Homoplastic evolution of proximal podomere patterning across Arthropoda

Lim1 and SoxNeuro spatially mark the developing coxa and trochanter in the harvestman Phalangium opilio, and Lim1 RNAi deletes both segments.

• Why it matters: Lineage-specific shifts in appendage gene expression complicate aligning leg segments across arthropods, especially when segment number differs between subphyla.
• What they did: The study compared proximal podomere patterning in a sea spider and an arachnid, using RNAi depletion of Lim1 and cross-taxon expression comparisons of Lim1 and SoxNeuro.
• The result: Conflicting homology interpretations are linked to homoplastic evolution of SoxNeuro across Arthropoda, with additional homoplasy suggested for Sp6-9 and Irx2.

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