Genomic analyses point to several bouts of past gene flow.
Waterfowl hybridize like there is no tomorrow. Between 30% and 40% of ducks, geese and swans are known to interbreed. For some species combinations, hybrids are regularly observed because they are easy to recognize and occur close to humans. A nice example concerns Greylag Goose (Anser anser) x Canada Goose (Branta canadensis) hybrids in the Netherlands. The aberrant morphology of these crosses allows for easy identification and they reside around several Dutch cities (see this paper for more details). But what about sea ducks? This group of waterfowl – classified in the tribe Mergini – spend most of their lives at sea, making them difficult to observe. Some hybrids have been reported, such as Barrow’s Goldeneye (Bucephala islandica) x Common Goldeneye (B. clangula). However, the incidence of hybridization among the 15 species in this tribe remains largely unknown. A recent study in the journal Molecular Phylogenetics and Evolution addressed this knowledge gap with genomic data. Sampling more than 350 individuals, the researchers searched for genetic signatures of hybridization.
Before starting their quest for sea duck hybrids, the researchers needed to be sure that they could confidently discriminate between all species. A principal component analysis pointed to three main groups: (1) Harlequin Ducks (Histronicus histronicus), (2) eider species and Long-tailed Ducks (Clangula hyemalis), and (3) Bufflehead (Bucephala albeola), scoters, mergansers, and goldeneyes. Within these three groups, population assignment analyses (using ADMIXTURE) could identify the different species and even provided some insights into more fine-scale population structure within certain species.
Now, we can explore the dataset for possible hybrids. The ADMIXTURE results indicated a first-generation hybrid between Barrow’s Goldeneye and Common Goldeneye. Moreover, there were 18 individuals with signs of ancestry from several species, suggesting possible backcrosses. The ADMIXTURE analyses had too low resolution to pinpoint the species involved, so the researchers turned to a more powerful co-ancestry analysis (using fineRADstructure). Here is an overview of the uncovered hybrids with number of individuals between parentheses:
- Barrow’s Goldeneye x Common Goldeneye (1)
- Barrow’s Goldeneye backcrossed with Bufflehead (1)
- Bufflehead backcrossed with White-winged Scoter (3)
- Common Eider backcrossed with Common Goldeneye (1)
- Common Eider backcrossed with Harlequin Duck (3)
- Harlequin Duck backcrossed with Common Merganser and Bufflehead (2)
- King Eider backcrossed with Surf Scoter (1)
- Long-tailed Duck backcrossed with Black Scoter (1)
- Long-tailed Duck backcrossed with Bufflehead (2)
- Long-tailed Duck backcrossed with Harlequin Duck and a scoter species (1)
- Long-tailed Duck backcrossed with eider species (1)
- Red-breasted Merganser backcrossed with King or Spectacled Eider (1)
Ancient Gene Flow
The results described above nicely fit with general hybridization patterns in birds: rare on an individual basis (only 18 out of 363 individuals), but common at a species level (just count the number of species in the list). But the sea duck story does not end here. Next, the researchers explored the genomes of the sea duck species for signs of ancient introgression. These analyses suggested gene flow between the ancestors of scoters and mergansers, and between the ancestors of scoters and goldeneyes. However, it is difficult to pinpoint the exact species involved in these ancient introgression events. It could entail hybridization between the ancestors of extant species, or between extinct lineages that left their genetic signature in present-day species (i.e. ghost introgression). Regardless of the details, introgression clearly played a pivotal role in the evolutionary history of sea ducks.
A Hybrid Species?
And then there is the Steller’s Eider (Polysticta stelleri). The genetic analyses indicated that this species contains genetic ancestry from Long-tailed Ducks and several eider species. Specifically, 94-98% of genetic variation in the Steller’s Eider came from three other eider species, while the remaining 2-6% was assigned to Long-tailed Ducks. These patterns raise the possibility that the Steller’s Eider is a hybrid species. If that is the case, its unique plumage might be the result of transgressive segregation (i.e. hybrids showing extreme phenotypes). However, the hybrid species hypothesis remains to be tested with more detailed analyses. There is still the possibility that the genetic make-up of the Steller’s Eider is the outcome of several, independent hybridization events (similar to the situation in Red-breasted Goose, Branta ruficollis). Indeed, the researchers indicate that “while we cannot definitively identify the source of the shared genetic variation, we provide strong evidence for a complex evolutionary history of shared variation between true Eiders, long-tailed ducks, and Steller’s eider that will require additional sampling of individuals and genomes to fully understand.” To be continued.
Lavretsky, P., Wilson, R. E., Talbot, S. L., & Sonsthagen, S. A. (2021). Phylogenomics reveals ancient and contemporary gene flow contributing to the evolutionary history of sea ducks (Tribe Mergini). Molecular Phylogenetics and Evolution, 161, 107164.
Featured image: Steller’s Eider (Polysticta stelleri) © Ron Knight | Wikimedia Commons