Two recent studies explore a hummingbird hybrid zone from morphological, behavioral and genetic perspectives.
Allen’s Hummingbird (Selasphorus sasin) and Rufous Hummingbird (S. rufus) are almost indistinghuisable in the field. Identifying these fast-flying birds might even be more tricky than expected, because several authors argued that these species hybridize on the west coast of the US. A recent study by Brian Myers and his colleagues confirmed these suspicions: using behavioral and morphological data, they showed that Allen’s Hummingbird and Rufous Hummingbird interbreed along a contact zone in Oregon.
Hummingbirds are known for their acrobatic courtship displays. And the Selasphorus hummingbirds are no exception. Males fly in complicated patterns above potential mates, trying to woo them with daring dives, zig-zags and special sounds. Allen’s Hummingbird and Rufous Hummingbird produce fluttering sounds with their tail feathers, and they do this in slightly different ways. Rufous Hummingbird uses the tip of the second tail feather (or retrix 2) while Allen’s Hummingbird relies on the third tail feather (or retrix 3). This difference strategy is also reflected in the morphology of these feathers. A useful feature to discriminate between these species.
Courtship displays can be broken up into several elements, such as dives, shuttles and pendulums (illustrated in the figure below). These elements often occur in a species-specific order. For instance, if an Allen’s Hummingbird performs a series of pendulums, it often ends this series with a dive. Interestingly, birds from the contact zone performed displays that differed from both parental species. Some putative hybrids showed a series of pendulums but instead of ending with a single dive, they finished with multiple dives. This move is characteristic for Rufous Hummingbirds.
Using this morphological and behavioral information, the researchers were able confidently identify hybrids and pinpoint the exact location of hybrid zone. But these results raise another question: does hybridization also result in introgression (i.e. gene flow between the species)? You could imagine that females are not impressed by the abnormal courtship displays of the hybrids. So, hybrid males might be unable to find a partner and backcross.
This question can be resolved with genetic data. And that is exactly what Christopher Battey did in a recent Evolution paper. He collected DNA samples from both species and performed a suite of genetic analyses to reconstruct the evolutionary history of these hummingbirds. He concluded that “demographic models, introgression tests, and genotype clustering analyses support a reticulate evolutionary history consistent with divergence during the late Pleistocene followed by gene flow across migrant Rufous and Allen’s Hummingbirds during the Holocene.”
The high levels of introgression lead to another mystery: how do these hummingbird species remain distinct in the face of gene flow? To unravel this mystery, Battey compared the level of genetic differentiation across the genomes of these species. Genomic regions that are highly differentiated might hold genes related to reproductive isolation. This search revealed some interesting regions – so-called islands of differentiation – on the Z-chromosome. Moreover, these regions showed an unexpected decrease in genetic diversity, suggesting strong linked selection.
To explain the background about these patterns, I will quote from an excellent digest (i.e. the news-articles accompanying some Evolution papers) that was written about this paper by… me.
[Battey] uncovered several islands of differentiation on the Z-chromosome, which were accompanied by a significant drop in nucleotide diversity, suggesting that they were formed by linked selection. However, the Z-chromosome is expected to show lower nucleotide diversity compared to autosomes (Irwin 2018). This sex chromosome has a lower effective population size (3/4 of the autosomes) and is heavily influenced by differential reproductive success between males (who carry two Z-chromosomes) and females (who carry one Z-chromosome). If the diversity on the Z-chromosome is affected only by its lower effective population size, the diversity ratio between Z-chromosome and autosomes would be around 0.75. Taking into account variation in reproductive success can, in theory, push this ratio down to 0.56 (Charlesworth 2001). In the hummingbirds, the ratio was even lower (ranging from 0.44 to 0.58), indicating that additional linked selection might have occurred.
Z-linked genes involved in reproductive isolation mechanisms, such as male plumage traits and female preference, have been found in other bird species. Whether the Z-chromosome of these hummingbirds also harbors such barrier loci remains to be determined. If so, these genomic regions can be used as an extra line of evidence to support the species status of Allen’s Hummingbird and Rufous Hummingbird.
Battey, C. J. (2019). Evidence of linked selection on the z chromosome of hybridizing hummingbirds. Evolution.
Myers, B. M., Rankin, D. T., Burns, K. J., & Clark, C. J. (2019). Behavioral and morphological evidence of an Allen’s× Rufous hummingbird (Selasphorus sasin× S. rufus) hybrid zone in southern Oregon and northern California. The Auk, 136(4), ukz049.
Ottenburghs, J. (2020). Exploring genomic islands of differentiation on the Z‐chromosome of hummingbirds. Evolution.