Antero-posterior patterning in the brittle star Amphipholis squamata and the evolution of echinoderm body plans.

Although the adult pentaradial body plan of echinoderms evolved from a bilateral ancestor, identifying axial homologies between the morphologically divergent echinoderms and their bilaterian relatives has been an enduring problem in zoology. The expression of conserved bilaterian patterning genes in echinoderms provides a molecular framework for resolving this puzzle. Recent studies in juvenile asteroids suggest that the bilaterian antero-posterior axis maps onto the medio-lateral axis of the arms, perpendicular to the proximo-distal axis of each of the five rays of the pentaradial body plan. Here, we test this hypothesis in another echinoderm class, the ophiuroids, using the cosmopolitan brittle star Amphipholis squamata. Our results show that the general principles of axial patterning are similar to those described in asteroids, and comparisons with existing molecular data from other echinoderm taxa support the idea that medio-lateral deployment of the bilaterian AP patterning program across the rays predates the evolution of the asterozoans, and likely the echinoderm crown-group. Our data also reveal expression differences between A. squamata and asteroids, which we attribute to secondary modifications specific to ophiuroids. Together, this work provides important comparative data to reconstruct the evolution of axial properties in echinoderm body plans.