Genes & Development

Part 1: The Debate

Gene Theory

Nuclear vs Cytoplasmic InheritanceIs control over development tied to the nucleus

(chromatin) or to the cytoplasm?

T Boveri & EB Wilson – Nuclear controlTH Morgan – cytoplasmic control

Gene Theory

Boveri’s support for nuclear controlPolyspermy in sea urchins

• Embryos developing with multiple sets of chromosomes had defects

• Chromosome number (nucleus) important

EB Wilson & Nettie StevensCorrelated chromosome absence/presence with

sex determination• Drosophila

Gene Theory

XO & XY = maleXO & XY = male

XX = femaleXX = female

Gene Theory

TH Morgan Chief proponent of cytoplasmic inheritance1910 his lab accumulated data that supported

chromosomal inheritanceDiscovered and characterized Drosophila white (w)

mutant (has white eye)w phenotype had sex linked inheritance pattern –

• w male x wt female F1 all wt offspring

• wt male x F1 female only w-eyed males

Gene Theory

Morgan’s conversionSince mutation was inherited together with the

X chromosome, Morgan accepted the chromosomal inheritance theory wholeheartedly

Went further to hypothesize that genes were arranged linearly on chromosomes

Gene Theory

Nettie Stevens was a graduate student with Morgan at Columbia University

Did postdoctoral studies with WilsonWilson and Morgan were very good friendsHOMEWORK: go online to devbio

website and read material at website 4.1Quiz on Monday!

Geneticist vs Embryologist

Wilson and Morgan were embryologistsTheir combined support of the

chromosomal inheritance theory brought more geneticists into embryological systems

Influx of geneticists was disdained by classical embryologistsEE Just, H Spemann, F Lillie et al

Geneticists vs Embryologists

Embryologists set forth 3 criteria that must be satisfied by genetics in order to accept the dominance of the gene theory

1. How can identical chromosomes give rise to differentiated cell types

2. Demonstrate that genes control early developmental processes

3. Explain environmentally influenced phenomena such as temperature dependent sex determination

Gene Action in Early Development

Brachyury (brachy = short; ury = tail) Salome Gluecksohn-SchoenheimerCharacterized the early embryos of mice with

the Bra mutantAdult phenotype – deformed tails/pelvisEmbryo phenotype – lack posterior notochord

Drosophila wing mutationsConrad WaddingtonDemonstrated defects in the imaginal disk

formation

Gene Action in Early Development

Both Waddington’s and Gluecksohn’s experiments established that genes did effect early developmental processes

1 down 2 to go

Gene Action in Early Development

Genomic Equivalence

Explain differential gene expression

1. Establish that genomes of differentiated cells are equivalent

2. Determine why only certain genes are expressed

Genomic Equivalence

Regeneration of newt lens Remove lens Iris cells trans-differentiate to regenerate the

lens Series of changes in iris cells

1. Ribosome synthesis2. DNA replication3. mitosis 4. exocytose melanosomes5. form a group of undifferentiated cells6. turn on crystalline genes

Genomic Equivalence

Cloning – the ultimate equivalence testGenerate an entire, normal animal from a

the nucleus of a somatic cellRequires that somatic nucleus is totipotent

Genomic Equivalence

Cloning of the frog Xenopus laevis by nuclear transplantation of albino gut cell nuclei into enucleated, wt oocytes. All progeny are albino & female

Genomic Equivalence

Procedure for cloning frogs from differentiated nuclei. Successful cloning requires serial passage of donor nuclei through activated oocytes.

Genomic Equivalence

Enucleate oocyte

Isolate donor nuclei

Inject nuclei into oocyte

Genomic Equivalence

Totipotency of donor nuclei appears to decline with age

Genomic Equivalence

Dolly1st cloned mammalMammary epithelial cellCultured and maintained in G0

Fused with enucleated oocyte by electric shock1/434 success rate (0.23%)

Genomic Equivalence

Cloned Mice