Back on the Ground: Genome Analysis of Convergent Flight Loss in Rails

Similar phenotypic traits can evolve independently in response to comparable environmental challenges. A striking example of this process is the repeated and irreversible loss of flight in birds, particularly on islands. The rail family (Rallidae) provides an exceptional model for studying this phenomenon, as nearly a quarter of the 130 extant species have independently become flightless. Here, we present the first genome-wide comparative analysis of multiple independent flightless rail lineages to identify the molecular basis of flight loss. We compared coding regions from seven rail species (four flightless and three volant) using more than 11,000 alignments and multiple phylogeny-based tests, including branch-site models of selection, relative evolutionary rate analyses, and assessments of function-altering amino acid substitutions. Across all analyses, 116 genes showed significant associations with flightlessness, of which 37 were linked to biological functions related to flight capacity–such as muscle, bone, limb, and heart development–or to traits reflecting ecological consequences of flight loss, including immune response, renal function, lipid metabolism, cognition, and sensory perception. Many genes under selection in flightless species were also involved in gene regulation and post-translational modification. These findings suggest that convergent loss of flight in rails arises not from major mutations in a few key loci but from numerous small, repeated genetic changes affecting both developmental pathways and regulatory mechanisms.