Developmental Biology Exam 2 Flashcards
Primary sex determination
gonads: testees or ovaries
genetics
Secondary sex determiantion
Phenotype- male vs female internal and external organs
Hormones and paracrine factors from the gonads
Bipotential gonads
genital ridge: gonad rudiment will become gentile duct
bipotent: can become ovaries and testees
Have indifferent development takes place followed by differentiation into either testees or ovaries (based on XX and XY genotype
Have both Mullerian and Wolffian ducts
Hormones made by genotype will fully generate one duct and disintegrate the other
How genitals are affected with XY genotype
Gonads become the testees
Wolffian duct differentiates into sperm transport duct
Mullerian duct is degenerated
How genitals are affected with XX genotype
Gonads become ovaries
Mullerian duct becomes oviduct which become the fallopian tubes
Wolffian duct is degenerated
Primary Sex determination for XY
Gene SRY promotes testis formation
SRY is a transcription factor which binds enhancer region of Sox9 gene. Sox9 is then expressed by activating itself via SRY
Sox9 also activates anti-Mullerian hormone (AMH) and promotes degradation of beta-catenin.
Primary Sex determination for XX
no SRY gene present
wnt4 paracrine factor leads to wnt/beta-catenin signaling and target gene expression
wnt4 leads to beta-catenin stabilization
Results in expression of ovary differentiation genes
Beta-catenin promotes maintenance of ovarian structures and blocks expression of Sox9
Secondary sex determination female
Bipotential gonad leads to wnt4 factor leads to ovary leads to granula cells and thecal cells leads to follicles leads to estrogen leads to differentiation of Mullerian duct leads to female sex phenotype
In the absence of testosterone the Wolffian duct regresses
Secondary Sex determination male
Bipotential gonad leads to SRY, Sox9 leads to testees leads to:
1. Sertoli cells which make AMH which leads to regression of Mullerian duct
AND
2. Leydig cells which make testosterone which causes differentiation of Wolffian duct which causes male sex phenotype
Germ Plasma theory (1892)
Weismann proposed
Some germ cells contain heritable information and somatic cells carry out ordinary body functions
germ plasm, which is independent from all other cells of the body (somatoplasm), is the essential element of germ cells (eggs and sperm) and is the hereditary material that is passed from generation to generation.
Background of Test of Germ Plasma Theory (1910)
Done by Theodore Boveri
Used parascaris aequorum which is a round worm parasite with chunky chromosomes you can see under a microscope
Have two chromosomes per haploid cell
Cleavage of first embryonic development separates animal half from vegetal half of the zygote.
Animal chromosomes during first two blastomeres: chromosome diminution
vegetal blastomere: chromosomes remain normal
During second cleavage animal cells split meridionally and the vegetal cell divides equatorially. Both vegetal cells created have normal chromosomes
Therefore at the fourth cleavage only one cell (vegetal) will have a full set of genes
At the 16th cell stage there will only be two cells with undiminished chromosomes. One of them becomes a germ cell and the other does chromosomes diminution to form stomatic cell
Chromosome diminution
When chromosome blastomere ends fragment before cell division
Only portion of original chromosome survives
Genes are lost and are therefore not present in new nuclei
Meridional division
a type of cell division that occurs during the early stages of embryonic development when a furrow cuts through the center of an egg, bisecting both poles
Equatorial division
is a term used to describe the process of cell division where chromosomes are divided equally into two daughter cells during metaphase of mitosis or meiosis:
Test of Germ Plasma Theory (1910)
Boveri set out to study the part of the cell plasma that caused the chromosomes to not diminish and see if this so called section of the cytoplasm existed
He centrifuged the eggs before their first cleavage to shift orientation of mitotic spindle. Therefore each cell formed should have portion of the vegetal portion
After first division no nucleus underwent chromosomal diminution. Only the animal ones for the second division underwent chromosomal diminution.
He concluded the vegetal cytoplasm contains a factor that protects nuclei from chromosomal diminution and determines germ cells
Primordial germ cells (PGCs)
Germ cells (aka. germ line) = cells that make gametes. Totipotent
Germ cells derived from PGCs. Can become either sperm or eggs. Therefore PGCs are bipotent
Where do PGCs come from?
