Researchers have uncovered a fascinating new mechanism behind the skeletal reduction and asymmetry of emu wings, revealing the crucial role that embryonic and fetal movement plays in shaping the evolution of skeletal morphology.
Unexpected Structural Anomalies in Emu Wings
Emus, those iconic flightless birds, have been the focus of fascination by evolutionary biologists for a long time. Their wings, that would have been used to fly, are greatly reduced over hundreds of thousands of generations. Nonetheless, the exact molecular mechanisms underlying these remodeling processes are not well-characterized.
A new study published in Nature Communications reveals, for the first time, a developmental mechanism that explains the long known reduction and asymmetry of emu wings. This explains why emu wings fail to form distal muscles, and the complete loss of muscle leads to almost no mechanostress development during its development which in turn results in the bone abnormalities.
The skeletal reduction of emu wings included not just changes to the bones, but also the fusion of particular bones at asymmetrical positions, which the researchers say has never before been seen in other limb-reduced… What is new about these results are that specific wing reducing features are interpreted as resulting from heterotopic suppression (Figure 1), rather than due to simple growth cessation as previously suggested. That unique bone pattern emerges because the muscles that would normally form on what is the distal portion of a bird’s wing never establish themselves, removing the mechanical stress necessary for proper bone development. These data demonstrate the tremendous influence of differences in embryonic and fetalmovements on skeletal morphology throughout the course of evolution.
Emu Wing Muscle Progenitor Cells: A Story of Two Stains
In one of their most striking findings, the researchers discovered muscle progenitor cells in emu wings that have a dual identity arising from both somite-derived muscle progenitor cells and lateral plate mesoderm cells.
These double-dipping cells, however, die as they try to become muscle fibers and thus no muscle is ever formed. This muscle dysgenesis is the prime cause of the absence of mechanostress and consequently the characteristic cessation of morphogenesis in emu wings.
The researchers say this aspect is essentially important, since their find points out that the early movements of an embryo and fetus are required not only for the elongating of some skeletal transcriptions but also to drive and balance patterning throughout bones. The results highlight the pronounced morphological effects of embryonic immobility, and specifically muscle form defects, on skeletal evolution.
Conclusion
In short, this pioneering work on the evolution of emu wings presents intriguing new avenues by which the multiple scales of movement during embryonic and fetal life can transform skeletal development. The researchers have shown that changes in embryonic and fetal movement can materially bias morphological evolution, a finding with serious ramifications for our understanding of adaptive radiation — life’s capacity to innovate across islands and continents powerful enough to threaten long-held assumptions about how the planet fills up with life.
