Researchers have been able to unravel the genetic causes of rare hybrid warblers with mismatched colors using a very rare tool. Credit: Marcella Baiz at Penn StateResearchers have been able to unravel the genetic drivers of two rare traits in hybrid warblers, the black face mask and black throat patch in blue and golden-winged warblers. Ecology online publishes a new study that describes the bird and pinpoints the genetic location that controls its throat patch and face mask.Marcella Baiz (postdoctoral researcher at Penn State, and the first author of this study), stated that golden-winged and blue-winged birds have both a dark face mask and a deep black throat patch. "Hybrid offspring of both species have the same traits so either one or both of the black plumage traits is present when they mate. We captured this strange bird, which looks almost identical to a golden-winged Warbler but has no black throat patch.Researchers believe that this rare combination is found in less than 0.5% hybrid warblers. This hybrid was not previously described, according to the researchers. It was only documented once in a bird that had been collected in 1934 and then described in 1951 by a museum collection.David Toews (Penn State biology professor and author of the study) said that they first found this bird because of a tip from Shaver's Creek Environmental Center birder. He suggested that a nearby area would be a good habitat for warblers and also uploaded observations of warblers to the app eBird which can be used by scientists in their research. When we saw this amazing bird, we went to the area to find out more. We carefully photographed the bird to document its plumage and took a blood sample to sequence its genome. Then we released it."Blue-wing and golden-wing warblers and hybrids between them, which are often matched in face and throat traits, have either a black or a white face mask, with a black throat patch, or both. A rare hybrid, however (right), carries the black mask and the throat patch but not the face mask. Credit: Illustrations by Liz Clayton Fuller; Photo by David ToewsThe researchers previously sequenced the genomes blue- and gold-winged warblers, and discovered a small region responsible for the black coloration. This gene is located upstream from the Agouti signaling protein (ASIP). The two traits are almost always inherited together so it is not clear if ASIP regulates the traits separately or together.Baiz stated that "Because we already had DNA sequencing data from the parent and hybrid species with the matched throat color and face, sequencing the genomes of the mismatched bird allowed for us to distinguish the genetic regions under the face mask and the throat patch."The team confirmed the connection between the black throat area and the previously identified location. They also found a new location that is near, but farther upstream in the genome, which they believe is related to the black mask. These birds must have two copies of the blue-winged warbler's chromosomes for black pigment to exist. This suggests that they are "recessive" genes. These spots can be found with one or both of the copies from blue-winged warblers. This results in no throat patch or face mask.Toews stated that John Treadwell Nichols, a biologist, hypothesized that the black coloration of the throat was a recessive trait more than 100 years ago. Kenneth Parkes, who described the rare hybrid in 1951 said that throat and mask color could be controlled separately. He suggested that they should be linked or placed very closely on the genome. Parkes described his theory in 1951 as a "genetic problem for future research" and both theories were confirmed using modern genetic tools.Researchers suggest that the two locations they found might be within separate promoter regions of the ASIP gene. These regions control whether the gene is turned on or off in different contexts. They are so close together on the chromosome that they would be inherited together even if genetic material is moved between chromosomes during reproductive. This would explain why many hybrids have both or none of these black plumage characteristics. However, the mismatched bird was most likely the result of a rare case in which this was not true, and then several generations of backcrossing of golden-winged Warblers.Baiz stated that coloration genes in warblers share a similar genetic structure, with multiple promoters controlling the location of pigment. This makes it easy to see how a few mutations could result in a variety songbird species with different colors. This may explain why warblers come in so many species.It is possible that a mismatched hybrid may only have one trait, as coloration of warblers is an important cue to mating behaviors. It may attract females from both species, as it has both the qualities of one and the other. Researchers hope to see this bird and determine if it is a mate. Future research linking plumage traits and reproductive success will help clarify these implications.Toews stated that the bird has been observed for two years consecutively, which means it has survived at most two migration events. "This story and the study that led to it is a great example of citizen scientists and birders making a difference in research."Marcella D. Baiz and colleagues, Rare hybrid solves the "genetic problem of linked plumage characteristics", Ecology (2021). Information from Ecology Marcella D. Baiz and colleagues, Rare hybrid solves the "genetic problem of linked plumage characteristics", (2021). DOI: 10.1002/ecy.3424
