Developmental Biology Exam 2
Parasegements
"transegmental" units that do not affect actual adult segments. They affect the posterior compartment of one segment and the anterior compartment of the immediately posterior segment The expression patterns in the early embryo are delineated by parasegmental boundaries, not by the boundaries of the segments. Parasegment appears to be the fundamental unit of embryonic gene expression
Ex of control through activation of transcription:
-In this case, hunchback is an activator - it binds giant enhancer The giant enhancer has low affinity for hunchback Consequently, giant transcription is activated by hunchback where hunchback concentration is highest
What is stripe placement (pair-rule genes) a result of?
1) The modular cis-regulatory enhancer elements of the pair-rule genes 2) The trans-regulatory gap gene and maternal gene proteins that bind these enhancer sites
What are the two things that dorsal does to create a Dpp activity gradient?
1. It represses the transcription of Dpp in ventral cells 2. It activates the expression of a Dpp-inhibitor: Sog Dpp is expressed in the dorsal region and it is inhibited by Sog, so it does not spread to the ventral side. Sog binds Dpp and Tld at gradient overlap
What are the giants gene two methods of activation?
1. One for its anterior expression band (through Bicoid and hunchback) 2. One for its posterior expression band (by Caudal)
What does Gurken do inside the ovary?
1. The oocyte nucleus travels to anterior dorsal side of oocyte where it localizes gurken mRNA 2. Translated gurken is recieved by torpedo proteins 3a. Torpedo signals causes follicle cells to differentiate to a dorsal morphology 3b. Pipe synthesis is inhibited in dorsal follicle cells 4. Gurken does not diffuse to ventral follicle cells 5a. Ventral follicle cells synthesize Pipe. 5b. Pipe signals sulfates ventral vitelline proteins. 6. Sulfated vitelline membrane proteins bind gastrulation-defective (GD)
What do segment polarity genes do?
1. They maintain segment boundaries 2. Establish cell fate within each segment Pair-rule genes activate segment polarity genes and each segment polarity gene is expressed at identical positions within each segment The en gene is expressed in cells that contain high concentrations of either eve or ftz. The wg gene is expressed in cells that contain high concentrations of slp. sloppy paired (slp) even skipped (eve) fushi tarazu (ftz)
Different enhancers have different affinities for bicoid
1. transcription of a gene may take place if its enhancers can be activated by transcription factors that are present in the nucleus 2. Some transcription factors activate gene expression; others repress gene expression 3. Different enhancers have different affinities for a transcription factor
The discovery of the 8 drosophila Hox genes happened in what year?
1984 Using a homeobox probe, Bill McGinnis, Mike Levine and Ernst Hafen identified eight genes, which turned out to be the eight homeotic genes in flies.
What does gurken do in the developing embryo?
7a. GD cleaves Snake to its active form and forms a complex with snake and uncleaved Easter proteins. 7b. Easter protien is cleaved into its active form 8. Cleaved easter binds to and cleaves Spätzle; activated Spätzle binds to Toll receptor protein. 9. Toll activation activates Tube and Pelle, which phosphorylate the Cactus protein. Cactus is degraded, releasing it from Dorsal, 10. Dorsal protein enters the nucleus and ventralizes the cell
Hox gene expression overlap: What regulates gene expression of hox genes in Drosophila?
Gap genes and pair-rule genes regulate expression of HOx genes in Drosophila
Gurken
A ligand secreted by the oocyte dorsally Follicle cells are competent to receive this signal Follicle cells NOT receiving gurken modify the vitelline membrane (for later use by embryo)
Predict the protein distributions resulting from diffusion after translation of maternal mRNAs. bicoid and nanos mRNAs are translated first and proteins diffuse freely in syncytial embryo.
Answer 1
predict krüppel expression pattern based on the activity of maternal genes and other gap genes. Rules: 1. bicoid activates krüppel transcription. 2. hunchback and knirps repress krüppel transcription at high concentrations. 3. hunchback activates krüppel transcription at low concentrations.
