Selection on Phalanx Development in the Evolution of the Bird Wing

Area of Science:

• Evolutionary developmental biology
• Comparative genomics
• Paleontology

Background:

• The frameshift hypothesis explains bird wing evolution via altered positional values and digit phenotype.
• The "most anterior digit" (MAD) hypothesis suggests changes in limb transcriptional regulation.
• Both hypotheses rely on conserved Hoxd gene expression patterns in digit I.

Purpose of the Study:

• To investigate Hoxd expression patterns in a diverse phylogenetic sample of amniotes.
• To examine the molecular and developmental mechanisms underlying phalanx loss in bird wing evolution.
• To test the validity of the frameshift and MAD hypotheses.

Main Methods:

• Studied Hoxd expression patterns in 18 amniote species.
• Analyzed patterns of apoptosis and Sox9 expression.
• Examined paleontological data and recent dinosaur phylogenies.

Main Results:

• Found significant variation in Hoxd11, Hoxd12, and Hoxd13 expression in digit I across amniotes, refuting a conserved molecular signature.
• Evidence suggests phalanx loss in wing digit IV resulted from early arrest of the phalanx-forming region and subsequent cell death.
• Demonstrated that multiple amniote lineages lost phalanges without a frameshift event.

Conclusions:

• Bird wing evolution likely occurred through targeted phalanx loss driven by natural selection.
• The findings support a gradualist model of evolution involving modifications in developmental gene expression.
• Re-evaluation of the Archaeopteryx lithographica wing phenotype is consistent with phalanx loss, not a frameshift.