Evolutionary rewiring of the endomesoderm gene regulatory networks specify skeleton secreting cells in stony corals.

This study reveals that stony corals evolved their unique ability to secrete calcium carbonate skeletons by co-opting and rewiring ancestral endomesoderm gene regulatory networks, specifically through the novel cis-regulatory control of the *Brachyury* gene, to specify skeleton-secreting cells.

The Gist

Imagine a coral reef as a bustling underwater city. The buildings of this city are the coral skeletons, made of calcium carbonate, which provide homes for thousands of other sea creatures. But how do these tiny, soft-bodied animals (corals) know how to build such hard, rocky structures? And how did they learn to do something that their cousins, the sea anemones, simply don't do?

This paper is like a detective story that uncovers the "instruction manual" inside coral DNA that tells them how to become architects.

The Mystery: The "Soft" Cousin vs. The "Hard" Architect

Corals and sea anemones are distant relatives, like cousins who grew up in the same family but took very different career paths.

• Sea Anemones are soft, squishy, and don't build skeletons.
• Stony Corals are the hard-working builders of the reef.

Scientists have long wondered: What changed in the coral's genetic code to make them start building houses?

The Discovery: A Case of "Job Theft"

The researchers discovered that corals didn't invent a brand-new set of instructions from scratch. Instead, they stole a job description from a different department and gave it to a new team.

Think of a coral embryo as a construction site with two main teams:

1. The "Inner Team" (Endomesoderm): These cells are supposed to build the internal organs (like the stomach).
2. The "Outer Team" (Ectoderm): These cells are supposed to be the skin.

In sea anemones, the "Inner Team" stays inside, and the "Outer Team" stays outside. But in corals, the "Inner Team" got a new assignment. Some of these internal cells were told to leave their usual spot and move to the skin (the outer layer). Once there, they were told: "Stop being an organ-builder. Start being a skeleton-builder."

The scientists found that specific genes usually used to build the stomach (like a gene called Brachyury) were suddenly showing up in the skin cells of corals, telling them to secrete the hard skeleton.

The "Switch" That Flipped the Script

How did this happen? The paper explains that the hardware (the genes themselves) didn't change much. Instead, the software (the switches that turn genes on and off) was rewired.

Imagine a light switch in your house.

• In a sea anemone, the switch for the "Skeleton Light" is broken or missing, so the light never turns on.
• In a coral, the scientists found that the "wiring" around the Brachyury gene was changed. A new "switch" was installed that allowed this gene to turn on in the skin cells, not just the stomach cells.

To prove this, the researchers took the "wiring" (the DNA switch) from the coral and plugged it into a sea anemone. Even though the sea anemone doesn't build skeletons, the coral's switch successfully told the anemone's cells to turn on the "skeleton gene" in the wrong place! This proved that the secret wasn't the gene itself, but the regulatory switch controlling it.

The "Off" Button Experiment

To be sure this system was real, the scientists used a chemical "off button" to stop the main signal (called TCF) that usually tells cells to become internal organs.

• Result: When they turned off this signal, the corals stopped building skeletons.
• Conclusion: This confirmed that the same signal that tells a cell "You are an organ" also tells a coral cell "You are a skeleton builder." The coral just uses this old signal for a new purpose.

Why Does This Matter?

This study is a big deal because it shows how evolution works like a Lego set. You don't need to invent new bricks to build a new castle; you just take existing bricks and snap them together in a new way.

Corals evolved to build reefs not by inventing new genes, but by rewiring their existing genetic network. They took the instructions for making internal organs and redirected them to the skin to create the hard skeleton that supports entire ecosystems.

In a nutshell: Corals are like master architects who realized they could build a city by repurposing the blueprints for their own internal organs. They flipped a genetic switch, moved the construction crew to the outside, and built the foundation of the ocean's most diverse cities.

Technical Summary

1. Problem Statement

Stony corals (Scleractinia) are the only cnidarians capable of secreting a robust calcium carbonate skeleton, a trait that makes them keystone species for coral reef ecosystems. Despite their ecological importance, the genetic mechanisms driving the emergence of skeletogenic cells (calicoblasts) during early development remain undefined. While the expression of specific skeleton-associated genes (e.g., Galaxin, Ectin) has been documented, the upstream Gene Regulatory Networks (GRNs) and cis-regulatory architectures that specify these cells are unknown. Furthermore, it is unclear how the conserved developmental GRNs of cnidarians (specifically the endomesoderm network) were evolutionarily rewired to generate this novel cell type in corals, distinguishing them from non-skeletonizing relatives like sea anemones.

