Evidence for the prepattern/cooption model of vertebrate jaw evolution

Area of Science:

• Evolutionary developmental biology
• Comparative genomics
• Vertebrate paleontology

Background:

• The evolution of jaws in vertebrates was a pivotal event, enabling predation on larger prey.
• The precise developmental origins of the jaw joint from jawless ancestors remain incompletely understood.
• Understanding pharyngeal patterning in jawless vertebrates is key to reconstructing jaw evolution.

Purpose of the Study:

• To investigate the developmental genetic basis of pharyngeal patterning in the jawless lamprey.
• To compare gene expression patterns between lampreys and jawed vertebrates (gnathostomes).
• To propose a novel model for the evolutionary origin of the vertebrate jaw.

Main Methods:

• Examined the expression of 12 key developmental genes in the pharynx of the modern jawless fish, lamprey.
• Analyzed gene expression along the dorso-ventral (DV) axis of the lamprey pharynx.
• Compared gene expression patterns with known patterns in gnathostomes.

Main Results:

• Lamprey pharyngeal patterning exhibits nested Dlx gene expression and combinatorial Msx, Hand, and Gsc gene expression along the DV axis, similar to gnathostomes.
• Key gnathostome joint regulators (Bapx, Gdf5/6/7) are absent in the lamprey first arch.
• Barx, typically absent in the gnathostome intermediate first arch, is present in lamprey, marking this region.

Conclusions:

• Gnathostome-like pharyngeal DV patterning predates the evolution of the jaw.
• Jaw evolution likely resulted from the co-option of Bapx and Gdf5/6/7 into a pre-existing DV patterning system.
• This "Pre-pattern/Cooption" model challenges theories of de novo pharyngeal patterning driving jaw evolution.