Genes, Development, and Evolution: Homeotic Genes, Stem Cells, and Evo-Devo

Chapter 21: Genes, Development, and Evolution

Homeotic Mutants in Animals

Homeotic mutations are genetic changes that result in body parts forming in incorrect locations, providing insight into the genetic control of development.

• Homeotic mutations in Drosophila can cause a segment to develop like the one just in front of it.

• A second pair of wings may develop instead of balancing structures.

• Mutations can cause legs to grow in place of antennae.

Example: In fruit flies, homeotic mutants may have wings or legs in abnormal positions, demonstrating the role of specific genes in segment identity.

Homeotic Mutants in Plants

Homeotic mutations also occur in plants, affecting floral organ identity.

• A group of genes called MADS box genes code for transcription factors that regulate flower development.

• Mutation in these genes in Arabidopsis thaliana can cause petals and sepals to form in place of carpels and stamens.

Example: A mutant Arabidopsis flower may lack reproductive organs and instead have extra petals or sepals.

Homeotic (Hox) Genes: Structure and Function

Hox genes are a family of regulatory genes that determine the identity and arrangement of body segments in animals.

• Hox genes are used repeatedly at different stages of development.

• Hox gene products are transcription factors that activate effector genes.

• Each Hox gene contains a homeobox sequence encoding the DNA-binding domain of the transcription factor.

Definition: The homeobox is a conserved DNA sequence found in Hox genes, crucial for binding DNA and regulating gene expression.

Conservation of Hox Gene Organization

The organization and function of Hox genes are highly conserved across animal species.

• In Drosophila, Hox genes are found in clusters along the chromosome.

• The order of genes corresponds to their expression pattern in the embryo.

• Most animals have related sets of Hox genes arranged in clusters, with gene order aligning with expression order.

Example: Both flies and mice have Hox gene clusters that determine segment identity along the anterior-posterior axis.

Conservation of Hox Gene Function

Hox genes play a key role in specifying body part identity in most animals, including humans.

• Experimental introduction of mouse Hoxb6 gene into fly embryos caused similar phenotypes as the fly Antp gene, showing functional conservation.

• Hox genes are homologous, descended from a common ancestor.

• Pattern-formation genes have been highly conserved during animal evolution.

Example: The last shared ancestor of flies, mice, and humans lived about 600 million years ago, yet their Hox genes retain similar functions.

Hox Genes in Different Species: Organization and Expression

The arrangement and expression of Hox genes are similar in diverse species, reflecting evolutionary conservation.

• In both flies and mice, Hox genes are expressed in patterns that correspond to their order on the chromosome.

• This spatial and temporal expression determines segment identity during development.

Species	Hox Gene Cluster	Expression Pattern
Fly	Clustered	Head to Abdomen
Mouse	Clustered	Head to Tail

Stem Cells and Stem Cell Based Therapies

Stem cells are undifferentiated cells capable of self-renewal and differentiation into specialized cell types, with important applications in medicine.

• A single fertilized egg produces about 200 different cell types.

• Ernest McCulloch and James Till demonstrated stem cell differentiation using bone marrow transplants in mice.

• Stem cells can reproduce or differentiate into specialized cells.

Stem Cell Type	Source	Potential
Hematopoietic	Bone marrow, cord blood	Blood cells
Embryonic (hES)	Human embryos	Pluripotent (many cell types)
Induced Pluripotent (iPS)	Adult cells (e.g., skin)	Pluripotent

Human Embryonic Stem Cells

• Embryonic stem cells are pluripotent, able to differentiate into most cell types.

• Challenges include immune rejection and ethical concerns.

• Canadian policy restricts hES cell use to embryos originally created for reproduction and freely donated.

Induced Pluripotent Stem Cells (iPS)

• iPS cells are created by reprogramming adult cells using four key genes.

• Viral DNA inserts these genes into chromosomes, enabling cells to become pluripotent.

• Growth factors can direct differentiation into nerve, heart, or other cell types.

Example: Skin cells can be reprogrammed into iPS cells and then differentiated into insulin-producing pancreatic beta cells.

Treating Diabetes with Stem Cells

• Stem cells can be used to create insulin-producing cells for diabetes therapy.

• Researchers are working to turn hES and iPS cells into pancreatic beta cells for transplantation in type 1 and type 2 diabetes.

Changes in Developmental Gene Expression Drive Evolutionary Change

Alterations in developmental gene expression can lead to evolutionary changes in organismal form and function.

• Disruption of developmental processes can be lethal; modification can result in novel phenotypes.

• Evo-devo (evolutionary developmental biology) studies how changes in gene regulation drive evolution.

• Changes in the timing, location, or level of gene expression can produce new forms.

Example: Limb loss in snakes is due to changes in Hox gene expression and failure to produce the signalling molecule sonic hedgehog.

Case Study: Limb Loss in Snakes and Whales

• Most snakes lack limbs, but ancestors had four limbs.

• In chicken embryos, Hox6 and Hox8 are expressed in cells where ribs form; only Hox6 is expressed where forelimbs form.

• In snakes, Hox6 and Hox8 are both expressed in regions where forelimbs should form, leading to rib formation instead of limbs.

• Loss of sonic hedgehog signalling in whales led to hindlimb disappearance.

Summary Table: Hox Gene Expression and Limb Development

Organism	Gene Expression	Phenotype
Chicken	Hox6 only in forelimb region	Forelimbs develop
Snake	Hox6 & Hox8 in forelimb region	Ribs develop, limbs lost
Whale	Loss of sonic hedgehog	Hindlimbs lost

Check Your Understanding

• Evolutionary theory has emphasized genetic mutations as sources of variation, but evo-devo highlights the role of changes in gene regulation.

• Changes in cell-cell interactions and gene expression timing/location can lead to phenotypic variation and evolutionary change.

Learning Objectives

• Provide evidence for the conservation of signalling molecules and regulatory genes used during development.

• Explain how stem cells can be used in stem cell-based therapies to treat human disease.

• Describe how changes in developmental gene expression drive evolutionary change.

Additional info: Evo-devo is a modern field integrating genetics, development, and evolution, emphasizing regulatory changes over coding mutations in evolutionary innovation.