c. elegans lecture kaveh ashrafi [email protected] n412c genentech hall 415.514.4102
TRANSCRIPT
C. elegans lecture
Kaveh Ashrafi
N412C Genentech Hall415.514.4102
Genetics concepts:
-diploid genetics:* somatic tissue is diploid all the
time* hermaphrodite genetics
-multicellular organism:* when & where gene function is
required(mosaic analysis,
tissue/developmental stage specific promoters, cell ablation)
-forward and reverse screens
Sydney Brenner
Goldstein lab movie (http://www.bio.unc.edu/faculty/goldstein/lab/movies.html)
I. OVERVIEWC. elegans as an experimental system
Life cycleShort reproductive maturation time & large
number of progeny
From wormatlas: www.wormatlas.org
Basic anatomy: tube within a tube
Outer tube-body wall-cuticle-epithelial system-muscle system-excretory system-nervous system (hermaphrodite: 302 neurons, 5000 synaptic connections)only organism for which complete wiring diagram known
Pseudocoelomic cavity-fluid-filled; transport
Inner tube-alimentary system (pharynx/intestine)-reproductive system
ADULT MALE
Sex
autosomes (pairs) sex chromosome(s)
5 XX
5 XO
body plan of an adult hermaphrodite
Hermaphrodites are self fertilizing because they contain bothoocytes and sperm
Attractions for developmental biology & neurobiology:invariant somatic cell lineage
Cell divisions give rise to 1090 cells. 959 survive, 131 die==>discovery of genetic basis of programmed cell death.
How do cells adopt their fates? (cellular basis of asymmetry, differentiation
programs)
How do they end up in the right place at the right time? How do cell come together to form organs/tissues?
(3D migration, programmed cell death, developmental timing)
How do cells communicate with each other?(signaling cascades, neuroendocrine pathways)
Molecular genetic analysis of disease processes, physiology, & behavior
Genetics of Development, Physiology, & behavior
II. GENETIC BASICS
Self progeny vs. cross progeny
~100% XX
F1 have genotype of parent (clonal)
X
50% XX 50% XO
F1 hermaphrodites are heterozygous at all loci; F1 males are heterozygous at all autosomal loci, hemizygous on X
I, II, IIIIV, V, X
I, II, IIIIV, V, X
I, II, IIIIV, V, X
I, II, IIIIV, V, X
I, II, IIIIV, V, X
I, II, IIIIV, V
Example of a genetic cross in C. elegans
UNC=uncoordinated movementunc-40(e271) I a recessive mutation
X unc-40 (e271)/unc-40 (e271) +/+
F1 :
self progeny: 100% Unc, ~100% hermaphrodite unc-40/unc-40 cross progeny: 100% non-Unc (WT), unc-40/+
Example of a genetic cross in C. elegans
1/4 unc-40/unc-40 1/2 unc-40/+ 1/4 +/+
Phenotypic ratios for recessive alleles? Dominant alleles?
What are the sex ratios? What if mutation is on X?
unc-40 +
unc-40
+
Take F1 that is cross progeny, single onto a new plate, allow to self
F2 (from self fertilization of cross progeny)
III. GENETIC SCREENS
Point of entry into a biological process.
A simple screen that can produce informative, tractable mutations with strong
and specific phenotypes.
F1: m/+ can identify dominant mutations
F2: +/+; m/+; m/m can identify dominant & recessive mutations
Po
A simple forward genetics screening strategy
+/+
m1 ++
+ ++
m2
m3+
+
m5m4
+m6 m7
F3: can identify maternal effect mutations; shows of mutations identified in F2 breed true
From screen to gene identity
*Determine if the mutants breed true*Backcross*Determine nature of the mutation (e.g. dominant/recessive)*Determine # of complementation groups*Determine molecular identity: mapping
Po
F1
F2
Positional mapping using SNPs
X
Select F2 progeny with desired phenotype
Rescue & Transgenics
*Inject DNA fragments from wild type into mutant animals to identifyRescuing region.
*Sequence DNA region from mutant to identify mutation.
general considerations regarding screens
•Specificity of phenotype under study•Robustness of phenotype under study
You always have to balance the ease of screening scheme/assay with the desired targeting/specificity of desired phenotype/pathway