Expression of HOX Cluster Genes in Early Larval Development of Sea Urchin Strongylocentrotus intermedius (Echinodermata, Echinoidea)

Expression of 11 genes of the Hox cluster (SiHox1, 2, 3, 5, 6, 7, 8, 9/10, 11/13a, 11/13b, and 11/13c) was assessed in the sea urchin Strongylocentrotus intermedius at early developmental stages, including the blastula (13 h post fertilization (hpf)), gastrula (35 hpf), prism (46 hpf), and pluteus (4 and 9 days post fertilization (dpf)) stages. Expression of SiHox7, 11/13b, and 11/13c was observed at the blastula stage; early activation of 11/13c was detected for the first time in regular sea urchins. The expression level was very low at the gastrula and prism stages. The pluteus stage showed a significant increase in expression of all Hox cluster genes. Deviations from temporal collinearity of Hox gene expression in echinoderms are discussed.

Early post-metamorphic, Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms

Inferring the development of the earliest echinoderms is critical to uncovering the evolutionary assembly of the phylum-level body plan but has long proven problematic because early ontogenetic stages are rarely preserved as fossils. Here, we use synchrotron tomography to describe a new early post-metamorphic blastoid echinoderm from the Carboniferous (approx. 323 Ma) of China. The resulting three-dimensional reconstruction reveals a U-shaped tubular structure in the fossil interior, which is interpreted as the digestive tract. Comparisons with the developing gut of modern crinoids demonstrate that crinoids are an imperfect analogue for many extinct groups. Furthermore, consideration of our findings in a phylogenetic context allows us to reconstruct the evolution and development of the digestive system in echinoderms more broadly; there was a transition from a straight to a simple curved gut early in the phylum's evolution, but additional loops and coils of the digestive tract (as seen in crinoids) were not acquired until much later.