Expression levels of the estrogen receptor determine aggressive behavior in these songbirds.
White-throated sparrows (Zonotrichia albicollis) come in two distinct morphs: the white-striped (WS) and the tan-striped morph (TS). These morphs do not only differ in their plumage patterns, but also in behavior, such as the degree of parental care that they provide (which I discussed in this blog post). The differences between these morphs have a solid genetic basis. Already in 1966, Thorneycroft identified a chromosomal rearrangement that explains the occurrence of the two white-throated sparrow morphs. Recent molecular work showed that this rearrangement is an inversion (i.e. a flipped section of DNA, more on inversions in this blog post), giving rise to a so-called supergene which links numerous genes that influence the morphology and behavior of these birds. Tan morphs have the same version of the supergene (i.e. they are homozygous) whereas white-striped morphs have two different versions (i.e. they are heterozygous).
Knowing that a supergene underlies the different morphs is only the first step. Now, we can zoom in on the contents of this supergene and determine how these linked genes work together in shaping the plumage and behavior of white-throated sparrows. A recent study in the journal PNAS performed some clever experiments to understand the role of one particular gene.
As mentioned above, the white-striped and the tan-striped morphs behave differently. Studies in wild populations found that WS birds are more aggressive compared to TS birds when defending their territories. Given that territorial aggression in songbirds has been linked to steroid hormones, it makes sense to search for genes that are involved in the production or processing of these hormones. Interestingly, one of the genes (ESR1) in the supergene codes for an estrogen-receptor. Moreover, this gene comes in two different versions (ZAL2 and ZAL2m) that follow the genetic patterns underlying the two morphs. Tan morphs have the same version of gene (two times ZAL2) whereas white-striped morphs have two different versions (ZAL2 and ZAL2m). Sounds like the perfect candidate gene!
The researchers quantified the level of aggression of different birds in several behavioral trials. Next, they measured the expression levels of the different ESR1-versions in certain brain areas. They summarized their findings as follows: “the degree to which a bird engaged in territorial aggression, which was markedly higher in the WS birds than in the TS birds, was predicted by the relative expression of the ZAL2m allele.” In another experiment, the researchers knocked down the expression of the ESR1-gene in certain brain areas and assessed the aggression of the birds. This experiment revealed that the more aggressive birds became less aggressive when the ESR1-gene was turned off.
These findings provides direct evidence that the estrogen-receptor plays a crucial role in determining the behavior of these morphs. However, it remains to be determined how it actually works. This receptor is a transcription factor that interactions with a large number of other proteins as well as with numerous regulatory elements. A previous study reported that ESR1 lies within an interconnected module of 157 genes that are differentially expressed between the morphs. Of these 157 genes, 115 are located in the supergene. More experimental work is needed to disentangle this complex web. Slowly but steadily we are getting closer to the genetic underpinnings of these intriguing morphs.
Merritt, J. R., Grogan, K. E., Zinzow-Kramer, W. M., Sun, D., Ortlund, E. A., Soojin, V. Y., & Maney, D. L. (2020). A supergene-linked estrogen receptor drives alternative phenotypes in a polymorphic songbird. Proceedings of the National Academy of Sciences, 117(35), 21673-21680.
Featured image: White-throated sparrows (Zonotrichia albicollis) © Cephas | Wikimedia Commons