Genetic models Self-organization How do genetic approaches help to understand development? How can equivalent cells organize themselves into a pattern?

GeneFunction Phenotype

Gene Mutant Phenotype To find out what a particular gene does during development: 1) Make a targeted mutation in the gene (e.g. a knockout mouse). 2) Examine the resulting phenotypes. 3) Deduce the gene’s function. Developmental Function

Gene To identify genes that carry out a particular developmental process: 1) Screen for mutants in which the process is altered (i.e. with mutant phenotypes). 2) Identify the genes that have been mutated. 3) Deduce function from phenotypes. Developmental Function Mutant Phenotype

Developmental Function Protein Biochemical Function Gene Cloned genes can be used to analyze biochemical functions involved in a developmental process Mutant Phenotype

To understand a developmental process, figure out relations among genes and proteins affecting the process. Developmental Function Protein Biochemical Function Gene Developmental Function Protein Biochemical Function Gene Developmental Function Protein Biochemical Function Gene

Arabidopsis thaliana (Mouse-ear cress, Thale cress) Small weed, related to mustard - can fit thousands of plants in a small area Short generation time: 6-8 weeks when pushed fast Diploid, self-pollinating, transformable Small sequenced genome: About 125 Mb and 25,000 genes

Arabidopsis is closely related to these plants

Other model plants

Finding Arabidopsis mutants EMS (ethylmethane sulfonate) 3. Screen M2 plants for mutants having phenotypes of interest 2. Allow M1 plants to self- fertilize, collect M2 seed 1. Mutagenize seeds (M1)

Carrying out a mutant screen in a non- hermaphodite species

~1000 somatic cells Transparent Entire cell lineage described Genome sequenced Self-fertilizing hermaphrodite Caenorhabditis elegans – a model genetic species

Cell lineage of C. elegans

Full somatic cell lineage of C. elegans

C. elegans vulva formation

Ablate P6, another cell acquires vulval fate instead An equivalence group – P3-P8 cells have the same potential

Ablate anchor cell, no vulva forms

C. elegans vulva formation Three cells give rise to the vulva because: 1)They are close to the signal source 2)They communicate with each other

Where does the Anchor Cell (AC) come from?

(from Wilkinson et al. (1994), Cell 79: ) Anchor cell (AC) and Ventral uterine precursor cell (VU) – an equivalence group of 2 cells

lin12 or lag2 mutants: Both precursor cells become anchor cells Signal = Lag2 (similar to Delta) Receptor = Lin12 (similar to Notch) Stochastic asymmetry Positive feedback reinforcement

(from Wilkinson et al. (1994), Cell 79: ) Model for Anchor cell (AC) and Ventral uterine precursor cell (VU) specification

Delta - Notch signaling pathway

Lateral inhibition in a field of cells (such as Drosophila neurogenic ectoderm)

Drosophila neurogenic ectoderm: Bristles are neural cells Mutant sector with partial loss of Delta function

Action of Delta and Notch in Drosophila neurogenic ectoderm Neuroblast Epidermis

Action of Delta and Notch in Xenopus neural plate

Dominant-negative Delta mRNA injected into Xenopus embryo (Normally, other cells differentiate as neural tissue later)