How different are Mallards and Chinese Spot-billed Ducks on a genetic level?

A recent study detected minor differences on the sex-chromosomes.

Morphologically, Mallards (Anas platyrhynchos) and Chinese Spot-billed Ducks (A. zonorhyncha) are easy to tell apart. First of all, the sexes of the Mallard are drastically different whereas male and female Chinese Spot-billed Ducks look alike. In addition, the Chinese Spot-billed Duck can be recognized by its pale head which is marked by a whitish eyebrow and two black stripes. And it sports a yellow spot on the bill from which it derives its name. Interestingly, these morphological differences do not extend to the genetic level. Analyses of mitochondrial DNA and several nuclear markers could not discriminate between these species.

The observation of clear morphological disparity without genetic divergence is not uncommon in birds. I have covered several cases on this blog, such as redpolls and warblers. A mismatch between morphology and genetics can often be explained by a few differentiated genomic regions that underlie the phenotypic differences. Hence, Irina Kulikova and her colleagues took another look at the genetic make-up of the Mallard and the Chinese Spot-billed Duck. Did they find any genetic differences?

Genetic Outliers

The researchers scanned the genomes of 23 Spot-billed Ducks, 29 Mallards and 3 hybrids. In the end, they obtained more than 3000 genetic loci: 3130 on the autosomes and 194 on the Z-chromosome (i.e. one of the sex-chromosomes in birds). Most of the genetic variants at these loci were shared between the two species, confirming previous work that they are genetically similar. However, genetic differentiation was about 4.5 times higher on the Z-chromosome compared to the autosomes. A more detailed look at this sex-chromosome revealed three loci that were significantly different between Mallard and Chinese Spot-billed Duck. Moreover, these loci popped up when the researchers tested for signatures of divergent selection. There are thus genetic differences between these duck species. We just had to look really hard to find them.

The Z-chromosome is highly differentiated between Mallards and Chinse Spot-billed Ducks. It contains three clear outlier (depicted as triangles) that might underlie the morphological differences. From: Kulikova et al. (2022).

Future Work

Finding the genetic differences between these duck species is only the first step. Now, the researchers want to find out whether these genetic outliers directly contribute to the morphological differences that we observe. We know that genes regulating plumage coloration and bill color often reside on the sex-chromosomes (see this review by Darren Irwin). Mallards and Spot-billed Ducks might be another example. But this hypothesis remains to be tested with more fine-scale genomic analyses. Nonetheless, the researchers are confident that they are on the right track:

Whether these regions are involved in phenotypic differences between the species and sexual dimorphism is the prospect of future work. We believe that whole-genome sequencing along with plumage analyses will shed light on phenotypic evolution and help to identify speciation mechanisms in Mallard and Chinese Spot-billed Duck.


Kulikova, I. V., Shedko, S. V., Zhuravlev, Y. N., Lavretsky, P., & Peters, J. L. (2022). Z‐chromosome outliers as diagnostic markers to discriminate Mallard and Chinese Spot‐billed Duck (Anatidae). Zoologica Scripta.

Featured image: Chinese Spot-billed Duck (Anas zonorhyncha) © Alpsdake | Wikimedia Commons

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