The Monopolization Hypothesis and the dispersal-gene flow paradox in aquatic organisms

Many aquatic organisms rely on passive transport of resting stages for their dispersal. In this review, we provide evidence pointing to the high dispersal capacity of both animals (cladocerans, rotifers and bryozoans) and aquatic macrophytes inhabiting lentic habitats. This evidence includes direct observation of dispersal by vectors such as wind and waterfowl and the rapid colonization of new habitats. Such high dispersal capacity contrasts with the abundant evidence of pronounced genetic differentiation among neighbouring populations in many pond-dwelling organisms. We provide an overview of the potential mechanisms causing a discrepancy between high dispersal rates and reduced levels of gene flow. We argue that founder events combined with rapid local adaptation may underlie the striking patterns of genetic differentiation for neutral markers in many aquatic organisms. Rapid population growth and local adaptation upon colonization of a new habitat result in the effective monopolization of resources, yielding a strong priority effect. Once a population is locally adapted, the presence of a large resting propagule bank provides a powerful buffer against newly invading genotypes, so enhancing priority effects. Under this Monopolization Hypothesis, high genetic differentiation among nearby populations largely reflects founder events. Phylogeographic data support a scenario of low effective dispersal among populations and persistent effects of historical colonization in cyclical parthenogens. A comparison of patterns of gene flow in taxa with different life cycles suggests an important role of local adaptation in reducing gene flow among populations. We argue that patterns of regional genetic differentiation may often reflect historical colonization of new habitats rather than contemporary gene flow. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.