Answer 3 Kruppel expression would be where hunchback and knirps levels are low
How are morphogen gradients translated into a segmented body?
Anterior and posterior gradients activate gene expression patterns that progressively define the fly segments There are gap genes, pair-rule genes, and segment polarity genes
Morphogen gradient
Any substance which concentration varies in a graded fashion, and that causes cells to adopt different fates along this gradient Different cell types are specified at different concentrations (Diffusion- Hi to Lo)
Which of these phenotypes may result from a mutation in a Hox gene?
Arm in place of leg
What type of specification do embryos use?
Autonomous and Conditional specification
What are specific stripes of expression based on?
Based on each gap gene's enhancers and local abundance of other gap genes (which are all transcription factors but may activate or repress other genes)
Why do hh and wg need to maintain each other's expression?
Because pair-rule gene expression is lost after cellularization
Anterior determinant
Bicoid
autonomous specification
Cell fate is specified by determinants that are positioned at specific sites in the cell.
After segment polarity genes are activated by pair-rule genes, what takes place?
Cellularization It takes place at the end of cell cycle 14 It introduces a selective barrier to cell-to-cell communication Instead of a syncytial embryo, it is a cellularized embryo that uses ligands and receptors for cell-to-cell communication (no more morphogen gradient)
What segregates determinants in different cells?
Cleavage It separates the determinants into different cells
In vertebrates: how is segment identity patterned by Hox genes?
Concept of the Hox code: certain combinations of Hox genes specifiy the identity of particular segment/region along the anteriro-posteriro axis So Hox code segment identity is based on combinations
What do each pair-rule genes control?
Controls the development of altering segments Each stripe has its own enhancer region, with different binding sites for transcription factors (gap and maternal proteins)
What happens in the absence of dorsal?
Dorsal identity spreads ventrally in the absence of dorsal This means that Dorsal represses the dorsal fate - how does it do that? The dorsalizing factor -Dpp (decapentalegic), is transcribed in every cell in the absence of dorsal.
Dorsal specifies what side? and Dpp specifies what side?
Dorsal specifies the ventral side Dpp specifies the dorsal side
How are genes arranged in a hierarchy (2)? Based on expression patterns!
Genes expressed over larger domains are assumed to act upstream of genes with smaller domains of expression
How does the dorso-ventral axis start?
Gurken mRNA is produced by oocyte, and localizes to its anterior-dorsal end Gurken mRNA distribution is restricted in the nucleus but after translation, Gurken protein is present dorsally
What act as the master control genes?
Hoc genes They seem to be the master genes of development and act as switches for other genes
Homeotic transformations
Homeosis A body part is transformed into the likeness of another
What do mutations in Hox genes result in?
Homeotic transformations Wild type of the gene antennapedia results in a normal antenna placement Loss of function of antennapedia at T2 results in the legs becoming antenna A gain of function in T2 results in legs taking the place of the antenna Ultrabithorax loss of function mutation in T3 causes an extra pair of wings (4 total wings) Ultrabithorax gain of function mutation causes a phenotype with no wings and two pairs of halteres
Spatial Colinearity
How genes are organized on a chromosome reflect where they are placed in the embryo
Why does hox-gene loss-of-function result in anteriorizing transformations?
Hox genes specify segment identity at the anterior end o f their expression domain If a Hox gene looses its function, the next more anteriorly expressed gene specific the segment where function is lost
Example of ooplasmic determinant: germ cell determinant
In frogs, polar granules are left by the mother at the vegetal pole of the egg Cleavage segregates determinants in different cells
Example of ooplasmic determinant: somatic cell determinant
In snails, isolation experiments have shown that development is regulated by determinants In the image, the isolated trochoblast contained determinants that specified its cell fate (autonomous specification). It still became a ciliated cell
Where are hox genes expressed?
