# Evolutionary innovation within conserved gene regulatory networks underlying biomineralized skeletons in Bilateria

## TL;DR

This study explores how ancient gene networks are reused and modified to form mineralized skeletons in different animal groups, revealing how evolution balances conservation and innovation.

## Contribution

The study reveals a hierarchical model where chromatin dynamics allow modular innovations within conserved gene regulatory networks.

## Key findings

- A biphasic regulatory program involving ancient transcription factors and chromatin remodeling controls shell formation in bivalve molluscs.
- A conserved developmental toolkit was co-opted for larval exoskeleton formation in the common lophotrochozoan ancestor.
- Limited regulatory conservation exists between lophotrochozoans and echinoderms despite shared use of ancestral regulators.

## Abstract

Biomineralized skeletons have evolved convergently across animals and exhibit remarkable diversity in structure and development. However, the evolutionary origins of gene regulatory networks underlying biomineralized skeletons remain elusive. Here, we report comprehensive developmental profiling of transcriptomic and chromatin dynamics in a bivalve mollusc, Crassostrea nippona. We provide evidence for a biphasic regulatory program orchestrating larval and adult shell formation, involving the coordinated activity of ancient transcription factors and dynamic chromatin remodeling. Comparative analyses suggest a conserved developmental toolkit was co-opted for larval exoskeleton formation in the common lophotrochozoan ancestor. In contrast, limited regulatory conservation was observed between lophotrochozoans and echinoderms with regard to the formation of biomineralized skeletons, despite both relying on a heterochronic activation of ancestral regulators. Together, our findings support a hierarchical model in which dynamic chromatin decouples rapidly evolving effectors from deeply conserved regulators, allowing modular innovations within conserved gene regulatory networks. This study highlights how epigenetic dynamics bridge evolutionary conservation and novelty, offering a framework for understanding the independent evolution of biomineralization across Bilateria through combinatorial regulatory evolution.