In mammals they come from the ICM
ICM made primitive endoderm. epiblast are all cells of the embryo which also make PGCs
On P side becomes PGCs and are more likely to be mesoderm but signals tell them they are going to be germ cells
Therefore induction occurs
Migrate to gentile ridge and during this migration the PGCs also proliferate
Mammalian spermatogenesis
- Proliferation
- Meiosis
- Differentiation
Spermatogenesis: Proliferative Phase
- primordial germ cells (PGCs) migrate to the gentile ridge. While migrating they become gonocytes. Once at destination they become seminiferous tubules
- Contact with the seminiferous tubules leads to gonocyte differentiation into spermatogonial stem cells (SSCs). SSCs are type A spermatogonia.
The type A spermatogonia (stem cells) will either not divide, will divide to make asymmetric cell or through mitosis will make type B spermatogonia which will differentiate and mitosis to have two primary spermatocytes
sertoli cells
men mesodermal cells differentiate into these
Their role is to secrete AMH
Will also form seminiferous tubules
during week eight they surround germ cells to make testis chords
Testis chords
Form loops in central region of developing testis and are connected by thin canals called rete testis near developing kidney duct
What happens when male germ cells enter the gonads?
They develop within the testis chord, proliferate and then arrest in mitosis
When puberty hits, the testis cords develop into seminiferous tubules. The germ cells migrate to the periphery of the tubules to make SSC’s
Two directions men mesodermal cells can go?
Sertoli cells (epithelial) or Leydig cells (Mesenchymal)
Leydig cells
Secrete testosterone
What does fully developed testis have?
epithelial tubules of Sertoli cells surrounding germ cells and a mesenchymal cell population that secrete testosterone
To protect the testis each is surrounded by thick ECM which is called the tunica albuginea
XX fetus
Germ cells in gonad are organized in clusters surrounded by pre-granulosa cells
Germ cells enter meiosis
When XX is birthed, the pre-granulosa cells all degenerate only leaving the ones at the cortex of the gonad left
Each germ cell is surrounded by pre-granulosa cells
Germ cells will become oocytes
pre-granulosa cells will become granulosa cells. The rest of the mesenchymal cells will become thecal cells which forms follicles with granulosa cells.
The follicles envelop the oocyte and secrete steroid hormones such as estrogens and during pregnancy progesterone.
Germ cells and somatic cells of gonad
Germ cells are biopotential but are told what to do when they are in either male or female sex chords
They are told to begin mitosis and become eggs or arrest in mitosis and become spermatogonia (sperm stem cells)
Importance of germ cells in XX and XY
XX: the follicle cells would degenerate without germ cells
XY: The germ cells help support the differentiation of Sertoli cells but are not required for the maintenance of testis structure
Steps of spermogenesis
- Proliferative phase where sperm stem cells spermatogonia increase by mitosis
- Meiotic phase involves two divisions to create a haploid state
- postmeiotic phase called spermiogenesis during which round cells (spermatids) eject most of their cytoplasm and become the streamlines sperm
Proliferative phase of spermogenesis
PGCs arrive at genital ridge
Gonocytes are in sex chords that will be seminiferous tubules
Gonocytes become undifferentiated spermatogonia near the basal end of the tubular cells. Are then true stem cells
Spermatogonia reside in stem cell niches at the junction of Sertoli cells, the Leydig cells and the testicular blood vessels
Adhesion molecules join the spermatogonia directly to the Sertoli cells which will nourish the developing sperm
Mitotic proliferation of stem cells in XY
Produces type A spermatogonia which is held together by fragile cytoplasmic bridges
Glial derived neurotrophic factor (GDNF) which is secreted from Sertoli cells keeps the stem cells in mitosis
BMPs and Wnts can induce type A spermatogonia to differentiate into further sperm
Developmental process of sperm
Undifferentiated spermatogonia goes through differentiation to make differentiating spermatogonia. These cells initiate meiosis which makes spermocytes which undergo second meiosis. Then the cells undergo spermiogenesis to become spermatids
Type B spermatogonia
precursors of the spermocytes and contain high levels of Stra8
Spermocytes
last cells to undergo mitosis and divide out to generate primary spermatocytes which enter meiosis
primary spermatocytes
Undergoes meiotic division to male a pair of haploid secondary spermatocytes which complete the second division of meiosis
Spermatids
Formed after secondary spermatocytes complete a second division of meiosis
Are connected together by cytoplasmic bridges
Are haploid cells but since they are connected together they can diffuse into the cytoplasm of their neighbors and function as a diploid
As the divisions happen the cells go from the seminiferous tubule to its lumen
Oogenic meiosis
Primary oocyte goes through unequal cytokinesis after telophase I to generate second degree oocyte and a polar body
The secondary oocyte goes through another unequal cytokinesis after telophase II which gives mature ovum (egg) and another (2nd) polar body
Oogenesis
- PGCs (gonocytes) proliferate to make oogonia before birth
- In gentile ridge, oogonia associate with somatic cells to form follicles. One forms one follicle
- Surviving oogonia in a follicle enter meiosis I before birth. Become primary oocytes
- Primary oocytes pause in diplotene of prophase I
- Resumed at sexual maturity. ~12-40 years will be resumed meiosis in response to luteinizing hormone (LH) who is co-secreted along with follicle-stimulating hormone by the gonadotrophin cells in the adenohypophysis (anterior pituitary).