In the segmented embryo In the syncytial embryo: -there are morphogen gradients from the maternal effect genes + gap genes + pair rule genes Then the embryo becomes cellularized when segment-polarity genes become expressed then, the homeotic selector (Hox) genes become expressed. Hox gene mutants have no segmentation defects so they are expressed downstream
Example of control through repression of transcription:
In this case, kruppel is a repressor- it binds giant enhancer. giant transcription is repressed wherever kruppel is present.
What does differences in Hox gene expression patterns correlate with?
It correlates with species-specific patterns of vertebral identity
What does hox gene expression display in drsosophila?
It displays spatial colinearity hox genes are organized in complexes in the fruitfly genome Hox gene organization n the genome matches the order of organization on the fruitfly phenotype Expression is partly overlapped The ordeer i which Hox genes are expressed along the A/P axis corresponds to the order of the genes along the chromosomes But their expression patterns are partly overlapping
What is the result of the antagonism between Dorsal/Sog and Dpp?
It is a precise patterning of cell fates along the dorso-ventral axis. Dorsal specifies the ventral side Dpp specifies the dorsal side
What is the vertebrate dors-ventral axis also patterned by?
It is also patterened by opposing gradients of Dpp and Sog-homologs except that Dpp patterns the dorsal side in insects, vs the ventral side in vertebrates
How is the embryo segmentation pattern progressively laid down?
It is progressively laid down through successive waves of gene expression maternal effect genes initiate gap genes which initiate pair-rule genes, which initiate segment polarity genes, which initiate hox genes ----> results in fruitfly baby with all its segments :)
Bicoid protein is a transcription factor and it has a homeodomain. What does that mean?
It means bicoid has a DNA-binding domain Different concentrations of bicoid protein turn on different genes (transcription factors)
What does the gradient of Toll activation by spatzle lead to?
Leads to the gradient of Dorsal Activity The highest of dorsal activity is ventrally Dorsal gene is outside of the nucleus on the dorsal side Dorsal is in nuclei on the ventral side ON the ventral side, there is an activation of Toll by Spatzle
What happens to hunchback and kruppel if bicoid gradient is decreased overall?
Less hunchback because bicoid activates hb so there would be more kruppel and kruppel would expand to the anterior conclusion: Kr is downstream of hb and bcd
How are genes arranged in a hierarchy (4)? Based on altering expression in mutant embryos!
Mutant gene alters expression of genes acting below it, but not above it. ex: expression of hunchback and kruppel are abnormal in bicoid mutant embryos.
Lecture 9 Summary
Mutant phenotype severity, expression pattern size and timing, as well as changes in expression resulting from particular mutations are all used to arrange genes in a hierarchy. There are 8 pair rule genes that are each expressed in 7 non-overlapping stripes. Loss of a particular pair-rule gene function results in loss of a segment part every other segment. Each stripe of expression is controlled by its own enhancer. Each enhancer contains binding sites for maternal determinants and gap genes, some of which act as activators, while others act as repressors. Particular pair rule gene combinations activate segment polarity genes in each segment. Segment polarity genes need to maintain their own expression after cellularization, which they do by forming positive feedback loops across segment boundaries. Segment polarity gene products - hedgehog and wingless, are ligands that also pattern each segment along its anterior-posterior axis by forming morphogen gradients.
Gap Genes
Mutations in these genes cause "gaps" in Drosophila segmentation. The normal gene products map out the basic subdivisions along the anterior-posterior axis of the embryo. These are the first zygotic genes to be expressed because mutations in these cause gaps in the segmentation pattern These genes encode transcription factors, and differing combinations and concentrations of gap gene proteins regulate the transcription of pair-rule genes, which divide the embryo into periodic units.
Posterior determinant
Nanos
Are segments uniform along the anterior-posterior axis?
No Segments are polarized: externally --Hair points out posteriorly --Some segment subparts are hairy other are not Internally --muscles attach in specific segments subparts -Nerve endings are organized -Respiratory system is also organized in each segment Each segment is polarized by opposing gradients of hedgehog (hh) and wingless (wg)
What happens if there is no bicoid and no hunchback?
