But no signs of nuclear introgression (yet).
A few years ago, I wrote a blog post about the genetic population structure of Black-billed Gulls (Chroicocephalus bulleri) in New Zealand. Analyses of the mitochondrial DNA (mtDNA) revealed two major groups, of which one clustered with the Red-billed Gull (C. novaehollandiae scopulinus). This pattern suggests hybridization, but can also be explained by ancestral variation that has not been sorted into both species (i.e. incomplete lineage sorting, see this blog post for a detailed explanation). In other words, these gull species might share mitochondrial variants that were present in their common ancestor.
A recent study in the journal Ibis revisited this conundrum and investigated the genetic make-up of Black-billed and Red-billed Gulls with mitochondrial and nuclear markers. Are we dealing with introgression or incomplete lineage sorting?
When Andrew Given and his colleagues inspected the genetic make-up of 26 Black-billed Gulls, they found six individuals with mtDNA from Red-billed Gulls. In contrast, all Red-billed Gulls possessed mtDNA from their own species. This pattern argues against incomplete lineage sorting where you would expect shared mitochondrial variants in both species. Moreover, the comparison of different demographic models provided statistical support for a model of strict isolation followed by secondary contact. Hybridization is thus the most likely explanation.
Interestingly, there were no signs of introgression in the nuclear DNA (based on six microsatellites). The researchers suggest that extensive backcrossing with Black-billed Gulls has diluted the introgression signature in the nuclear DNA. However, it is also possible that the microsatellites – which only cover a small section of the genome – were not powerful enough to pick up subtle signals of past gene flow (see for example this blog post). A genomic analysis is thus warranted here.
The observation that only Black-billed Gulls have acquired foreign mtDNA can tell us something about the behavior of these birds. As you probably know, mtDNA is transferred through the maternal line. Hence, hybridization mainly occurred between female Red-billed Gulls (which supply the mtDNA) and male Black-billed Gulls. This mating pattern could be explained by the female-biased sex ratio in colonies of Red-billed Gulls. Males fledglings and adults show lower survival rates, resulting in an imbalanced sex ratio. Because there are insufficient males for Red-billed Gull females, they turn to Black-billed Gulls. A nice example of Hubbs Principle, which I covered in another blog post.
Despite clear genetic evidence for introgressive hybridization, field observations of hybrids are rare. During more than 50 years of monitoring, the authors found no mixed pairs in the large Kaikoura colony of Red-billed Gulls. And only five cases of hybridization have been reported in literature (see this paper). Nonetheless, you only need a few hybridization events for the exchange of genetic material between species. Or the transfer of mtDNA happened in the distant past. A genomic analysis might provide the answer.
Given, A. D., Mills, J. A., Momigliano, P., & Baker, A. J. (2023). Molecular evidence for introgressive hybridization in New Zealand masked gulls. Ibis, 165(1), 248-269.
Featured image: Black-billed Gull (Chroicocephalus bulleri) © Paul Davey | Wikimedia Commons