Genomic study unveils the true identity of Brewster’s and Lawrence’s Warbler

Are they first generation hybrids, backcrosses or something else?

Some bird hybrids were initially described as distinct species. I have covered some notable examples on this blog, such as Rawnsley’s Bowerbird (Ptilonorhynchus rawnsleyi) and Argus Bare-eye (Phlegopsis barringeri). In most cases, the species name disappears when the hybrid identity of the bird has been revealed, but sometimes the name stays around. In papers on hybridization dynamics between Golden-winged Warbler (Vermivora chrysoptera) and Blue-winged Warbler (V. pinus), you often come across Brewster’s Warblers and Lawrence’s Warblers. The latter two “species” turned out to be hybrids. In 1893, Sage already expressed his doubt by stating that ‘I am not inclined to believe leucobronchialis [i.e. Brewster’s Warbler] a hybrid, but hope to have more to say on this subject at another time.” However, the names are still used to indicate the characteristics of these birds.

“Lawrence’s” hybrids are similar to Blue-winged Warblers (i.e. yellow overall, with 2 narrow white wing bars) but have the black throat patch and face mask, similar to Golden-winged Warblers. “Brewster’s” hybrids, by contrast, lack a black throat patch, have little to no yellow on the underparts, and commonly have partially separated yellowish wing bars.

Based on these traits, Nichols (1908) and Parkes (1951) speculated that first generation hybrids would look like Brewster’s Warblers, while second generation hybrids and backcrosses would resemble Lawrence’s Warblers. With the advent of genomic data, we can put these hypotheses to the test. In a recent study in the journal The Auk, Marcella Baiz and her colleagues examined the genetic make-up of nine Vermivora warblers.

The different species and hybrids of Vermivora Warblers. From: Baiz et al. (2020) The Auk.

 

Hybrid Triangles

To figure out whether Nichols and Parkes were right, the researchers used triangle plots. Based on two statistics – heterozygosity and hybrid index – you can deduce what kind of hybrid or backcross you are dealing with. Pure individuals are located in the lower corners, while first generation hybrids are at the top. The sides of the triangles (D1 and D2) indicate backcrosses. You would thus expect that Brewster’s Warblers (F1) end up at the top and Lawrence’s Warblers at the sides (backcrosses) of these triangles.

This was, however, not the case. The sequenced individuals were scattered across the triangle and did not follow the predictions by Nichols and Parkes. The Lawrence’s Warbler in this study is not a backcross, but probably a multigenerational hybrid with mostly Blue-winged Warbler ancestry. Similarly, the Brewster’s hybrids are not F1 hybrid, but can trace the majority of their ancestry to either parental species. It thus seems that these hybrid types are quite variable and that F1 hybrids and backcrosses are not easy to distinguish based on the coloration of their underparts.

An example of a triangle plot (left, adapted from Pulido-Santacruz et al. 2018). In this case, you would expect Brewster’s Warblers (F1) at the top and Lawrence’s Warblers at the sides (backcrosses) of these triangles. The results show a different picture, indicating that the hybrids are quite variable (right, from Baiz et al. 2020).

 

Black Throat Patch

The story of the black throat patch is very different. Previous work by David Toews and his colleagues uncovered high genetic differentiation between Golden-winged and Blue-winged Warblers near the gene ASIP. This candidate gene has been linked to plumage differences in other bird species, such as Sporophila Seedeaters, Setophaga Warblers and Lonchura Munias. In this study, the researchers could zoom in on the genomic region where this gene resides. They found genetic variants in front of ASIP, suggesting that mutations in the regulatory sequences – the on-and-off switches – are responsible for the presence or absence of a black throat patch. Gene expression studies are needed to confirm this prediction. So, we moved on from one set of predictions (by Nichols and Parkes) to the next one. In science, we call that progress!

A clear signal of genetic differentiation at the ASIP gene (highlighted in grey). From: Baiz et al. (2020) The Auk.

 

References

Baiz, M. D., Kramer, G. R., Streby, H. M., Taylor, S. A., Lovette, I. J., & Toews, D. P. (2020). Genomic and plumage variation in Vermivora hybrids. The Auk, 137(3), ukaa027.

Featured image: Golden-winged Warbler (Vermivora chrysoptera) © Caleb Putnam | Wikimedia Commons

 

This paper has been added to the Parulidae page.