No Hb expression No Kr expression And the bcd gradient is not affected in Kr- or Hb- embryos. Conclusion: Kr is downstream of hb, bcd
What activated segment polarity genes?
Pair-rule genes
What cofactors do Hox protein DNA-binding specificity require?
Pbx and Meis (they are also transcription factors) The homeodomain has binding specificity for the sequence ATTA.... a very common sequence.. too common to account for the specificity of Hox gene function When associated with a Pbx protein, the sequence recognized becomes 12 bp long.
What is an example of an activator of Hox gene expression?
Retinoic acid is an activator of hox gene expression Accutane is used to treat acne and it is mostly retinoic acid AN excess of retinoic acid during pregnancy is likely to result in hox gene misexpression since retinoic acid is a teratogen
What mediates the transition from specification to determination in Drosophila?
Segmentation Genes These divide the early embryo into a repeated series of segmental primordia along the anterior-posterior axis. Mutations in these genes result in drosophila embryos that lack specific segments or parts of segments
In vertebrates, what other organisms are made of repetitions of the same segment identity?
Snake bodies A snake axial skeleton is nearly exclusively made of thoracic vertebrae Snakes evolved from tetrapods. The snakes have an expanded thoracic cativity
How are genes arranged in a hierarchy? Based on mutant phenotypes!
So mutations causing larger domains to be missing or are abnormal are assumed to inactivate earlier acting genes ex: Gap genes - delete approx. 7 segments Pair rule genes - delete part of every pair of segments Segment polarity genes - delete part of every segment
What activates stripes of gap genes?
Specific combinations of transcription factor concentration and enhancer affinity activate stripes of gap gene expression
The drosophila embryo starts as a ___________.
Syncytium (many nuclei, one cell) The nuclei start on the outside but they eventually go to the outside of the cell Syncytium helps the morphogen gradient Ex: Bicoid can bind to DNA without going through a cell membrane
Ooplasmic determinants controlling body plan
The body plan is first established by patterning of the body axis and it is set up in the embryo anterior--posterior dorsal--ventral left-right
What is involved in the dorsal-ventral system?
The dorsal-ventral system involves gradients of small molecules that can diffuse between cells Dorsal- and Dpp-activity gradients specify the dorso-ventral axis
Hox genes in the animal kingdom
The homeodomain sequence was used as a probe to discover Hox genes in other animals The more complex an organism is, the more hox genes there are
What does the loss of bicoid result in?
The loss of bicoid results in loss of structures under gradient influence No acron, head, or thorax segments in embryos from bcd -/- mothers
The ventrally modified vitelline membrane signals back to the developing envelope. What does the modified vitelline membrane activate?
The modified vitelline membrane activates Spatzle ventrally Active spatzle binds its receptor Toll, which is evenly distributed on the oocyte membrane. Toll activity is this graded - highest ventrally, lowest dorsally. Activated Toll Activates Dorsal - a transcription factor Dorsal activity turns on the ventral fate
How do we go about understanding the genetic basis of anterior-posterior patterning?
The mutant screen developed by the 1995 Nobel Prize winners for Physiology and Medicine Christiane Nüsslein-Volhard and Eric Wieschaus found mutations affecting segment number andpolarity in Drosophila The mutant phenotype analysis through denticle patterns The bicoid-deficient embryos lack anterior segments (missing head pieces)
Egg chamber
The ovary in which the Drosophila oocyte will develop, containing 15 interconnected nurse cells and a single oocyte. The 15 metabolically active nurse cells make mRNAs and Proteins that are transported into the single cell that will become the oocyte
What were the experiments demonstrating that the bicoid gene encodes the morphogen responsible for head structures
The type of experiment is referred to as the 'find it, lose it, move it" experimentation scheme. In normal development, there is one head and one tail (WT phenotype) In the bicoid dieficient mutant, there is no head and two tails. Researchers injected bicoid mRNA in the middle, and a head developed in the middle with two tails. Inject bicoid at the anterior and posterior end = two heads and no tail (tail structures in the middle)
What is missing in the bicoid mutant?
