Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Figure 7.1 The Human Infant Preformed in the Sperm, as Depicted by Nicolas Hartsoeker (1694)Slide 37Slide 381Model Organisms in Developmental Biology Plants Invertebrates VertebratesWhy use model organisms?What features do they have in common?Contrasting genome sizes in model organisms• SMALLMb=megabases =1,000,000 base pairs of DNA (million)• LARGEGb=gigabases=1,000,000,000 base pairs of DNA (billion)120 Mb3 Gb3Model Organisms: Arabidopsis • Small weed, member of the mustard family• can fit thousands of plants into a small growth room • Short generation time (=time from seed to seed) for a plant: 6 weeks • history--most intensely studied plant for past 15 years, genome sequenced, excellent genetics and experimental tools developedSmall Genome = 125 Mb~10 times yeast (12 Mb)4Model Organisms: C. elegans • Very small, so 10,000 worms can be kept on a Petri dish. • Short generation time (=time between equivalent stages in adult and progeny)- 3 days. • Embryo is transparent and develops outside the body in a short time (14 hours!!) - so can watch development with a microscope.• 1031 cells in an adult, and the lineage of each cell is known.• very cheap to keep.Small Genome = 100 Mb5Model Organisms: Drosophila• small (adult < 5 mm long). Can keep hundreds in a small vial.• short generation time - 8 days• embryo develops outside the body in a short time - so can easily study development• history - scientists have been doing genetics and collecting mutations for many years (since 1910)• very cheap to keep.Small Genome = 120 Mb6Model Organisms: Xenopus•vertebrates•embryos develop outside the body in a short time - reach tadpole stage in 2-3 days•embryos are HUGE!! So many historical (and modern) experiments have been done using frogs•relatively inexpensive because you can get lots of embryos from one female frog•DISADVANTAGE: generation time of Xenopus laevis = 1-2 years, so no genetics, but Xenopus tropicalis is < 5 months, so some genetics projects starting upGenome = 3.1 x 109 bp = 3.1 Gb7• also vertebrates• each fish is about 2 inches long, can keep many in a tank• generation time = 2 months• embryo is transparent and develops outside the body in a short time (24 hrs.)• not so cheap as worms and flies but cheaper than mice• DISADVANTAGE: only recently used, so little history and geneticsModel Organisms: Zebrafishlarge genome = 1.7 Gb8Model Organisms: Mice• also vertebrates!• mice are ~ 3 inches long, can keep many mice in a room.• generation time is ~ 3 months, so genetics can be done• history - scientists have worked with mice for 100 years• lots of ways to change the genetic program in mice - can introduce extra genes, or remove a specific gene, then study the effect on development•DISADVANTAGE: development inside the mother, hard to get at. Expensive!Large Genome = 3 Gb(30 times worms & flies300 times yeast)9Why Model Organisms MatterMutation of the Kit gene in humans and mice“Piebaldism” • Affected individuals are anemic, sterile, deaf, and lack pigment in certain skin cells• Kit encodes a receptor tyrosine kinase required for cell proliferation in neural crest, blood, and germ cellsFigure 1.2210Rules of Evidence in Developmental BiologyGilbert p. 39-40Three main types (KNOW THESE):1) Correlative Evidence• You see a gene expressed in a tissue• Weak, but relatively easy2) Loss of Function Evidence• You knock out the gene, and the tissue fails to form• Better, but a little harder.• Gain of Function Evidence1) You express a gene in an inappropriate tissue, and transform it to a different fate2) Best, but hardest of all“Find it, Lose it, Move it”11Rules of EvidenceWhat type of experiment is this?OogenesisBehind every successful embryo stands a hard-working mother.Eggs are enormous cellscarrying nutrients forthe developing embryoWhy? Mass of egg must equal the mass of the embryo that emerges!Even human eggsare huge cellsEggs are enormous cellscarrying nutrients forthe developing embryo• yolk=large stores of nutrients• large stores of macromolecules• most eggs surrounded by a coat or shellThe germ line is set aside early in development as a separate cell lineage This can sometimes be visualized by the segregation of putative "determinants”as in C. elegansFertilized egg"P granule"mother cell of germline = P4 P1HOW are P-granules segregated?PAR proteins & microfilaments see p. 195 and Fig. 5.43C. elegans:P granulesare segregated at each divisionand make thecell that inherits them the germlineFigure 5.44 cell nuclei P-granulesSimilar mechanisms lead to very early segregationof the germline in thefruit fly DrosophilaGerm cellprogenitorsAPHuman Oogenesis2 million oocytes at birth !Figure 16.29Only ~ 400 used during a woman’s lifetimeOocytes arrest in prophase of meiosis I for up to 50 years!How does a single egg cell make all the stuff needed to start development?…..with support from her friendsFly OogenesisNurse cells are mitotic sisters of the oocyteNurse cells synthesize macromolecules and pump them into the oocyte through the cytoplasmic bridgesoocytenurse cellsfollicle cellsThe Drosophila ovary is composed of both somatic follicle cells and germ line nurse cells and oocyteMost of the instructions for forming organisms are already in the egg!Asymmetric distribution of egg contents influences embryonic developmentdorsalventralanterior posteriorMolecules synthesized by the nurse cells and specifically localized in the oocyte determine the anterior/posterior axis of the embryoPosterior Determinant(nanos)Anterior Determinant(bicoid)OocyteSpecific mRNAs are bound by proteins and transported along microtubules by motor proteins to their destinationsWhat happens if the anterior/posterior axis isn’t established correctly??wildtype larvabicoid mutantFig. 6.24bicoid mutants have no head!!Localized mRNAs also influencevertebrate developmentVg1 mRNAVg1 mRNA localizes to the vegetal poleof the XenopusoocyteVentral follicle cells build a signal into the egg shell that determines the dorsal/ventral axis of the embryoVentral follicle cellsVentral
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