POPULATION STRUCTURE AND MITOCHONDRIAL POLYPHYLY IN NORTH AMERICAN GADWALLS (ANAS STREPERA)

Abstract

We examined population genetic structure in Gadwalls (Anas strepera) to test the prediction that female philopatry and fi delity to migratory fl yways have contributed to the partitioning of mitochondrial variation across North America. Sequencing a 658–659 base-pair fragment of the mitochondrial DNA (mtDNA) control region from 348 Gadwalls, we found two distinct clades that were broadly intermixed across both breeding and nonbreeding populations. Clade A was abundant in North America as well as among published sequences from Eurasia. Clade B was sequenced from 5.5% of North American Gadwalls and was more similar to Asian Falcated Duck (A. falcata) haplotypes than to clade A haplotypes. Maximum likelihood indicated that Gadwall clade B haplotypes were a monophyletic group nested within Falcated Duck haplotypes, which suggests mtDNA introgression of clade B into Gadwalls. However, that topology was weakly supported, and we could not reject topologies that were consistent with incomplete lineage-sorting as the cause of mitochondrial polyphyly. Migratory fl yways did not contribute signifi cantly to population structure and, in general, we found a lack of genetic structure among most populations. However, Gadwalls sampled in Alaska and Washington were well diff erentiated from other populations. Coalescent analyses supported a historical population expansion for clade A, and this expansion could have contributed to the high genetic similarity among some populations but the strong diff erentiation of others. Female-mediated gene fl ow, along with both historical and contemporary population and range expansions, has likely contributed to the overall weak mtDNA structure in North American Gadwalls.