There is a loss of the anterior structures No head or thoracic segments Only abdominal segments
maternal effect genes
These genes produce messenger RNAs localized to different regions of the egg The mRNAS encode transcriptional and translational regulatory proteins that diffuse through the syncytial blastoderm and activate or repress the expression of certain zygotic genes.
What do different levels of Dorsal activate?
They activate the transcription of different genes which triggers different cell fates There is no dorsal activity on the dorsal side
Hox gene are what kind of transcription factors?
They are homeobox transcription factors Ed Lewis won the Nobel Prize in Physiology and Medicine in 1995 based on his studies of drosophila homeotic mutants, ed lewis proposed in 1978 that all these homeotic genes arose from a common ancestral gene Ed Lewis was kind of the father of modern genetics and discovered hox genes before the genome was sequenced
What are the transcription patterns of the anterior gap genes initiated by?
They are initiated by the different concentrations of the hunchback and bicoid proteins. High levels of Bicoid and Hunchback induce the expression of giant, while Kruppel transcript appears over the region where hunchback begins to decline High levels of hunchback (in the absence of Bicoid) also prevent transcription of the posterior gap genes. Caudal (highest at the posterior end) is thought to activate abdominal gap genes -> Knirps and giant
hunchback and caudal mRNA
They are other maternal effect genes Their protein distributions are shaped by bicoid and nanos bicoid protein represses caudal mRNA translation nanos protein represses hunchback mRNA translation hunchback and caudal mRNA is distributed evenly throughout the embryo, but after translation the protein concentration changes. There is hugher concentrations of hunchback in the anterior and higher concentrations of caudal in the posterior
ooplasmic determinants
They are placed in the egg They are determinants are positioned at specific sites in the cell and it determines cell fate
Anterior-posterior axis determinants are placed in the ___________ as _____________.
They are placed in the oocyte as mRNA and these mRNA packets form protein gradients via diffusion The mRNA is tethered to one end of the egg and when it is translated, diffusion can take place
After the initial gap gene expression patterns have been established by the maternal effect gradients and Hunchback, how are they stabilized and maintained?
They are stabilized and maintained by repressive interactions between the different gap gene products themselves due to the embryo being a syncytium The boundary-forming inhibitions is thought to be directly mediated by the gap gene products because, because all four major gap genes encode for DNA-binding proteins. The repressive interactions create a precise system of overlapping mRNA expression patterns that are maintained by having redundant enhancers
Where are maternal effect genes transcribed? and where are they synthesized?
They are transcribed in ovary cells during oogenesis oocyte develops in the ovaries Maternal mRNAs are not synthesized in the oocyte They are synthesized in neighboring cells - the nurse cells Maternal mRNAs are first uniformly deposited in the oocyte but mRNA transfer from nurse cells to oocyte leads to uniform distribution
What do all drosphila Hox genes encode?
They encode a DNA-binding homeodomain the region of the gene is known as a homeobox and the region of the resulting protein is the homeodomain. The explanation for why it is so conserved across organisms, through hundreds of millions of years of independent evolution, is that its function restricts its evolution. Homeodomains function as DNA binding domains and are found in many transcription factors that control development and cell fate decisions. Many transcription factors have homeodomain - bicoid, even skipped, engrailed The homeodomain is ≈ 60aa The homeobox encoding the homeodomain is ≈ 180bp
What did the genetic screens pioneered by Nüsslein-Volhard and Wieschaus identify?
They identified a hierarchy of genes that: 1) Establish anterior-posterior polarity 2) Divide the embryo into a specific number of segments, each with a different identity The hierarchy is initiated by maternal effect genes that produce messenger RNAs localized to different regions of the egg
What do hedgehog and wingless do?
