Like trains running on separate but parallel tracks, sometimes the forces of evolution can affect different species running along these tracks in very similar ways. Take the evolution of a simple trait found in birds: foot feathering.
Lead researcher Chiara Bortoluzzi and colleagues at Wageningen University & Research in the Netherlands, investigated the genetic basis of foot feathering, a very recognizable trait that can easily be selected to fixation in breeds. The trait, known scientifically as ptilopody, can be observed in domesticated and wild avian species and is characterized by the partial or complete development of feathers on the skin of the ankles and feet.
Similar to pigeons?
Previous work in pigeon had demonstrated that regulatory mutations in the genes Tbx5 and Pitx1 contribute to foot feathering. Bortoluzzi wanted to investigate whether similar mutations might also be contributing to foot feathering in chickens, and if so, if also the same pathways might be altered by the same regulatory mechanisms.
In the advanced online edition of the journal Molecular Biology and Evolution, Bortoluzzi and co-authors report that indeed, similar mutations in the same set of genes are affecting similar molecular pathways contributing to foot feathering in domesticated chickens. To perform their study, they used a combination of genome wide analysis, followed by comparative genomics and gene expression analysis.
The authors demonstrated that a 17,000 base-pair deletion near Pitx1 affects the expression of the gene similarly to what was observed in pigeon. Interestingly, the deletion in chicken removes the same exact region in foot feathered pigeons. On top of the deletion, the authors also identified a 4,000 base-pair haplotype near Tbx5. The DNA sequence of this haplotype was identical among foot feathered birds and was linked to domesticated individuals.
The manuscript provides strong evidences that foot feathering has evolved by parallel evolution in chicken and pigeon. Although chicken and pigeon diverged more than 89 million years ago, the trait is determined by the exact same number of loci and exact same number of genes. Interestingly, foot feathering can also be observed in avian wild species, including snowy owl and golden eagle. In these wild raptor and boreal species, ptilopody has entirely evolved by natural selection. However, the occurrence of the phenotype suggests that a trait can evolve in different species under different types of selection and selection pressure. It is likely that Tbx5 and Pitx1 are involved in ptilopody in wild avian species. However, future studies on both wild and domesticated avian species are required to validate this hypothesis.