Ch. 15- Developmental genetic of Drosophila

How do embryonic cells become committed to one cell fate?

Is this controlled by genetics?

I. cell fate control
	A. essential for multicellular development
		1. first few cells are totipotent or pleuripotent
		2. as they develop, cells potential is restricted
			a. cell fate decisions
				i. irreversible
					a. cell determination
				ii. potentially reversible
					a. cell specifications
	B. systems that determine cell fates (figure 15.3)
		1. cytoplasmic (mosaic) determinants
			a. molecule present in cytoplasm
				i. mRNA or proteins
				ii. NOT dependent upon concentration of 
				molecule
		2. regulative development
			a. positional information
				i.signal that informs cell of location in embryo
				ii. usually a gradient of gene products
					a. due to localized transcription of gene
				iii. dependent upon concentration of molecule

II. 5 life cycle stages of Drosophila
	A. oocyte, embryo, larva, pupa, adult
        B. oogonial cell in mother's ovaries
                1. 4 mitotic divisions with incomplete cytokinesis
                        a. 16 cells connected with cytoplasmic channels
                                i. one becomes oocyte
                                ii. others become nurse cells
                                        a. produce proteins and mRNAs
                                                i. transferred into oocyte
                                        b. nurse cells degenerate
                                iii. surrounded by 1000 follicle cells
                                        a. collect yolk proteins from blood
                                                i. transfer to oocyte
                                        b. these follicle cells form vitelline 
                                         membrane & egg shell
                                        c. outer layer of cytoplasm is 
                                         cortical cytoplasm
                2. when oocyte is fertilized & laid (embryo)
                        a. oocyte nucleus completes meiosis
                        b. oocyte & sperm nuclei fuse
                        c. new zygotic nucleus begins to divide
                                resulting in nuclear syncytium
                                        one cell, many nuclei
                3. after ninth division of nuclei
                        a. zygotic nuclei migrat outward toward 
                         cortical cytoplasm
                        b. embryo now called syncytial blastoderm
                4. nuclei that reach posterior end form first separate 
                 embryonic cells (pole cells)
                        a. pole cells give rise to germ cells
                5. other nuclei continue to divide 3 more times
                        a. then cell membranes form simultaneously
                         around cortical nuclei
                                i. embryo now a cellular blastoderm
                6. gastrulation now begins
                        a. generates 3 primary cell types
                                i. endoderm
                                ii. mesoderm
                                iii. ectoderm
                        b. give rise to all larval & adult cell types
III. cell determination and pattern formation occur at cellular 
 blastoderm stage
        A. cells transplanted at syncytial blastoderm stage do not yet 
         have cell fates determined
        B. cells transplanted at cellular blastoderm stage have committed to form certain structures

IV. maternal gene products regulate gene action
	A. maternal genes regulate cell fates in embryo
		1. transcribed by maternal genotype during oogenesis
		2. transported as mRNA or protein to oocyte
	B. body axis formation
		1. due to maternal effect genes
			a. anterior
				i. bicoid
					a. mRNA localized to anterior
					 i. docked to anter. cytoskeleton 
					elements
					b. translated during early cleavages
					c. bicoid a TF for hunchback
						i. early zygotic gene
						ii. hbk has 5 binding regions in 
						 promoter for bicoid
					d. leads to hunchback gradient too
					e. hbk stimulates transcription of
					 genes for head & thorax, represses
					 activity of genes for abdomen
				ii. torso
					a. located at extreme ant. end
					b. represses hbk expression
						i. by sitting on promoter
			b. posterior
				i. nanos is maternal gene
					a. stored as mRNA in post.
					b. prevents transl. of hbk
					c. therefore, nanos regulated by 
					 maternal genes in both ant & post
				
	C. germline formation
		1. controlled by cytoplasmic determinants
			a. packaged in posterior