CHAPTER 19 DEVELOPMENTAL GENETICS Brenda Leady, University of Toledo Prepared by Brenda Leady, University of Toledo

Development Refers to a series of changes in the state of the cell, tissue, organ, or organism Underlying process that gives rise to the structure and function of living organisms Developmental genetics aimed at understanding how gene expression controls this process

Sperm and egg unite to produce a zygote That diploid cell divides and develops into the embryo Cells divide and begin to arrange themselves Each cell becomes determined – destined to become a particular cell type Commitment occurs before differentiation – cell’s function and morphology have permanently changed into a highly specialized cell type

Genome is a set of genes that constitute the program of development Unicellular organisms – controls structure and function of the single cell Multicellular – controls cellular features and the arrangement of cells

Model organisms Fruit fly Drosophila melanogaster Advanced techniques for generating and analyzing mutants Large enough for easy study but small enough to determine where genes are expressed Nematode worm Caenorhabditis elegans Simplicity – only about a thousand somatic cells Pattern of cell division and fate of each cell known

House mouse Mus musculus Zebrafish Brachydanio rerio Thale cress Arabidopsis thaliana Wild mustard family Short generation time, small genome

Pattern formation Coordination of events leading to the formation of a body with a particular pattern Formation of an adult body with 3 axes Dorsoventral, anteroposterior, and right-left May also be segmented Plant bodies are formed along a root-shoot axis in a radial pattern

Positional information Each cell in the body must become the appropriate cell type based on its relative position Each cell receives positional information that tells it where to go and what to become Cell may respond by Cell division, cell migration, cell differentiation or cell death (apoptosis)

2 main mechanisms used to communicate positional information Morphogens Cell adhesion

Morphogens Give positional information and promote cellular changes Act in a concentration dependent manner with a critical threshold concentration Distributed asymmetrically In the oocyte or egg precursor In the embryo by secretion and transport Induction – cells govern fate of neighboring cells

Cell adhesion Each cell makes its own cell adhesion molecules (CAMs) Positioning of a cell within a multicellular organism is strongly influenced by the combination of contacts it makes with other cells and with the extracellular matrix

Hierarchy of transcription factors Four general phases for body formation Organize body along major axes Organize into smaller regions (organs, legs) Cells organize to produce body parts Cells themselves change morphologies and become differentiated Differential gene regulation – certain genes expressed at specific phase of development in a particular cell type Parallel between phases and expression of specific transcription factors

Animal development Drosophila model Oocyte establishes pattern for adult Elongated cell with positional information After fertilization, zygote develops into blastoderm Series of nuclear divisions without cytoplasmic division (produces many free nuclei) synctial blastoderm Individual cells are created after nuclei line up along cell membrane (cellular blastoderm)

Gastrulation involves cells migrating to the interior 3 cell layers formed- ectoderm, mesoderm and endoderm Segmented body plan develops Head, thorax and abdomen Larva – free living Pupa – undergoes metamorphosis Adult Egg to adult in 10 days

Phase 1 Pattern development First phase is establishment of body axes Morphogens are distributed prior to fertilization Bicoid, example morphogen Mutation results in larva with 2 posterior ends Nurse cells are located near anterior end of oocyte Bicoid gene transcribed in nurse cells and mRNA transported into anterior end of oocyte Maternal effect Transcription factor that activates particular genes at specific times Asymmetrical distribution means activated only in certain regions

Phase 2 Segments Normal Drosophila embryo divided into 15 segments 3 head, 3 thoracic and 9 abdominal Each will give rise to unique morphological features in adult

Bithorax gene complex Normal – wings on 2nd thoracic segment and 2 halteres on 3rd thoracic segment Mutant – 3rd segment has wings so 2 sets of wings and no halteres

3 classes of segmentation genes Gap genes Mutation – several adjacent segments are missing Pair-rule genes Mutation – alternating segments or parts of segments deleted Segment-polarity genes Mutation – portions of segments to be missing either anterior or posterior region and adjacent regions to mirror each other

To create a segment in phase 2, a group of genes acts sequentially to govern the fate of a given body region Maternal effect genes, which promote phase 1 pattern development, activate gap genes Seen as broad bands of gap gene expression in the embryo Gap genes and maternal effect genes then activate the pair-rule genes in alternating stripes in the embryo Once the pair-rule genes are activated, their gene products then regulate the segment-polarity genes Expression of a segment-polarity gene corresponds to portions of segments in the adult fly

