One population or many? Evaluating genetic differentiation and phenotypic plasticity with reciprocal transplants of mussels from spatially and environmentally separated populations.

A.F. Casper. Québec-Océan et Département de Biologie, Pavillon Vachon, Université Laval. Ste-Foy, Quebec J1K 7P4

Reproductive isolation, selection pressure, and local adaptations frequently lead to population differentiation in heterogeneous environments like large rivers. This experiment uses zebra mussels (Dreissena polymorpha) as a model of an animal that has rapidly colonized divergent habitats within the St. Lawrence River (e.g. estuarine, high turbidity, and hypo-ionic). Do the populations that persist in these environments rely on a species–wide plasticity or are they just a sub–set of the global population (i.e. survivors of selection)? To explore this question, populations from three different source environments of the St. Lawrence River (Great Lakes, estuarine, and low Ca/Mg/K) were reciprocally transplanted then compared in terms of growth rate. Because larvae can drift the length of the river within 20 days, the amount of gene flow between these locations is believed to be quite large, thus constituting a single more or less homogenous population. Instead, results show broad differences in growth rate at transplant destinations based on source populations. This challenges the idea that broad scale passive dispersal necessarily leads to a single phenotypically plastic population. Instead population divergence and microevolutionary mechanisms in balance with strong gene flow may mean that population connectivity can be ephemeral in space and time.