BG16 Flashcards
directional patterns
patterns in particular parts
one place rather than another
periodic patterns
segments and spots
wolpert FFM
1970 french flag model
- cells aquire positional info from morphogens that are secreted from a point source.
describe the FFM
morphogen diffuses becomig lower in concentration as it moves away from the source
cells read this gradiet and respond discretely to it
differentiating into different kinds of cells accordingly.
what is the fruit fly AP axis a result of
a genetic cascade
bicoid mutation drosophila
can cause loss of head to tail patterning etc.
bicoid experiments
bicoid gene encodes morphogen responsible for head structures in drosophila
bicoid (-/-) embryos are injected with bicoid mRNA, point of injection forms head structures.
– injection of posterior pole of early cleavage wt embryo with bicoid mrna = head structures at both poles.
what is nanos
gradient
posteriror equivalent of bicoid
high gradient posteriorly low anteriorly.
AP gradient of nanos
nanos mutants
lack telson - last segment in abdomen
caudal and hunch back gradients
both have constant mRNA expression across the embryo however this is truned into a pair of protein gradients
hunchback is highly anteriror, low posterior
caudal vv.
hunch back expression regulation
nanons represses translation of hunchback so hunchback mRNA only translated where nanos is low = anteirorly.
bicoid acts as a transcriptional activator of zygotic hunchback causing anterior expression.
caudal regualtion
bicoid represses translation of caudal mRNA giving rise to cadual protein gradient.
simplist explanation for butterfly eyespots
2D diffusion gradient from morphogenic centre of focus
- cell respond depending on conc gradeitn.
may give rise to diferent sized spots
- non linear FFM.
how can you expermientally change spot formation in lepidoptera
- cauterize (burn away) focus of the morphogen in a caterpillar before pupation - spot fails to form
- transplant spots to get extra.
TF expressed in imaginal diks near site of future spots
distal-less
spallt
engrailed
spot mutants in Bicyclus spp
can be interpreted in terms of altering production of or response to such a morphogen.
what is the morphogen for spot formation
unknonw might be wingless a wnt dpp tgf-beta etc.
frenchflag model flaws
cant give periodic patterns
repeated patterns
annelids etc.
turing model described
model supposed two morphogens with feedback loops
inhibiting or activating themselves and eachother
- when modelled with differential equation under some conditions stationary waves of morphogen conc could be obtained read by cells.
turing model
assume have two ligands u and v that interact with each other
du/dt = F(u,v) - duv + Du( changein)u
rate of conc change = production - degredation + diffusion.
same for dv.
depending on the parameters of the turing model what states can you get
- uniform/stationary
- uniform/oscillating
- stationary waves with extreme short wave-length
- oscillatory cases withe extreme short wave lengths
- oscillatory cases with finite wave length
- stationary waves with finite wave length = turing pattern.`
How the turing model leads to periodic patterns
- conc of activator is higher in other regions by random fluctuations.
- by self-enhancing property its concentration increases at centre region followed by an increase of inhibitor in neighbouring regions.
- as diffusion rate of inhibitor is larger than activator, inhibitor moves to lateral regions
- depresses the activator function, resulting in decrease of activator conc,
- decrease causes decrease of inhibitor in wider area
- so inhibitor conc becomes lower, and then activator becomes dominant
- leading to local self-activation again
and cyclical pattern.
fruit fly segmentation
segmentation genes are expressed in alternate parasegments
tfs bind to CRE regions in varying strengths chaing expression
- every segment is specified individually
- does not show turing periodic patterning.
mutations are stripe specific.
if turing would kill of all stripes equally.
example of turing model
angel and zebrafish
angelfish juveniles stripes change as they grow.
zebrafish have mosaic like arrangement of three pigment cells
zebra fish pigment cells
interaction
melanophores - black
xanthophores - yellow
iridiophores - shiny
interaction between melanophores and xanthophores are critical to patterning process.
turing model in zebra fish stripes
- activation of melanophors inhiits xanthophores in the local region.
- this results in furhter activation of melanophores in this region.
- inhibition of xanthophores causes decrease of melanophores in distant regions, creating long range inhibitory loop and a stripe.
zebra fish mutant stripes
can produce different mutational phenotypes
jaguar: encodes k-channel
leopard: encodes connexin 4.28 involved in gap junctions.
- phenotypes simulated by turing model
destruction of pigment and turing model zebrafish
zebrafish stripes dont alter as the fish grows
however if you kill pigmentation cells with a laser beam they will regenerate.
- can also be stimualted using turing models.
how are snail patterns generated
by the mantle of snails as it grows.
turing model can generate realistic patterns.
snail shell patterns and turing idea.
idea that cells in the mantle produce pigments in response to standing waves of morphogens and that these waves vary as the animal grows, producing patterns.
snail patterning mechanism
shell patterns are under neurosecretory control
- mechanistic basis of shell pattern formation as a neurosecretory mech of inhibiton and activation.
sensory cells taste the previously laid pigment patterns that are processed by central ganglion and send to mantle netowrk that controls the pigment secreting cells.
turings return?
no strong evidence for turing model in any single system
- any system with activator/inhibitor kinetics = turing process
what properties is the neural net model of snail formation based on
spatial lateral inhibition
delayed temporal inhibition.
- neural field equations describe the local pattern of neuron spiking
describe the neural net model
- local activity of excitory neurons induces activity of inhibitiory interneurons in the surrouding tissue.
what cant he neural net paramter be used for
can look at the parameters for real shells, estimate and map onto a phylogeny
and the parameter values and hence the patterns of extinct ancestors can then be inferred.
