A glimpse of the complicated colonization history of the ruddy duck.
Somewhere, high up in the mountains, there lives a population of hybrid ducks. It sounds like the start of an exciting adventure story or the caption of a Gary Larson cartoon (I added one of my favorite cartoons below). It is, however, a serious research question. Several authors have suggested that a Colombian population of ruddy ducks (Oxyura jamaicensis) in the Andes is of hybrid origin. A recent study in Ecology and Evolution tried to solve this mystery and reconstruct the colonization history of the ruddy duck.
Black and White Heads
The ruddy duck is a diving duck that is found throughout the New World, ranging from Canada all the way down to Tierra de Fuego. Based on facial plumage of the males, it is divided into three subspecies: the North American jamaicensis with all-white cheeks, the Andean-Patagonian ferruginea with a black head, and the Colombian (hybrid?) andina with a variable black-and-white head.
North to South?
A previous study indicated that ruddy ducks spread from North America to the tropical Andes and eventually to the southern Andes. This scenario entails that ducks from North America adapted to the high altitude environment of the tropical Andes, only to go downhill again and consequently “re-adapt” to the lower altitude conditions of the southern Andes. However, this previous study – published in Molecular Ecology – did not have samples from the Colombian population that are needed to confirm this scenario.
Therefore, Maria Lozano-Jaramillo and her colleagues sequenced several genes across the range of the ruddy duck, including the population in Colombia. The genetic analyses revealed gene flow from the southern Andes and from North America into Colombia. The patterns are consistent with a hybrid origin of the andina subspecies. Surprisingly, these results also point to a “south-to-north colonization of the highland areas from the southern South American lowlands,” contrary to the scenario outlined above.
Low Levels of Oxygen
Another important difference with previous work is the evolution of the hemoglobin genes. This protein – which carries oxygen around in the body – has a higher affinity for oxygen in species adapted to high altitude environments where oxygen levels are low. The previous study found an association between certain variants of these genes and elevation.
The expectation in the present study was that the high altitude Colombian population would have hemoglobin genes adapted to low oxygen levels. Surprisingly, this was not the case. Not all individuals from this population carried the “high-elevation” variants identified in the previous study. In addition, a study in PLoS One found that ruddy ducks in North America also possess hemoglobin with high affinity for hemoglobin although they do not live at high altitudes. The authors speculate that “high-affinity hemoglobin might have been pre-adaptive and facilitated geographic expansion in montane areas even if it did not evolve de novo in the Andes.” Clearly, there is a lot to discover about the evolutionary history of the ruddy duck.
Lozano-Jaramillo, M., McCracken, K.G. & Cadena, C.D. (2018) Neutral and functionally important genes shed light on phylogeography and the history of high-altitude colonization in a widespread New Worlds duck. Ecology and Evolution 8, 6515-6528.
This paper has been added to the Anseriformes page.