Recapitulation-like developmental transitions of chromatin accessibility in vertebrates

Area of Science:

• Evolutionary developmental biology
• Genomics
• Comparative genomics

Background:

• Gene expression profiles in vertebrates show high conservation during mid-embryonic organogenesis, with divergence at earlier and later stages.
• The "hourglass model" describes this pattern, but doesn't exclude later appearance of evolutionarily novel gene regulations.
• Molecular evidence for evolutionarily derived gene expression appearing later in development was previously lacking.

Purpose of the Study:

• To investigate the evolutionary origins and developmental timing of gene regulatory elements.
• To test if evolutionarily newer gene regulations are activated at later developmental stages.
• To explore the relationship between phylogenetic history and the activation of gene regulatory elements during embryogenesis.

Main Methods:

• Genome-wide assessment of accessible chromatin regions during embryogenesis in mouse, chicken, and medaka.
• Estimation of evolutionary ages for these genomic regions using phylogenetic analysis.
• Correlation of chromatin accessibility timing with phylogenetic age across developmental stages.

Main Results:

• In all three species, the accessibility of genomic regions followed their phylogenetic history.
• Evolutionarily newer gene regulations were predominantly activated at later developmental stages.
• This pattern of delayed activation for newer regulations was observed after the mid-embryonic, phylotypic period.

Conclusions:

• A phylogenetic hierarchy exists for putative regulatory regions, with activation timing reflecting their evolutionary appearance.
• Newly introduced regulatory elements may be more likely to persist when activated later in embryogenesis.
• The findings suggest a potential mechanism linking evolutionary novelty and developmental timing, with implications for understanding recapitulation.