Week 8 Flashcards
Epistasis
Epistatic interactions are assyed by comparing the phenotype of a double mutant organism with that of the singly mutant organism
Epistasis criteria for the two mutations
Have related phenotypes
- growth control
- sex determination
- dorsal ventral axis determination
Work on a pathway that makes a distinct decision
- growth/non-growth
- male/female
- expression/non-expression
The two mutation have distinct/opposite phenotypes
- all males versus all females
- expression always ON versus always OFF
- Ventrilized versus dorsalized
Reproduction and cell phenotypes
Sperm fertilizes an egg forming the zygote and zygotic genome. The cells divide, during early division the maternal information coming form the maternal genome is important.
zygote starts out as totipotent but as the cells divide they become differentiated into specific cell types
Order of differentiation
Pluripotent cell undergoes determination to become a determined cell the determined cell will differentiate into a differentiated cell with a distinct identity (has an identity now we can excute to give rise to a cell with the identity)
Body plan
Anterior posterior axis
the body plan is where cells will end up in the fully developed organisms
the cells are being assigned identities
this nucleus divides
The nucleus divides
This gives rise to a single layer of cells called the blastoderm (5,000 cells) monolayer of periphery cells.
at this stage the cells know exactly what they are going to become
the anterior posterior axis has already been determined
Number of genes involved in the body plan
120 genes associated with the determination of the anterior posterior axis is the segmentation of the body plan
Genetic hierarchy regulating determination of the A-P
Maternal coordinate genes (bicoid and nanos)
Zygotic Gap genes (hunchback)
Pair rule genes (fushi tarazu)
Segment polarity genes
Homeotic selector genes antennapedia
Maternal coordinate genes
determines the development of the axis
Zygotic gap genes
regulates the expression of pair ruled genes
Pair rule genes
determine the number of segments that will form
Segment polarity genes
patterns the axis within each segment
Homeotic selector gene
determine what the segments will become
Maternal to zygotic genome (humans)
transition form a developmental program being run off of information provided by the maternal genome and placed in the egg to the information that’s going to be expressed from the zygotic genome
transition occurs at the first cleavage cycles
Maternal to zygotic genome (drosophila)
transition occurs at the blastoderm stage
Maternal effect screens
Create larvae that are homozygous for a DNA seuqence change and ask whether the DNA seuqence change affects the development of the zygote.
in a maternal screen establish a homozygous mother who cannot pass down info to her progeny
Generation timing of maternal effect screen vs zygotic screen
Maternal screen: F4 embryo/ F3 homozygous mother
Zygotic screen: F3 dead embryo
bcd/bcd mother vs bcd/bcd (lf)
bcd/bcd mother normal egg
bcd/bcd (lf) larva missing a head
Body plan and the anterior posterior axis
coordinate genes determine the basis axis of the head.
bicoid mRNA is located at the future anterior end of the egg, nanos is at the posterior end
Nanos located in the posterior end
Nanos is required to surpress HB protein expression.
HB is present in the anterior end
nanos (lf) results in HB being expressed in both axis: development of larvae lacking abdominal segments
Nanos located in the posterior end
Nanos is required to surpress HB protein expression.
HB is present in the anterior end
nanos (lf) results in HB being expressed in both axis: development of larvae lacking abdominal segments
Bicoid transcription factor activates zygotic HB expression
Encodes a transcription factor that binds to DNA binding sites of the hunchback protein
Dual regulation
Bicoid increases HB expression in the anterior portion of the cell by activating zygotic expression.
Nanos surpresses the expression of hunchback form a messenger RNA deposited by the mother.
Gap genes
first genes zygotically expressed
maternal affect gene determine the axis. They interact with gap genes that are expressed in broad domains of the cell
they determine other large developmental domains of the body plan, they tell the cells to become a specific segment
Gap genes (mutation)
Lf mutations in gap genes results in deletion of segments
In gap lf mutants, contiguous set of segments
Gap genes….
Encode transcription factors defining broad regions of the body plan, this expression pattern defines large domains along the anterior-posterior axis.