- Meiosis pauses again at meiosis II until fertilization
Dictyate resting phase
Meiosis arrested at first meiotic prophase and reinitiated in a smaller population of cells
Resumes during puberty
Retinoic acid (RA)
Determines timing of meiosis and sexual differentiation of mammalian germ cells
How Retinoic acid (RA) affects XX
RA activates Stra8 which is a txn factor what causes initiation of meiosis
After meiosis female germ cell fate/female differentiation occurs in the gentile ridge
How Retinoic acid (RA) affects XY
Cyp26b1 causes degradation of an acid which causes RA to not work
Nanos (late) degrades Stra8
These two factors causes meiosis to not occur which causes male fate and delayed meiosis
Fertilization goals
- Sexual reproduction. Genetic information from parents to offspring
- Initiate the development of the egg metabolism (maturation)
What happens during fertilization?
Sperm and egg contact and recognition
Regulation of sperm entry
Fusion of genetic material
Activation of the egg metabolism
Anatomy of sperm
From flagella to acrosomal vesicle:
Axoneme (made of tubulin and mitochondria), Mitochondria, Centriole, Nucleus (haploid), Cell membrane, Acrosomal vesicle (derived from Golgi; contains digestive enzymes)
Egg (ovum) Anatomy
Female pronucleus, Plasms membrane, vitelline envelope (outer jelly layer which is ECM around the egg; is important for sperm and egg recognition. Called Zona Pellucida in mammals), Jelly coat
What will ovulated eggs have before fertilization?
Cumulus, polar body, ovum and zona pellucida
Spermatogenesis
Happens when organism reaches maturity
Long flagellum develops
Mitochondria accumulated to power the swimming
Nucleus condenses
Cytoplasm is released
Acrosomal vesicle forms
Vitelline envelope, Zona Pellucida
ECM surrounds the egg
Composed of glycoproteins
Important for sperm/egg recognition
Recognition of egg and sperm
- Chemoattraction of the sperm is attracted to the egg. Due to soluble molecules secreted by the egg
- Binding of the sperm to the ECM (zona pellucida) of the egg
- Exocytosis of the sperm acrosomal vesicle and release of its enzymes
- Passage of the sperm through the ECM to the egg cell membrane
- Fusion of the egg and sperm cell membranes
What happens after the egg and sperm recognize each other?
The haploid sperm and egg nuclei can meet and the reactions that initiate development can begin
Sperm translocation (mammalian)
- Sperm motility- Use flagella
- Sperm rheotaxis- movement in response to current. Environment works against them because the outward flow comes down fallopian tube to allow the egg to leave the tube to get into the uterus. Sperm responds to this directional cues and orients itself against outward flow
- Uterine muscle contractions help the sperm move to its destination
Where do egg and sperm mostly meet?
In the ampulla of the fallopian tubes.
The acrosome reaction
When sperm and egg meet
Sperm binds zona pellucida (ZP) proteins. ZP2 responsible for human sperm to bind to oocytes. ZP2 binds bindin.
1. The acrosomal membrane fuses with eggs membrane.
2. enzymes from inside sperm membrane are released to reveal bindin. Bindin is an egg binding protein
Are species specific polysaccharides in the egg jelly that bind receptors on the sperm membrane. Acrosomal membrane fuses and contents are released to reveal bindin.
What happens after sperm-egg binding?
