Vertebrate paired fins are specified by an ancient dorso-ventral patterning system.

LARM-Lmx1b dorsal determination is essential for dorsoventral patterning of zebrafish paired pectoral and pelvic fins, while being dispensable for unpaired dorsal and anal fins.

• Why it matters: Paired fins are dorsoventrally asymmetric, but median fins are symmetric, leaving unclear how an ancient dorsoventral patterning system was redeployed during fin evolution.
• What they did: Zebrafish genetic lineage tracing of lmx1bb and comparative expression analyses across ray-finned and cartilaginous fishes were combined with deletion mutants of the mouse limb enhancers LARM1 and LARM2.
• The result: LARM enhancer function is partially conserved upstream of zebrafish lmx1bb and specifies dorsal fate in paired fins, supporting conservation of an ancient regulatory module rather than simple co-option of a median network.

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