Gap TFs regulates the expression of other gap genes and the expression of pair rule genes
Kruppel and Knirps
Knirps regulates expression of Kruppel
Kruppel and Knirps
Knirps regulates expression of Kruppel
Pair-rule genes
segment the body plan expressed from the zygotic genome encode transcription factors pair-rule proteins define the segments of the body plan very specific pattern of expression
Pair rule gene (lf)
portion of every other segment is missing.
alternative deletion of segments. the segments deleted are the segments the pair-rule gene is expressed
regulate pair rule genes and segment polarity genes
Segment Polarity genes
pattern the segments
expressed from the zygotic genome
expressed in every segment
segment polarity proteins pattern the structure of every segment
segment polarity genes (lf)
every segment is present and every segment is defective. anterioir posterior polarity within a segmnet is often affected
parts of the segment are deleted and duplicated
Why do the segments look different from one another
homeosis: the transformation of one body part into the likeness of another
homeotic selector gene
Antennapedia
a switch for antenna versus leg development
WT: antennapedia is expressed in leg primordia
Antp(lf): no antennapedia resulting in legs becoming antenna
Antp(gf): antenna results in antenna becoming legs
This tells us that the atennapedia gene encodes a factor important for leg development but is not important to tell other cells to become a leg.
Proboscipedia
homeotic selector genes are switch genes, Pb lf results in the transformation of the feeding tube into a pair of tarci, no proboscic and no supression of development of tarsus
The origin of animals and relationship between animal groups
look for evidence in the fossil record
porifera: sponges
sponge genome is the second closest to the most recent common ancestor
spongest are the base group of animals
Hypothesis
what the most recent common ancestor looked like and what machinery did this ancient animal have that allowed the diversification into all these animal gorups
Animal phylogeny
Placozoa (two layers of cells)
difficult to group organism based on morphology
bilaterians: symmetric; head and tail; axis of symmetry
more related to starfish
Common ancestor of bilaterians
Urbilateria
had all the genetic machinery we consider in terms of the body plan; axis and organs
a complex organism existed at the proteosome deutorostome split
complex machinery of developmental pathways back 650,000,000 years ago. it had the complex machinery to set up a central nerovus system, segmentation, regionlized gut, primitive heart and eyes, as well as body out growths
Experimental/Observational approach of evo/devo
conservation of structure: similar amino acid sequence
conservation of expression: genes of different organisms expressed similarly
conservation of requirement: are the different genes required for similar things
conservation of function (functional equivalence): can i take one gene in one organism and replace it in another and have that work
Conservation of sequence (eyeless and pax6)
eyeless proteins in drosophila are very similar in sequence to the Pax6 protein in vertebrates
Conservation of expression (eyeless and Pax6)
expression of eyeless and pax6 occurs in group of cells that give rise to the eye
phyogenetic comparison of expression
expressed in the tissues that give rise to an array of photoreceptors
Conservation of requirement
Eyeless is required for drosophila eye development
800 facet with photoreceptor
compund eye is reduced in drosophila with no eyeless gene
Pax6 is required for mouse eye development; homozygous knockout leads to no eye development
Aniridia
Mutantions in human Pax6 result it aniridia; lack of an iris when heterozygous
the functional allele shows haploinsufficiency resulting in a missing iris.
Conservation of function (functional equivalence)
take the eyeless gene and place it in a vertebrate to see if there is the formation of the vertebrat eye; express eyeless gene in other parts of the body
UAS GAL4 ectopic expression system
Take a yeast transcription factor and placed it in drosophila, seperate gal4 expression from the uasgal4 which is this cis element upon which gal4 will bind to activate transcription of galactose gene.
we get extopic expression only when when we bring them together
Binary or two component system
Fly stock A: UAS line; GAL4 bindging sites UASGAL4 next to YFG
Fly stock B: Driver line; enhancer and GAL4 gene that expresses the GAL4 activation/DNA binding domain proteins
mate the flies togehter in cells of the progeny where both genes are expressed you get ectopic expression of your gene
Why a 2 component system
The ectopic expression of the gene may be lethal so are able to establish the UAS stock first with no expression
Expression occurs only after we have crossed the UAS line with GAL4 driver line
Ectopic expression of eyeless
Induces ectopic eye formation
expression of eyeless in cells other than the eye
wherever gal4 is expressed we see the formation of eye tissue
Expressing Pax6
Create a UAS that contains the UAS dused to mouse Pax6
induction of ectopic eyes
Pax6 from mice and eyeless can both induce ectopic eye formation in drosophila indicating the mouse pax6 is functionally equivalent to drosophila eyeless function
Urbiletaria
experiments conclude that urbilaterian had a complicated set of geentic pathways that controlled the development of the primitive eye, heart structures, cns, segmentation, body wall out growth, body plan and throughoutput cut
this common ancestor radiated through divergence to create all of the biletarian organism
