distance dispersal events are biologically very important for plants because they affect colonization probabilities, the probabilities of popul ation persistence in a fragmented habitat, and metapopulation structure. Th ey are, however, very difficult to investigate because of their low frequen cy. We reviewed the use of molecular markers in the population genetics app roach to studying dispersal. With these methods the consequences of long-di stance dispersal are studied, rather than the frequency of the dispersal ev ents themselves. 2 Molecular markers vary, displaying different amounts of variation and dif ferent modes of inheritance: they may be either dominant or codominant, and may or may not be subjected to genetic recombination. Use of markers has i nspired the development of maximum likelihood techniques that take the evol utionary history of alleles into account while estimating gene flow. 3 Inferring seed dispersal rates from indirect measurements of gene flow in volves three steps: (i) quantifying genetic differentiation among populatio ns and using this to estimate the rate of gene flow; (ii) producing a genet ic dispersal curve by regressing geographical distance among populations ag ainst the amount of gene flow; and (iii) separating seed-mediated from poll en-mediated gene flow, by comparing differentiation in nuclear vs. cytoplas mic molecular markers. In this way, potentially very low levels of gene flo w can be detected. 4 The indirect approach is based on a number of assumptions. The validity o f each assumption should be assessed by independent methods or the estimate s of gene flow and dispersal should be mainly used in a comparative context . In metapopulations, with frequent extinction and colonization, the relati onship between genetic differentiation and gene flow is not straightforward , and other methods should be used. 5 Highly variable molecular markers, especially microsatellites, have facil itated a direct genetic approach to measuring gene flow, based on parental analyses. 6 The population genetic approach provides different information about disp ersal than ecological methods. Thus population genetic and ecological metho ds may supplement each other, and together lead to a better insight into th e dispersal process than either of the methods on its own.