Evolution and Connectivity in the World-Wide Migration System of the Mallard: Inferences from Mitochondrial DNA

Background: Main waterfowl migration systems are well understood through ringing activities. However, inmallards (Anas platyrhynchos) ringing studies suggest deviations from general migratory trends and traditions inwaterfowl. Furthermore, surprisingly little is known about the population genetic structure of mallards, andstudying it may yield insight into the spread of diseases such as Avian Influenza, and in management andconservation of wetlands. The study of evolution of genetic diversity and subsequent partitioning thereof duringthe last glaciation adds to ongoing discussions on the general evolution of waterfowl populations and flywayevolution. Hypothesised mallard flyways are tested explicitly by analysing mitochondrial mallard DNA from thewhole northern hemisphere.Results: Phylogenetic analyses confirm two mitochondrial mallard clades. Genetic differentiation within Eurasia andNorth-America is low, on a continental scale, but large differences occur between these two land masses (FST =0.51). Half the genetic variance lies within sampling locations, and a negligible portion between currentlyrecognised waterfowl flyways, within Eurasia and North-America. Analysis of molecular variance (AMOVA) atcontinent scale, incorporating sampling localities as smallest units, also shows the absence of population structureon the flyway level. Finally, demographic modelling by coalescence simulation proposes a split between Eurasiaand North-America 43,000 to 74,000 years ago and strong population growth (~100fold) since then and littlemigration (not statistically different from zero).Conclusions: Based on this first complete assessment of the mallard’s world-wide population genetic structure weconfirm that no more than two mtDNA clades exist. Clade A is characteristic for Eurasia, and clade B for North-America although some representatives of clade A are also found in North-America. We explain this pattern byevaluating competing hypotheses and conclude that a complex mix of historical, recent and anthropogenic factorsshaped the current mallard populations. We refute population classification based on flyways proposed byornithologists and managers, because they seem to have little biological meaning. Our results have implications forwetland management and conservation, with special regard to the release of farmed mallards for hunting, as wellas for the possible transmission of Avian Influenza by mallards due to migration.