Phase 3 Segment characteristics Each segment begins to develop its own unique characteristics Cell fate – ultimate morphological features that a cell or group of cells will adopt Mutations in homeotic genes alter cell fate Bithorax is an example of a homeotic mutation Order of homeotic genes along chromosome corresponds to their expression along the anteroposterior axis of body Colinearity rule

Role of homeotic genes to determine identity of particular segments

Homeobox – coding sequence of homeotic genes contains 180-bp sequence Homeotic genes encode homeotic proteins that function as transcription factors Activate transcription of specific genes that promote developmental changes Homeobox – coding sequence of homeotic genes contains 180-bp sequence Encodes homeodomain for DNA binding

A Homologous Group of Homeotic Genes Is Found in All Animals Identify vertebrate genes that are homologous to those that control development in simpler organisms such as Drosophila Hybridization techniques Homologous genes are evolutionarily derived from the same ancestral gene and have similar DNA sequences Hox genes in mice Follow colinearity rule Key role in patterning anteroposterior axis

Phase 4 Cell differentiation Emphasis shifts to cell differentiation Studied in mammalian cell culture lines Differential gene expression underlies cell differentiation Stem cell characteristics Capacity to divide Daughter cells can differentiate into 1 or more cell types

Stem cell categories Totipotent Pluripotent Ultimate stem cell is fertilized egg Can produce all adult cell types Pluripotent Embryonic stem cells (ES cells) Embryonic germ cells (EG cells) Can differentiate into almost any cell but a single cell has lost the ability to produce an entire individual

Multipotent Unipotent Can differentiate far fewer types of cells Hematopoietic stem cells (HScs) Unipotent Daughter cells become only one cell type Stem cells in testis produce only sperm

Robert Davis, Harold Weintraub, and Andrew Lasser Identified Genes Encoding Transcription Factors That Promote Muscle Cell Differentiation What causes stem cells to differentiate into a particular cell type? Certain proteins function as “master transcription factors” Initial experimental strategy to identify genes expressed only in differentiating muscle cells Narrowed down to 3 genes Would any of these 3 genes cause nonmuscle cells to differentiate into muscle cells?

Belongs to myogenic bHLH genes MyoD was the only one to cause fibroblasts to differentiate into muscle cells Belongs to myogenic bHLH genes Found in all vertebrates and activated during skeletal muscle development Features promoting muscle cell differentiation Basic domain binds specifically to an enhancer DNA sequence that is adjacent to genes that are expressed only in muscle cells Protein contains an activation domain that stimulates the ability of RNA polymerase to initiate transcription Interacts with other cellular proteins Id – prevents muscle differentiation too soon E – activates gene expression

Plant development 2 key features of complex plant morphology Root-shoot axis Radial pattern

Differs from animal development No cell migration No morphogens An entirely new individual can be regenerated from somatic cells (totipotent) Similarities to animal development Use differential gene expression Use of transcription factors

Arabidopsis model for development After fertilization, the first cellular division is asymmetrical and produces a smaller apical cell and a larger basal cell Apical cell – gives rise to most of embryo and shoot Basal cell – root and suspensor cell for seed formation Heart stage – about 100 cells Basic organization established Root meristem – gives rise only to root Shoot meristem – all aerial parts of plant – stem, leaves, flowers 2 cotyledons to store nutrients

Seedling 3 main regions Shoot meristem organizing center Apical region – leaves and flowers Central region – stem Basal region – roots Shoot meristem organizing center Central zone – maintains undifferentiated stem cells Peripheral zone – dividing cells that will differentiate

Apical, central and basal regions express different sets of genes Apical-basal patterning genes Defects can cause dramatic effects

Plant homeotic genes First known homeotic genes discovered in plants Normal Arabidopsis flower composed of 4 concentric whorls Sepals – outer whorl, protects bud Petals Stamens – make pollen (male gametophyte) Carpel – produces female gametophyte

ABC model for flower development 3 gene classes A, B, C and E Whorl 1 – gene A product = sepals Whorl 2 – gene A, B and E products = petals Whorl 3 – gene B, C and E products = stamens Whorl 4 – gene C and E products = carpel Leaf structure is default pathway If A,B,C and E are defective, leaves result