Unidirectional Gene Flow: A Computer Model and a Population Genetic Survey of the Water-Dispersed Annual, Impatiens capensis
author: Benjamin Polacco
advisor: Keith Karoly
ABSTRACT: Few, if any, of the published gene flow models have examined the effects of unidirectional gene flow which may be typical for many plant species whose seeds or pollen are dispersed by flowing water or a predominant wind direction. To study the effects of unidirectional dispersal in a natural population, pattems of genetic variability were surveyed using isozyme and common garden techniques among six populations (five within a single watershed) of the likely hydrochorous annual plant, Impatiens capensis. Results at two polymorphic loci showed high divergence over all populations (FST=0.85), but low divergence between populations on the same tributary (FST=0.07 and 0.16). This divergence between streams provides evidence for predominant unidirectional dispersal by water. Genetic structure was also examined with morphological measurements of plants grown in a common garden experiment. Morphological genetic distances were difficult to interpret due to small sample size. However, morphological data did seem to agree with isozyme data. To examine the theoretical effects of unidirectional gene flow, a computer simulation of gene flow along a linear array of populations was created. The model incorporates both unidirectional gene flow equal between all populations, and bi-directional "stepping-stone" gene flow only between adjacent populations. Two models were simulated, one which includes only a single nonbranching linear array of populations, and another which includes populations arranged in a branched stream or "V". The simulations showed that unidirectional gene flow can lower the effective population size of a linear system by increasing the chance of allele fixations. In the V model, downstream gene flow can not decrease overall FST beyond a level determined by the rate of bi-directional gene flow, and FST is even increased in some cases depending on relative values of downstream and adjacent gene flow. In some cases, especially when a branch exists upstream, downstream populations will have higher allelic diversity compared to upstream populations. The results from the genetic survey of I. capensis agree well with results from the model simulation, though I. capensis appears to be recently introduced to northwestern Oregon. The effects of founder events and possible hybridization with I. ecalcarata are discussed in light of the genetic data.