The nocturnal avian predator
Most birds go about their daily activities during the light of daytime. When evening comes, they retire to their habitat for sleep and wake the next day to do it all over again. An exception to this rule is the owl, a nocturnal avian predator that likes to catch its prey in the dark of the night.
This unusual attribute of owls has led scientists to study their vision to further understand the molecular mechanisms that enable night vision. We know that their retina are densely packed with rod photoreceptors which are highly sensitive and help owls to see better in the dim light. But a new study by researchers at the Max Planck Institute for Ornithology suggests that the way in which DNA is packaged within these retinal cells is what enhances owls' vision to help them see at night. The research is published in Genome Biology and Evolution and was led by post-doctoral student Pamela Espíndola-Hernández.
Which genes have been positively selected for in owls?
Owls (Strigiformes) are believed to have diverged from their sister species, the Coraciimorphs during the Paleocene era before dividing into two families, Strigidae and Tytonidae. Many studies have explored the genetic basis of owls' adaptation to become nocturnal avian predators, but these works have adopted candidate-gene approaches rather than genome-wide scans. As such, limited information has been gathered on exactly how specific combinations of traits have evolved throughout time. In this study, Espíndola-Hernández et al. compared the genomes of 20 different bird species including 11 owls, five of which were newly sequenced for this research. When analyzing the genomes, the researchers measured nucleotide substitution rates for individual genes and also groups of genes to assess which genes had been positively selected for.
A special lens?
As anticipated, Espíndola-Hernández and colleagues found positive selection for genes relating to sensory perception, such as photosensitivity, dim-light vision and inner-ear development. However, they also found 32 genes relating to DNA packaging and chromosome condensation. Writing in the paper, the authors note, "We found consistent evidence that 32 genes related to DNA conformation change, chromosome condensation, and chromatid segregation have an accelerated substitution rate in the origin of the owls. From these genes, ATRX, SMC2, and SMC5 had also genome-wide significant selection signals, and the latter two had nominal significant selection signals from both models."
The finding suggests that owls could have evolved a specific type of DNA packaging in the retina, which has been observed in the rod cells of other nocturnal creatures such as mice and primates. "Nocturnal mammals show an unusual radially inverted pattern of hetero- and euchromatin in the nuclei of the rod photoreceptor cells, which acts as a collecting lens channeling the light efficiently toward the light-sensing outer segments, thereby increasing light availability in the deep layers of the retina," they write. Further exploration is required to confirm the theories suggested here, but the work certainly emphasizes we have much more to learn regarding owls' evolutionary advantages.
Reference: Espíndola-Hernández P, Mueller JC, Carrete M, Boerno S, Kempenaers B. Genomic Evidence for Sensorial Adaptations to a Nocturnal Predatory Lifestyle in Owls. Genome Biology and Evolution. 2020;12(10):1895-1908. doi:10.1093/gbe/evaa166.