Evolving Hox activity profiles govern diversity in locomotor systems

Area of Science:

• Evolutionary developmental biology
• Neuroscience
• Genetics

Background:

• Limb development is crucial for vertebrate evolution, enabling the transition to terrestrial life.
• The precise positioning of motor neurons is essential for coordinating limb movements.
• Hox genes play a significant role in establishing body pattern and appendage development.

Purpose of the Study:

• To investigate the genetic mechanisms controlling the development and positioning of limb-projecting motor neurons.
• To understand the role of Hox gene clusters (HoxA and HoxC) in vertebrate locomotion evolution.
• To elucidate the function of Hoxc9 in the evolution of ambulatory versus undulatory movement.

Main Methods:

• Analysis of transcriptional autoregulatory modules in mouse models.
• Investigating gene expression patterns within HoxA and HoxC clusters.
• Studying the regulatory domain of Hoxc9 proteins in appendage-bearing and limbless vertebrates.

Main Results:

• A transcriptional module initiated by HoxA and HoxC genes controls the generation of limb-projecting lateral motor column (LMC) neurons.
• Repression of this module at thoracic levels dictates LMC neuron positioning relative to limbs.
• A specific regulatory domain in Hoxc9 proteins of limbed vertebrates mediates this suppression, correlating inversely with LMC position.

Conclusions:

• Hox gene regulation, particularly Hoxc9 function and expression, is key to the evolution of limb-based locomotion.
• Modulation of Hoxc9 likely facilitated the transition from aquatic, undulatory movement to terrestrial, ambulatory locomotion.
• The findings provide insights into the genetic basis for the diversification of vertebrate motor control.