Regulatory heterochronies and loose temporal scaling between sea star and sea urchin regulatory circuits

It has long been argued that heterochrony, a change in relative timing of a developmental process, is a major source of evolutionary innovation. Heterochronic changes of regulatory gene activation could be the underlying molecular mechanism driving heterochronic changes through evolution. Here, we compare the temporal expression profiles of key regulatory circuits between sea urchin and sea star, representative of two classes of Echinoderms that shared a common ancestor about 500 million years ago. The morphologies of the sea urchin and sea star embryos are largely comparable, yet, differences in certain mesodermal cell types and ectodermal patterning result in distinct larval body plans. We generated high resolution temporal profiles of 17 mesodermally- ,endodermally- and ectodermally-expressed regulatory genes in the sea star, Patiria miniata, and compared these to their orthologs in the Mediterranean sea urchin, Paracentrotus lividus. We found that the maternal to zygotic transition is delayed in the sea star compared to the sea urchin, in agreement with the longer cleavage stage in the sea star. Interestingly, the order of gene activation shows the highest variation in the relatively diverged mesodermal circuit, while the correlations of expression dynamics are the highest in the strongly conserved endodermal circuit. We detected loose scaling of the developmental rates of these species and observed interspecies heterochronies within all studied regulatory circuits. Thus, after 500 million years of parallel evolution, mild heterochronies between the species are frequently observed and the tight temporal scaling observed for closely related species no longer holds.