A new asymmetry, an old toolkit: An emerging developmental enigma in appendage evolution.

lmx1b shows dorsal expression in paired fin mesenchyme but posterior expression in median fins, revealing a developmental asymmetry linked to appendage evolution.

• Why it matters: Dorsoventral polarity is central to paired appendages, yet median fins that predate them are symmetric, leaving the evolutionary origin unclear.
• What they did: The study used developmental expression analysis in fin mesenchyme to compare lmx1b patterning between paired fins and median fins.
• The result: The dorsal-versus-posterior shift in lmx1b implies genetic rewiring of an ancient developmental toolkit during the evolution of appendage polarity.

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