A conserved Hox9-10 code underlies the early establishment of hind appendage progenitors in zebrafish and the Iberian ribbed newt.

Hox9-10 genes are essential for early hind appendage progenitor establishment in zebrafish and the Iberian ribbed newt, indicating a conserved evolutionary role.

• Why it matters: Understanding the origin of paired appendage patterning systems is crucial for insights into vertebrate evolution, yet their developmental and evolutionary origins remain unclear.
• What they did: The study used gene disruption techniques, including CRISPR, to show that five HoxA/D PG9-10 genes in zebrafish and all seven Hox9-10 paralogs in newts are necessary for hind appendage progenitor formation, with reporter labeling confirming early cell involvement.
• The result: Findings suggest that the early requirement for Hox9-10 genes predates vertebrate divergence, linking posterior Hox codes to appendage patterning and supporting their role in the evolution of vertebrate paired appendages.