Low levels of hybridization between domestic and wild Mallards wintering in the lower Mississippi Flyway

ABSTRACT The Mallard (Anas platyrhynchos) duck is a ubiquitous and socio-economically important game bird in North America. Despite their generally abundant midcontinent population, Mallards in eastern North America are declining, which may be partially explained by extensive hybridization with human-released domestically derived game-farm Mallards. We investigated the genetic composition of Mallards in the middle and lower Mississippi flyway, key wintering regions for the species. We found that nearly 30% of wild Mallards carried mitochondrial (mtDNA) haplotypes derived from domestic Mallards present in North America, indicating that the individuals had female game-farm Mallard lineage in their past; however, nuclear results identified only 4% of the same sample set as putative hybrids. Recovering 30% of samples with Old World (OW) A mtDNA haplotypes is concordant with general trends across the Mississippi flyway and this percentage was stable across Mallards we sampled a decade apart. The capture and perpetuation of OW A mtDNA haplotypes are likely due to female breeding structure, whereas reversal of the nuclear signal back to wild ancestry is due to sequential backcrossing and lower and/or declining admixture with game-farm Mallards. Future studies of wild ancestry of Mississippi flyway Mallards will benefit from coupling molecular and spatial technology across flyways, seasons, and years to search for potential transitions of Mallard populations with different genetic ancestry, and whether the genetic ancestry is somehow linked to an individual's natal and subsequent breeding location. LAY SUMMARY Mallard ducks are common worldwide but are declining in the Atlantic flyway of eastern North America, a decline that may be influenced by widespread hybridization between genetically wild Mallards and domestic game-farm birds that are released for hunting. We used hunter-harvested birds to investigate possible westward expansion and hybridization rates in the lower Mississippi flyway. Despite recovering 30% of samples possessing game-farm Mallard-derived mitochondrial DNA, only 4% were identified as hybrids across thousands of nuclear loci. Prevalence of game-farm Mallard-derived mitochondrial haplotypes were consistent in Mallards sampled a decade apart, suggesting mitochondrial introgression can be captured and persist within lineages far longer than nuclear DNA. Whereas the prevalence of wild × game-farm Mallard hybrids remains significantly less in the lower Mississippi relative to the Atlantic flyway, continued genetic monitoring and development of management strategies to abate future hybridization will be required. RESUMEN Anas platyrhynchos es un ave de caza ubicua y socio-económicamente importante en América del Norte. A pesar de su población generalmente abundante en el centro del continente, la especie está disminuyendo en el este de América del Norte, lo que puede explicarse en parte por la hibridación extensiva con individuos de A. platyrhynchos liberados de granjas de caza de origen doméstico. Investigamos la composición genética de A. platyrhynchos en la ruta migratoria media e inferior del Mississippi, regiones clave de invernada para la especie. Descubrimos que casi el 30% de los individuos silvestres de A. platyrhynchos portaban haplotipos mitocondriales derivados de los individuos domésticos presentes en América del Norte, indicando que los individuos tenían un linaje femenino de individuos de granja en el pasado; sin embargo, los resultados nucleares identificaron solo un 4% del mismo conjunto de muestras como híbridos putativos. La recuperación del 30% de las muestras con haplotipos de ADNmt A del Viejo Mundo (VM) concuerda con las tendencias generales a lo largo de la ruta migratoria del Mississippi y este porcentaje se mantuvo estable en los individuos de A. platyrhynchos que muestreamos con una década de diferencia. La captura y perpetuación de los haplotipos de ADNmt A VM probablemente se debe a la estructura reproductiva de las hembras, mientras que la reversión de la señal nuclear hacia la ascendencia silvestre se debe al retro-cruzamiento secuencial y a una mezcla menor o en declive con individuos de A. platyrhynchos provenientes de las granjas de caza. Los estudios futuros sobre la ascendencia silvestre de los individuos de A. platyrhynchos de la ruta migratoria del Mississippi se beneficiarán de la unión de la tecnología molecular con la espacial a través de las rutas migratorias, las estaciones y los años para buscar transiciones potenciales de poblaciones de A. platyrhynchos con diferente ascendencia genética, y para determinar si la ascendencia genética está de alguna manera vinculada al lugar de nacimiento y subsecuente lugar de cría de un individuo.