They polarize each segment and maintain each other's expressions through a positive feedback loop hedgehog and engrailed are expressed at the anterior border of each parasegment wingless is expressed at the posterior border of each parasegment Patched = Hedgehog receptor Frizzled = Wingless receptor This positive feedback loop maintains hh and wg expression and only happens at segment boundary hh- anterior of each segment wg- posterior of each segment Transcription of hh group and wg group are first activated by pair-rule genes and once cellularization takes place, hh and wg engage in a positive feedback loop that maintain each others expression Opposing gradients of hh and wg also polarize each segment
Multiple segments become anteriorized when multiple Hox genes are mutated. Tribolium example?
Tribolium (the beetle) is the second most-used insect model organism after fruitflies mutation is the Abs-A(-/-), Abd-B(-/-) (Double muatant) causes the abdominal segmebts to adopt a thoracic fate! Complete loss of the entir Hox complex creates a mutant where all the segments adopt an antennal fate!
Drosophila Hox gene complexes are conserved in?
Vertebrates
Lecture 9 study Guide
What 4 kinds of data are used to arrange genes in a hierarchy? What does a typical pair rule gene expression pattern look like? What transcription factors activate pair rule gene expression? How is the striped expression pattern of a pair rule gene activated? How is enhancer function tested? How is transcription factor binding site function tested? How does cellularization of the drosophila embryo affect communication from nucleus to nucleus? What are the 2 functions of segment polarity genes? How do segment polarity genes polarize each segment? How do segment polarity genes maintain their own expression? Why is it necessary? How is segment polarity gene expression activated by pair-rule genes? What 2 ligands are encoded by segment polarity genes?
Bicoid
a maternal effect gene that codes for a protein responsible for specifying the anterior end in Drosophila melanogaster It is a transcription factor and it binds enhancers for zygotic genes like the (head gap genes, hunchback, and kruppel) (these zygotic genes are al turned on by bicoid to make segments) Different concentrations of bicoid activate (or repress) different genes
syncytium
a single cell or cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of nuclei. Multiple nuclei in a single cell, where the cell is not divided by a cell membrane
bicoid mRNA and protein localization
bcd mRNA, detected by in situ hybridization, is present in egg. Gradient of bicoid protein, detected by antibody staining, is translated from bcd mRNA after fertilization. Bicoid protein diffuses freely in the syncytial blastoderm stage embryo. localized mRNA (deposited by mom, detected by in situ hybridization) ----> gradient of protein (detected by antibody staining)
How do bicoid and nanos end up at opposite ends?
bicoid and nanos mRNAs are transported to opposite ends of the oocyte along the microtubule cytoskeleton Bicoid mRNA is transported towards the minus end of microtubules by dynein Nanos mRNA is transported towards the plus end of microtubules by kinesin Sequences bind dyenin and kinesin to help move it along
How does bicoid and nanos form gradients?
bicoid and nanos proteins form gradients by diffusing from nucleus to nucelus in the syncytial embryo Morphogen gradients acting in multicellular embryos use ligands and receptors to spread across fields of cells
Spatially unique combinations of what control pair-rule gene expression?
combinations of maternal and gap genes A specific combination of binding sites for transcription activators and repressors controls stripe 2 expression of even skipped
Lecture 7 Summary
determinants placed in the egg are called ooplasmic determinants (ooplasm = cytoplasm of the oocyte). ooplasmic determinants may have different roles for specifying different cell lineages of the embryo - they may specify germ cells, somatic cells, but also the body axes. In many organisms, including drosophila, the anterior-posterior- (AP) and dorsal-ventral (DV) axes of the organism are specified by the ooplasmic determinants deposited by the mother in the egg. In drosophila, both AP and DV axes are specified by morphogen gradients. Maternal mRNAs are placed by the mother in the egg, which are translated at egg activation, followed by diffusion. This results in gradients. Drosophila maternal morphogen gradients are transcription factors that activate or repress target genes at specific concentrations, due to differences in binding affinity for enhancers. bicoid is a maternal determinant that forms a morphogen gradient that specifies anterior structures. Nanos does the same for posterior structures. In the case of maternal effect genes, embryos inherit the genotype of their mother, without any input from the father.
