Meeting Abstract
Developmental systems drift (DSD) is a form of evolution where a trait or developmental process remains conserved despite changes in the underlying gene regulatory network. Regulatory elements in developmental gene networks often undergo substantial drift, including changes in the order, number or position of transcription factor binding sites. Examining the drift or conservation of regulatory elements across large periods of evolutionary divergence can provide insights into the structure and function of these elements in a network. We compared the heart gene regulatory networks of two tunicate species Ciona robusta and Corella inflata to explore how a long period of DSD, ~ 250my, has altered a functionally conserved gene network. One of the primary nodes in this network is Mesp, an initial cranial-cardiac transcription factor in both tunicates and vertebrates. Here we show that the Mesp enhancer is regulated by conserved upstream transcription factors in both species despite significant changes in sequence. Through serial minimization of a Corella Mesp reporter construct, we found that the location of the enhancer had shifted considerably in comparison to the characterized Ciona enhancer. Cross-species testing of the Ciona and Corella enhancers indicated that conserved upstream transcription factors regulate mesp expression in both species. Mutation of predicted binding sites for these conserved transcription factors revealed that functional important sites had shifted in their spacing within the enhancer while number and order appeared more constrained. These studies indicate that specific structural features of initial regulatory elements in vital development networks are highly constrained, perhaps due to rigorous temporal or spatial expression requirements.
