Development: Cell Division, Cell Differentiation, and Morphogenesis Cell Differentiation – change in pattern of gene expression (gene expression profiles) Morphogenesis –change in shape (creation of form) Cell Differentiation to specific cell fates: capacity to differentiate into different types of cells Totipotency to less than totipotent Differentiation occurs in stages: totipotent cell; stem cell with restricted potency; terminally differentiated cell capable of giving rise only to other cells of the same type Variable gene activity theory: differentiation is a result of limiting the gene expression profiles of cells with the same genome Dedifferentiation: return to totipotency – required for animal cloning (sheep mammary cell nucleus used to clone Dolly) Cell Differentiation is due to intracellular and extracellular signals Specific subcellular localization of Intracellular signals leads to asymmetric cell divisions, daughter cells have different gene expression profiles Specific extracellular signals determine a cells pattern of gene expression Change in gene expression profiles Coordinates gene expression profiles of neighboring cells – common cell type, common tissue type… Both types lead to Determination When cell is committed “irreversibly” to a particular fate or set of fates Types of developmental decisions: Binary decision: cell adopts one of two alternative (mutuallly exclusive) cell fates Germ line versus soma Male versus female Decision among multiple options Determination of cell’s fate within one of the main axes of the body Anterior – Posterior Dorsal – Ventral Left - Right

Levels of Control of Gene Expression

Cell Differentiation: A Binary Switch Intracellular Signals Asymmetric subcellular localization Extracellular Signals cell-to-cell contacts paracrine signalling Master Switch is usually one or two genes that are directly regulated by the signal Example Binary Switch Mechanisms: Sex determination in Drosophila melanogaster and Homo sapiens Signal is the ratio of X chromosomes to autosomes in Drosophila Signal is the presence of a Y chromosome in humans Mater switch = Sxl in flies; SRY (sex reversal gene) also known as TDF (testis determining factor) in humans

Sterile male Klinefelter Syndrome Turner Syndrome Drosophila sex is determined by the ratio of the number of X chromosome to number or sets of autosomes In Human, sex is determined by presence or absence of an intact Y chromosome Drosophila is diploid, with n = 4 3 autosomes = 1 “autosome set”, 1 sex chromosomes XX female 2 sex chromosomes, 2 autosome sets = X/A = 1 XY male X/1 = ½ XXX AAA triploid X/A = 1; female XYY AAA triploid X/A = 0.3; male XXY AAA triploid X/A = 0.67; intersex Klinefelter Syndrome Turner Syndrome Sterile male

Also some activation Also some activation of male-specific Fly Sex determination: Overall pathway through Sxl Sxl activation in early female development = transcriptional control Maintenance of Sxl expression later in female development is due to regulation alternative pre-mRNA splicing Regulation of tra and dsx = regulation of alternative splicing Also some activation of male-specific genes Also some activation of female-specific genes

Signal = X chromosome/Autosome Ratio

Signal Activates “Early” Promoter of Sxl X/A = 1 NUM > DEM X/A = .5 NUM < DEM

LOSS OF FUNCTION MUTANT PHENOTYPES XX male phenotype (Lethal) Viable XX male phenotype Viable Intersex Intersex Fly Sex determination: Overall pathway through Sxl Sxl activation in early female development = transcriptional control Maintenance of Sxl expression later in female development is due to regulation alternative pre-mRNA splicing Regulation of tra and dsx = regulation of alternative splicing

Another aspect of sex determination: Dosage Compensation Human dosage compensation: decrease expression from XX females by X chromosome inactivation Flies: increase expression from XY or XO males by expression of transcriptional activators Is why sex-lethal is a female lethal - XX sxl mutants overexpress genes on both X chromosomes – imbalance in gene expression Dosage compensation probably acts through Sxl…