Pair-rule genes
divide the embryo into units of two segments each paired, even-skipped, and odd-skipped The first indication of segmentation in the fly embryo comes when the pair-rule genes are expressed during the nuclear division cycle 13 Transcription patterns of these genes divide the embryo into regions that are precursors of the segmental body plan "zebra stripe" pattern (Hairy, even-skipped, and runt) that use distinct enhancers and regulatory mechanisms for each stripe. It is thought that the different concentrations of gap proteins determine whether or not a pair-rule gene is transcribed. 1. mutation in enhancer can delete a particular stripe and no other 2. if a reporter gene is fused with an enhancer, the reporter gene is only expressed in that particular stripe 3. placement of stripes can be altered by deleting the gap genes that regulate them. The products of the pair-rule genes activate the next level of segmentation genes, the segment polarity genes.
segment polarity genes
establish anterior-posterior gradient within each segment The number of segments is not affected but the direction of the belly hair changes The interaction between cells is mediated by the segment polarity genes They accomplish: 1. they reinforce the parasegmental periodicity established by the earlier transcription factors 2. Through this cell-to-cell signaling, cell fates are established within each parasegment The genes encode proteins that are constituents of the Wnt and Hedgehog signaling pathways. Mutations lead to defects in segmentation and in gene expression patterns across each parasegment
How are genes arranged in a hierarchy (3)? Based on time of expression!
expressed early on means -- the gene is high in hierarchy expressed later on means the gene is lower in the hierarchy
Hox gene misexpression results in?
homeotic transformations in vertebrates Example image: mice
will any of these embryos have a [bicoid -] phenotype?- the bicoid mutation is recessive
mom: +/- dad: +/- Zygotic genotype: (-/-), (+/+), (+/-), (+/-) The answer is no since the mom is +/- and the bicoid (-) phenotype is recessive If the mom was -/-, then all the embryos would have a bicoid phenotype
Predict the maternal mRNA genotype (s) in this offsping?
mom: gene A -/- dad: gene A +/+ zygotic genotype: 100% +/- maternal mRNA genotype: 100% -/-
gap genes
organize cells into groups of segments along the anterior-posterior axis Kruppel, hunchback, and knirps They are activated or repressed by the maternal effect genes and are expressed in one or two broad domains along the anterior-posterior axis. These genes usually show low levels of transcriptional activity across the entire embryo, but consolidate into discrete regions of high activity as the nuclear division continues. hunchback gene - important for establishing the initial gap gene expression pattern (highest activity/levels at the anterior side)
Hox genes
series of genes that controls the differentiation of cells and tissues in an embryo They specify segment identity (aka give each segment its identity)
A specific combination of binding sites for transcription activators and repressors controls?
stripe 2 expression of even skipped To test it experimentally, researchers used a reporter gene construct with Green fluorescent protein (GFP) in a Transgenic fly and saw where the expression of GFP was
Homeotic selecter (Hox) genes specify segment identity along the which axis>
the anterior-posterior axis An illustration of the control Hox genes exert on segment identity is the millipede body plan: -The millipede body is made of the same segment ideneity repeated over and over -hox gene expression is predicted to be the same in each segment Less complex organisms used the same repeated segments but over the course of evolution different hox genes started to be expressed in different segments. There are nest expression patterns of hox genes organized in the genome, and stay in order in the chromosme.