2. Methodology

The study utilized the temperate coral Astrangia poculata as a model system due to its amenability to laboratory spawning and genetic manipulation. The research employed a multi-omics and functional genomics approach:

• Developmental Profiling:
  • RNA-seq: Performed on developing larvae at various stages (blastula, gastrula, planula) to map the expression of conserved developmental GRN genes.
  • In Situ Hybridization Chain Reaction (HCR): Used to visualize the spatial expression patterns of key transcription factors (e.g., Brachyury, FoxA, TCF) and skeleton-associated genes (Ectin, Coadhesin) with high resolution.
• Functional Perturbation:
  • Pharmacological Inhibition: Larvae were treated with iCRT-14, a potent inhibitor of the $\beta$-catenin/TCF signaling pathway, to test the dependency of skeletogenic gene expression on the endomesoderm network.
  • RNA-seq Analysis: Conducted on treated vs. control larvae to identify downregulated gene sets, specifically focusing on Gene Ontology (GO) terms related to skeletogenesis.
• Epigenetic and Regulatory Mapping:
  • ATAC-seq (Assay for Transposase-Accessible Chromatin): Performed at 12hpf, 24hpf, and 36hpf to identify open chromatin regions (OCRs) and putative cis-regulatory elements (CREs) during embryogenesis.
  • Motif Analysis: Scanning of accessible regions for transcription factor binding sites (TFBS).
• Cross-Species Transgenesis (Reporter Assays):
  • A. poculata Brachyury Promoter: A 2kb proximal promoter region from A. poculata Brachyury was cloned upstream of a fluorescent reporter (mNeon) and injected into fertilized eggs of the sea anemone Nematostella vectensis (which lacks a skeleton).
  • N. vectensis Brachyury Promoter: A control construct using the N. vectensis promoter was injected into N. vectensis to establish baseline expression patterns.

3. Key Contributions

• Discovery of a Novel Subnetwork: The study identifies that elements of the ancestral cnidarian endomesoderm GRN (specifically involving Brachyury, FoxA, TCF, and Klf1) have been co-opted to form a subnetwork responsible for specifying skeleton-secreting cells in the ectoderm of stony corals.
• Identification of the cis-Regulatory Switch: The authors pinpointed the specific proximal promoter of the Brachyury gene as the evolvable unit responsible for the "ectopic" expression of this gene in the coral ectoderm.
• Functional Validation of Regulatory Rewiring: By demonstrating that the A. poculata Brachyury promoter drives ectodermal expression in N. vectensis (a species that does not produce a skeleton), the study proves that the regulatory logic for this novel cell type is encoded within the coral's cis-regulatory architecture and is sufficient to override the ancestral expression pattern.

4. Key Results

• Divergent Expression Patterns:
  • In A. poculata, endomesodermal genes (Brachyury, FoxA, TCF) are expressed not only in the invaginating pharynx (oral domain) but also in scattered cells throughout the aboral ectoderm (a "salt-and-pepper" pattern).
  • These ectodermal cells co-express known skeletogenic markers (Ectin, Coadhesin), suggesting they are the precursors to calicoblasts.
  • In contrast, N. vectensis restricts Brachyury expression strictly to the oral pole/pharynx with no ectodermal expression.
• Dependency on Endomesoderm Signaling:
  • Treatment with iCRT-14 (blocking TCF signaling) prevented gastrulation and, crucially, abolished the expression of both the endomesodermal markers and the skeletogenic genes (Ectin, Coadhesin) in the ectoderm.
  • RNA-seq confirmed a significant downregulation of skeletogenesis-related GO terms in treated larvae, confirming the endomesoderm GRN's dual role in germ layer specification and skeleton cell fate.
• Regulatory Architecture:
  • ATAC-seq revealed a dramatic increase in chromatin accessibility between the blastula and gastrula stages, particularly in promoter regions.
  • The A. poculata Brachyury promoter contains specific CREs that drive expression in both the oral pharynx and the ectoderm.
  • Cross-species assay: When the A. poculata Brachyury promoter was introduced into N. vectensis, it successfully drove ectopic expression in the ectoderm of the sea anemone larvae, recapitulating the coral pattern. Conversely, the N. vectensis promoter failed to drive ectodermal expression in N. vectensis.

5. Significance

• Evolutionary Mechanism of Novelty: This paper provides a concrete molecular mechanism for how a major evolutionary innovation (the ability to secrete a calcium carbonate skeleton) arose. It demonstrates that GRN rewiring via cis-regulatory changes (specifically in the Brachyury promoter) allowed the co-option of an ancient endomesoderm network to specify a completely new cell type (calicoblasts).
• Dual Role of GRNs: It challenges the view of GRNs as static, showing that the endomesoderm network in stony corals has a dual function: establishing germ layer identity (oral/endomesoderm) and specifying a lineage-specific cell type (skeletogenic cells).
• Model for Coral Diversification: The findings suggest that the diversification of stony corals from other anthozoans was driven by the evolution of specific regulatory elements that re-wired conserved developmental pathways. This framework helps explain the rapid diversification of coral lineages and their unique ability to build reefs.
• Methodological Advancement: The successful use of A. poculata for transgenesis and functional GRN analysis establishes it as a robust model system for studying coral development and evolution, bridging the gap between tropical coral genomics and functional developmental biology.