As the only birds with a nocturnal, predatory lifestyle, owls occupy a unique niche in the avian realm. Hunting prey in the dark comes with a number of challenges, and owls have evolved several features that leave them well-suited to this task. Owls uniquely combine traits common among predators, like acute vision and sharp talons, with adaptations for a nocturnal lifestyle, such as enhanced hearing and night vision. Pamela Espíndola-Hernández, a doctoral student at the Max Planck Institute for Ornithology, and her colleagues recently reported the results of a genome-wide scan designed to reveal the genetic mechanisms underlying these adaptations. In addition to confirming the important role of the visual and auditory systems, the study suggests the existence of an unusual adaptation not yet described in birds—a special way of packaging DNA in retinal cells to act as a light-channeling lens—shedding new light on the evolutionary history of this nighttime predator.
The majority of birds have a diurnal lifestyle, meaning they are primarily active during the day. Owls are thought to have diverged from their sister group, which includes mousebirds, woodpeckers, and kingfishers, during the Paleocene, when a radiation of small mammals may have led to an increased availability of nocturnal prey. To take advantage of this nightly feast, owls presumably retained predatory features shared with other raptors like eagles and hawks. At the same time, they developed nocturnal traits that have been observed in other birds that evolved nocturnality independently, such as kiwis and oilbirds. This culminated in a selection of features that make owls uniquely suited to fill the nocturnal predator niche, including retinas adapted for better night vision, asymmetrical ears and facial discs for enhanced hearing, and soft feathers that enable silent flight.
In order to identify the evolutionary forces contributing to this confluence of traits, Espíndola-Hernández and colleagues compared the genomes of 20 bird species, including 11 owls (five of which were newly sequenced for the study) and analyzed the nucleotide substitution rates of individual genes to identify those that experienced positive selection during the evolution of the owl clade.
As predicted, a primary finding of the study was that genes involved in sensory perception showed a genome-wide signal of positive selection. This category included genes involved in acoustic and light perception, photosensitivity, phototransduction, dim-light vision, and the development of the retina and inner ear. Genes involved in circadian rhythms, which regulate the body’s internal clock, also showed evidence of accelerated evolution, as did some genes related to feather production.
While these findings were expected, the analysis revealed another category of genes that was wholly unexpected: 32 genes related to DNA and chromosome packaging exhibited an accelerated substitution rate in the owl lineage. As a potential explanation for this surprising result, the authors point out that the DNA in the retinas of nocturnal mice and primates forms an unusual structure that acts as a sort of collecting lens and increases light detection in the deep layers of the retina. The study’s findings may therefore indicate that owls independently evolved a similar DNA packaging mechanism in the retina that enhances light channeling in photoreceptors, a feature that has not been observed in any bird species to date.
Espíndola-Hernández notes that the validity of these findings rely on the accuracy of the functional gene annotations within the owl lineage, which represents an important challenge for any genomics-based research in non-model organisms. Thus, she and her colleagues hope to verify the existence of these light-channeling DNA structures in the owl eye by studying owl photoreceptor cells. Direct investigations like these are critical, Espíndola-Hernández points out, to validate the findings of computational research.
Feature image by Zdeněk Macháček