Mutant Phenotypes of maternal effect genes are observed in what progeny?
the progeny of homozygous mutant females The females genotype determines the maternal mRNA genotype If the female has +/-, then the maternal mRNA genotype is 100% +/- If the female is -/-, then maternal mRNA genotype is 100% -/-
Lecture 7 Study Guide
what process segregates ooplasmic determinants in distinct cells? what developmental events are specified by ooplasmic determinants? what axes define the body plan? what maternal effect genes specify the anterior-posterior axis in drosophila? what are they deposited as? How do they give rise to graded distributions? what is a morphogen gradient? how do drosophila maternal morphogens activate the transcription of target genes? what phenotype results from loss of bicoid? nanos? how do parental genotypes relate to oocyte maternal effect gene genotype? what evidence supports the role of bicoid as specifier of anterior structures?
Lecture 8 Summary
• Bicoid and Nanos mRNAs are produced by nurse cells in fly ovaries. Their distribution in the oocyte is first uniform, but this is changed by transport along the microtubule skeleton. • Bicoid mRNA is transported to the anterior side by dynein, while nanos mRNA is transported to the posterior side by kinesin. • The dorso-ventral axis is also patterned by two opposing morphogen gradients: Dorsal and Dpp. • The ventral activity of Dorsal results from a chain of events that starts in the ovary with the dorsal secretion of Gurken. Gurken binds its receptor on dorsal follicle cells, which will later (during embryo development) not promote spatzle activity. • In contrast, Spatzle activity is promoted ventrally (Gurken absent): it binds its receptor (Toll), leading to the activation of Dorsal in ventral cells of the embryo. Active Dorsal moves into the nucleus, and activates expression of certain genes, based on its concentration. • On the dorsal side, a gradient of Dpp specifies dorsal identities. The Dpp gradient is regulated in 2 ways by Dorsal: Dorsal represses Dpp transcription, and Dorsal activates the expression of a Dpp activity inhibitor - Sog. • Once maternal determinants (bicoid, nanos, hunchback, caudal) have adopted their graded distributions, they activate stripes of Gap gene expression along the anterior-posterior axis. • Expression of a specific Gap gene may be promoted (or repressed) by maternal determinants or products of other Gap genes, based on its enhancers and their affinity for any of these transcription factors.
Lecture 10 Summary
• Hox genes specify segment identity along the anterior posterior axis. • Hox gene loss- or gain-of-function mutations result in homeosis (=homeotic transformations). • Homeosis is a change of identity: one body part is transformed into another body part. • Hox genes are organized in clusters in the genome, and they display spatial colinearity in drosophila. • In drosophila, Hox gene expression is activated by Gap genes and Pair-rule genes. • Hox gene loss of function results in anteriorizing homeotic transformations. • Vertebrates have more Hox genes than insects - each body segment is specified by a specific combination of Hox gene expression. • Loss- or gain- of Hox gene function also results in homeosis in vertebrates. • Hox genes specify segment activity by regulating the transcription of many genes - they act as master switches of gene expression.
Lecture 10 Study Guide
• What is the function of a Hox gene? • What kind of phenotype results from loss or gain of Hox gene function? • What is spatial colinearity? • Why does loss of Hox gene function result in anteriorization? • What might the body of an ancestral animal with a single Hox gene in its genome look like? • How is the genomic organization of Hox genes different between drosophila and tetrapod vertebrates? • What is the Hox code? • What is spatio-temporal colinearity? • Give an example of homeotic transformation arising from loss of Hox gene function in mice.
Lecture 8 Study Guide
• Where are bicoid and nanos mRNAs produced? How do they endup packed at the anterior and posterior ends of the oocyte? • What 2 opposing gradients specify the dorso-ventral axis? • How is the gradient of dorsal activity established? Make sure you distinguish what happens in the ovary from what happens later during development of the embryo. • What is the role of each of these factors in establishing the dorsal activity gradient: Gurken, the vitelline membrane, Spatzle, Toll. • How does Dorsal shape the Dpp gradient? • How do bicoid and nanos proteins shape the distribution of other maternal mRNAs (hunchback and caudal)? • How do maternal determinants shape the expression patterns of Gap genes? • How does the affinity of an enhancer for a particular transcription factor influence whether a gene is transcribed or not? Illustrate your answer with high vs low concentrations of transcription factor.