Izumo protein is displayed on sperm membrane
Izumo binds to Juno/CD9 protein on egg membrane -> Initiates membrane fusion of the two sperm and initiates sperm entry
How is sperm entry regulated? How to prevent polyspermy?
Fast block and Slow block
Polyspermy
Entry of more than one sperm
Ex. Fert. by 2 sperm = 3x DNA = triploid nucleus 3N = get multiple mitotic spindles
Each sperm brings one centriole = after duplication is 4 centrioles
With two mitotic spindles, cell tries to divide 3N genetic material into 4 cells
Fast block (Urchins, frogs and amphibians; not mammals)
Change in electrical potential inside the oocyte. Immediately after sperm entry. Sperm will not want to bind to a positive membrane potential of the egg
Egg is 70 mV which is -70 mV because the inside of the egg is negatively charged in relation to the exterior
Chemicals from the fusing sperm cytoplasm alter the sodium ion channels.
Within 1-3 seconds after binding the membrane potential shifts to a positive one of +20 mV and therefore the sperm will not fuse to a positive membrane potential.
Slow Block
aka cortical granule reaction (urchins and most mammals)
Mechanical removal of sperm
Enzymes are released by cortical granules and modify (cut off) ZP proteins. Enzymes are released when sperm binds egg. They are released between the cell membrane and the fibrous mat of vitelline envelope proteins. Enzyme called cortical granule serine protease.
Egg proteins (membrane receptors. Juno/CD9) are cut off as well
Therefore sperm no longer binds
Happens about one minute after sperm-egg fusion
What elevates the fertilization envelope from the cell during slow block?
glycosaminoglycans which are released by cortical granules
They absorb water to make more space between the cell membrane and the fertilization envelope
How is the fertilization envelope stabilized during slow block?
Crosslinking adjacent proteins through egg-specific peroxidase enzymes and a transglutaminase released from the cortical granules
This crosslinking allows resistance of shear forces of the oceans waves
As this is happening a fourth set of cortical granule proteins, including hyalin, forms a coating around the egg. The egg then extends elongated microvilli whose tips attach to the hyaline layer, which provides support for the blastomeres during cleavage
What activates cortical granule reaction?
Calcium ions
Upon fertilization the [Ca2+] increases
Cortical granule membranes fuse with egg cell membrane, releasing its contents. Starts at point of sperm entry.
A wave of cortical granules exocytosis goes around the entire egg to the other side
Rise in Ca2+ is which causes cortical granule reaction is not from the rise in Ca2+ but from the endoplasmic reticulum of the egg
What happens as cortical granules undergo exocytosis?
They release cortical granule serine protease (CGSP) which cleaves proteins linking to the vitelline envelope to the CM
Glycosaminoglycans form osmotic gradient, making space between the vitelline envelope and the CM
Enzyme Udx1 catalyzes formation of hydrogen peroxide (H2O2) which is a substrate for soluble ovoperoxidase (OVOP). OVOP and transglutaminases (TG) harden the vitelline envelope, now called the fertilization envelope
What happens after the egg is activated?
Effects of cytoplasmic calcium wave:
-Causes it to finish meiosis II (urchins already did meiosis II)
- Cortical granule release
- Translating maternal mRNA
Glucose is used to give energy for biosynthesis
TCA produces ATP + NADH + FADH –> e- carriers needed for metabolic rxns
Events of/after fertilization (Early Events, 1st minute)
Sperm-egg binding
Fertilization potential rise (fast block to polyspermy)
Sper-egg membrane fusion
Calcium increase first detected
Cortical granule exocytosis (slow block to polyspermy)
Events of/after fertilization (Late Events, 1-5 minutes later)
Activation of NAD kinase
Increase in NADP+ and NADPH
Increase in O2 consumption
Sperm entry
Acid efflux
Increased in pH (remains high)
Events of/after fertilization (Later Events, from 5 minutes after until the first cell division)
Sperm chromatin decondensation
Sperm nucleus migration to egg center
Egg nucleus migration to sperm nucleus
Activation of protein synthesis
Activation of amino acid transport
Initiation of DNA synthesis (replication and txn occurs)
Mitosis and cytokinesis
First cleavage
What happens after cytoplasmic Ca2+ wave?
Resumption of meiosis (except in urchins and cnidarians
Cortical granule release
NAD Kinase activation
How does metabolism work?
