Lecture Notes on Hox Code and Evolutionary Developmental Biology

Lecture Overview

• Title: Cells to Organisms - Lecture 5

• Date: February 23, 2025

• Instructor: Prof. Paula Murphy

• University: Trinity College Dublin

• Focus: Evolutionary Developmental Biology (Evo-Devo).

Introduction to Evo-Devo

• Evo-Devo: The intersection of evolutionary biology and developmental biology.

  • Understanding the relationship between evolution and developmental mechanisms.

  • Insights into body plan changes through evolution, influenced by environmental pressures and genetic variation.

The Hox Code

• Hox Genes:

  • Essential for generating complex body plans of animals.

  • Highly conserved over millions of years, indicating a fundamental role in development.

  • Crucial for conveying positional information and organizing body plans during embryonic development.

  • Key Features of Hox Genes:

  • Contain homeobox sequences encoding transcription factors for gene regulation.

  • Organize body plans and determine identity and fate of segments.

  • Found in clustered arrangements in the genome, reflecting spatial and temporal expression patterns during development.

Hox Genes and Limb Development

• Hox gene expression in limb buds is overlapping and combinatorial, dictating limb formation.

• Simplifies identity specification along the proximo-distal (P/D) axis critical for limb morphology.

• Colinearity:

  • Hox gene expression corresponds to limb position, with the chromosome order matching spatial expression patterns.

• Effects of Mutations:

  • Mutations (e.g., in Hoxa11 and Hoxd11) lead to missing limb elements like the radius and ulna in mice.

  • Human mutation in HOXD13 results in syndactyly, highlighting small genetic changes affecting phenotypes.

Positional Information in Plants vs Animals

• Evidence of positional coding in plants (e.g., ABC model for flower morphology).

• Different gene families (e.g., MADS-box) encode spatial information in plants, indicating diversity in developmental strategies.

The Evolution of Body Plans

• Study Approach:

  • Comparative studies relate developmental observations with morphological structures in various species.

  • Emphasizes molecular mechanisms governing development and evolution of body plans.

• Neo-Darwinism:

  • Integrates molecular inheritance with Darwinian evolution for understanding genetic and morphological changes through natural selection.

Mechanisms of Evolutionary Change in Development

1. Change in Number of Genes (Gene Duplication):

   • Gene duplication facilitates evolutionary change (Ohno, 1970).

   • Ancestral Hox clusters in multicellular animals indicate history of duplication events, correlating with vertebrate complexity.

2. Change in Timing and Spatial Expression:

   • Alterations in gene expression timing/location result in morphological changes.

   • Examples:

     • Snakes exhibit limb loss due to expanded thoracic vertebrae expression at inappropriate locations.

     • Sticklebacks show reduced spines due to absence of Pitx1 expression, demonstrating adaptive morphological changes.

3. Change in Gene Interactions:

   • Regulatory genes essential for diverse structures across taxa.

   • Example: Pax6 gene’s role in eye development illustrates varied structures from a single gene.

   • Ubx gene interaction influences leg absence in insects compared to crustaceans, showcasing evolutionary innovation through gene networks.

Morphological Variation Through Differential Growth

• Homologous structures show varied morphologies due to differences in growth rates and gene expression patterns.

• Differential signaling pathway activation affects mammalian skeletal development, leading to different evolutionary trajectories.

Key Takeaways

• Understanding genetic regulation yields insights into evolution and morphological changes.

• Gene number, expression patterns, and interactome dynamics are vital in shaping body plan evolution.

• Connections between molecular mechanisms, evolutionary patterns, and ecological adaptations enhance comprehension of life’s diversity.

Next Lecture

• Topic: Organogenesis

• Date/Time: February 24, 2025, at 9 am

• Contact: Paula.murphy@tcd.ie