Body plan evolution

Most bilaterian animals use shared developmental mechanisms to pattern their body plan along the antero-posterior and dorso-ventral axis. What happened to these conserved developmental programs in animals that have secondarily evolved radically distinct basic axial coordinates?

Echinoderms are one of the most intriguing of the metazoan phyla. As deuterostomes, they are the sister group to hemichordates and closely related to chordates. The extant members are divided into 5 classes, the sea urchins (echinoids), sea stars (asteroids), brittle stars (ophiuroids), sea cucumbers (holothuroids), and sea lilies or feather stars (crinoids). The adult body plan of these animals is highly modified and derived in relation to the other deuterostome groups: the most striking element of their body plan is the five-fold radial symmetry (also referred to as pentaradial); most apparent in the asteroids and ophiuroids. The evolutionary relationship between the five-fold radial body plan of echinoderms and the ancestral bilateral body plan has puzzled zoologists for more than a century. In the lab, we are revisiting this question by investigating how the highly conserved gene regulatory networks that pattern the body of most bilarian animals are deployed in the derived pentaradial body plan of echinoderms, with the goal to understand which steps led to the unusual evolution of these animals. This project involves comparative molecular and computational approaches in several echinoderm species, in particular the sea star Patiria miniata and the sea cucumber Parastichopus parvimensis.

Using a combination of RNA tomography and HCR in situ hybridization, we found that the antero-posterior program is deployed in echinoderms along the medio-lateral axis of the rays, but is missing a posterior (trunk) region. This suggests that echinoderms like sea stars are best seen as just head-like animals.

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