Ancestral state reconstruction by comparative analysis of a GRN kernel operating in echinoderms.

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genomics

Background:

• Echinoderm gene regulatory networks (GRNs) offer insights into development and evolution.
• Sea urchins, particularly euechinoids, exhibit significant rewiring in their skeletogenic GRN.
• Developmental differences between cidaroid and eueuchinoid sea urchins may stem from GRN rewiring.

Purpose of the Study:

• Investigate the impact of GRN rewiring on downstream developmental events.
• Characterize the spatiotemporal expression of key regulatory genes in the cidaroid sea urchin Eucidaris tribuloides.
• Determine the ancestral function and evolutionary trajectory of a specific GRN subcircuit.

Main Methods:

• Cloning of four downstream regulatory genes within the skeletogenic GRN.
• Quantitative Polymerase Chain Reaction (qPCR) for gene expression analysis.
• Whole-Mount In Situ Hybridization (WMISH) for spatiotemporal expression patterns.
• Phylogenetic analyses with homologs from other non-vertebrate deuterostomes.

Main Results:

• The erg-hex-tgif subcircuit displays a mesoderm-specific expression pattern early in Eucidaris development.
• This subcircuit appears to be directly downstream of initial mesodermal GRN circuitry.
• Comparative analysis across four echinoderm taxa suggests the ancestral function was to stabilize the mesodermal regulatory state.
• The subcircuit is co-opted as a unit in mesodermal subdomains in diverse echinoderms.

Conclusions:

• GRN kernels possess structural and functional modularity.
• These kernels stabilize clade-specific embryonic regulatory states.
• The erg-hex-tgif subcircuit exemplifies a modular kernel with conserved functions in echinoderm development.