An extensive study maps patterns of gene flow between Mallards and Black Ducks.
The king of avian hybrids? Clearly the Mallard (Anas platyrhynchos), this duck species has hybridized with at least 39 other species. Most of these hybridization events concern closely related species, such as the Mottled Duck (A. fulvigula) or the Spot-billed Duck (A. zonorhyncha). With all this interbreeding you would expect that the Mallard will assimilate all the other species, leading to genetic extinction. This has been proposed for the American Black Duck (A. rubripes) in the eastern United States (see Mank et al. 2004). A recent study in the journal Ecology and Evolution revisited the Mallard x Black Duck case with new genetic data.
Not Much Backcrossing
Philip Lavretsky (University of Texas) and his colleagues present “the most comprehensive molecular study of mallards and black ducks to date” by collecting 290 samples across the range of both species. The researchers quantified the degree of gene flow between Mallard and Black Duck by assigning each individual to a generational class. For example, pure species, first-generation hybrid (F1), backcross to Mallard, and so on.
This analysis revealed relatively few backcrosses. In fact, individuals that backcrossed into Black Duck or Mallard were genetically indistinguishable from “pure” individuals within one or two generations. This suggests that there is little gene flow between the species. In contrast to previous work, the Black Duck is not being assimilated by Mallards. However, this analysis is based on only 0.04% of the genome (using RADseq data), so they might have missed important genomic regions.
Detailed analyses of the Mallard samples revealed two clear populations, which the researchers refer to as western and non-western Mallards. Ducks from the non-western group, which occur mainly east of the Mississippi River, carry a genetic signal that suggests hybridization with feral ducks. To understand this signal, I need to provide some detailed information on the structure of mitochondrial DNA (mtDNA) in Mallards.
Based on mtDNA, Mallards belong to one of two groups, the so-called haplotypes A and B. Ducks in the Old World carry haplotype A, while both haplotypes can be found in populations of New Worlds ducks. The reason behind this pattern is not resolved yet. It could be that the New Worlds only consisted of haplotype B but that gene flow (historical or recent) has mixed things up. Or the New World populations have arisen relatively recent and haplotype B has not spread across the range yet (i.e. incomplete lineage sorting).
Anyway, most non-western Mallards carry haplotype A which originates from the Old World. The most likely source for this haplotype is feral ducks, which have been (and are still being) released in North America. The game-farm birds are originally from Eurasia and this carry haplotype A. Based on these patterns, the researchers propose the following scenario: hybrids tend to backcross with mallards and these backcrosses consequently interbreed with feral ducks. But samples from game-farm birds are needed to confirm this hypothesis. There will definitely be more studies on this system. So keep an eye on the Avian Hybrids Project.
Lavretsky, P., Janzen, T. & McCracken, K.G. (2019) Identifying hybrids & the genomics of hybridization: Mallards & American black ducks of Eastern North America. Ecology and Evolution. Early View.
This paper has been added to the Anseriformes page.