Abstract

Despite morphological diversification of chordates over > 550 million years of evolution, their shared basic anatomical pattern (or ‘body plan’) remains conserved by unknown mechanisms. The developmental hourglass model explains this by the conservation of organogenesis phase (= phylotypic period), however, no consensus has been made to why this phase has been conserved.   Here we collected early-to-late embryonic gene expression profiles from 8 chordate species, and analyzed their features to find a hint to this problem. We found that vertebrates’ conserved mid-embryonic developmental programs are intensively recruited to other developmental processes, and the degree of the recruitment positively correlates with their evolutionary conservation and essentiality for normal development.  Based on the results, we propose that the intensively recruited genetic system during vertebrates’ organogenesis period imposed constraints on its diversification through pleiotropic constraints, which ultimately led to the common anatomical pattern observed in vertebrates. Our findings not only imply that pleiotropic constraint also acts at regulatory level, but also highlights the “cost of complexity” hypothesis, which predicted that more complex organisms have less phenotypic evolvability.