Late embryonic expansion of a novel bone ridge underlies the evolutionary transformation of cylindrically shaped forelimb bones into the flattened skeleton of the penguin flipper
The evolution of flippers for wing-powered diving in penguins is a striking example of tetrapod limb specialization. The modern penguin flipper is structurally reinforced by a characteristic dorsoventral flattening of the long bones accompanied by a reduction in distal forelimb musculature, features which emerged convergently in flightless diving birds and aquatic mammals. While an extensive fossil record informs the morphological sequence through which these changes occurred, the evolutionary pressures and developmental mechanisms underlying these modifications are unknown. We find that in avian and mammalian forelimbs, a flattened bone morphology only emerged in aquatic lineages that lost ancestral modes of locomotion, including in flightless diving birds, pinnipeds, and cetaceans. Using penguin embryos as an accessible model for investigating flipper development, we demonstrate that early patterning of forelimb musculoskeletal morphology is similar to that seen in forelimbs of non-aquatic birds. Instead, later modifications of gene expression and cell and tissue behaviors underlie flipper phenotypes. Thus, we find that in the early penguin forelimb, the initial cues that pattern the muscle do not differ from other avian species, however late embryonic changes in proliferation result in dramatic reduction of muscle. Likewise, forelimb bones in penguins initially have similar cross-sectional proportions to those in flighted birds. The shape of these bones is, however, remodeled late in embryonic development through a process that shares molecular hallmarks with bone ridge formation at tendon attachment sites. In these bones, ridge-forming tissue initiates at the ends of the bones (the epiphyses) and extends into tendon-like connective tissue along the lateral edges of the bone, widening the long bones along the anterior-posterior axis and producing a flattened bone. Using spatial transcriptomics and comparative genomic tools we determine that differentially expressed genes between the ridge-forming tissues and long bone cartilage are significantly enriched for signals of selection in the penguin lineage and that these genes may also be convergently evolving in marine mammals. Together, these data show that the evolution of musculoskeletal morphology in the penguin flipper occurred through expansion or novel deployment of molecular programs typically associated with tendon-attachment sites during late embryonic development.