Glucose –glycolysis–> pyruvate –oxygen–> oxidative phosphorylation in mitochondrion to make ATP
Therefore with energy you get biosynthesis
What does the NAD kinase do after it is activated by Ca2+ release?
Converts NAD+ to NADp+ + NADP
What does Calcium release do?
Activation of NADK for lipid biosynthesis. From NAD+ to NADp+ and NADPH
Causes cortical granule exocytosis which causes slow block to polyspermy
Sperm nucleus entry (1-5 min)
Sperm nucleus enters the egg cytoplasm
pH increase in egg cytoplasm
Oocyte molecules decondense the sperm chromatin (release protamine’s) There will be no transcription from sperm DNA if you do not do this
Egg and sperm pronuclei migrate toward one another
Activation of protien-synthesis. Not from newly made genetic material, but material already in the cell (maternal mRNA in egg before fert.)
Maternally contributed mRNAs
Histones
Cytoskeleton proteins
Cell cycle regulators
Transcription factors
Body patterning proteins
Translational regulation in oocytes (“unmasking”)
mRNA gets masked so they don’t get degraded
1. BEFORE FERTILIZATION!!!! Maskin (inhibitory effect) links 5’ cap and 3’ cap end (via protein cytoplasmic polyadenylation element binding protein (CPEB)) at the short end of the poly A tail of mRNA in a repressive loop
2. FERTILIZATION!!!!! (or ovulation) activates a kinase that P’s CPEB to cause a change in CPEB shape which allows it to bind to other 3’ UTR binding proteins
3. Maskin is displaced and translation occurs
Ways to see if transcription has occurred after fertilization
Three radioactive tracers!!
Thymidine –> Can see if new DNA is being formed
Uracil –> Can see if new RNA is being transcribed
Sulfur/A.A. –> Can see if new protein is being made
What does actinomycin do the sea urchin zygotes?
It is an inhibitor and inhibits txn.
Initial burst of translation is from maternal mRNAs
Cleavage
Period of rapid (not rapid in mammals) cell division up until “mid-blastula transition” (MBT)
Goals of cleavage
Increase in number of cells
Make smaller cells for organism
decrease cell size
How can it be achieved to decrease cell size during cleavage?
Reduce or skip G1 and G2 phases
Faster than normal S phase/DNA rep.
How does the cell reduce or skip G1 and G2 phases?
Produce mitosis-promoting factor (MPF) which is made of cyclin B and cyclin dependent kinase (CDK)
Cyclin expression oscillates and cells enter mitosis when levels are high
During first cleavage MPF is regulated by maternal cytoplasmic factors such that it is constantly being made which leads to faster cell cycles/mitosies
How does the cell get faster S phase/DNA replication during cleavage?
More origins of replication than normal
More DNA pol. made
Maternal gene transcription
During diplotene (mammals pause 1st time), maternal RNAs are transcribed but not translated
When meiosis resumes, germinal vesicle breaks down, maternal RNAs enter cytoplasm. Maternal RNAs get distributed to different cells and not all get translated. Start to establish different differentiation
What happens in early development
Rapid synchronous cleavage
maternal mRNAs are deposited into different cells
activated maternal affect genes
Mid-Blastula transition (MBT)
maternal mRNAs are depleted
rate of mitosis slows down
activation of zygotic gene transcription
cellularization (in drosophila)
What happens to form the cellular blastoderm?
syncytium –> syncytium blastoderm –> cellular blastoderm
Nuclear elongation –> cellularization
occurs after cycle 13
coordinated by cytoskeleton (actin, myosin and microtubules)
syncytium blastoderm
all the cleavage nuclei are contained within a common cytoplasm. No cell membranes exist other than that of the egg itself.
Role of actin and myosin in blastoderm formation
pulls in membrane to pinch it off like cytokinesis
What does gastrulation lead to?
Segmentation
Anterior vs. Posterior
A/P axis during oogenesis
progressive developing structures/follicle
progressive movement goes top to bottom with bottom being more developed (at the oviduct) Top of ovariole is less developed
How is A/P polarity established in the oocyte?
Gurken is transcribed in nurse cells
Gurken mRNA is moved to oocyte and translated
At positive end of egg chamber, gurken signal binds to torpedo receptors on posterior of follicle cells (become posterior end where follicles are)
Posterior follicle cells send a singla back to oocyte which activates protein kinase A which recruits Par-1 protein to posterior edge of oocyte cytoplasm
Par-1 organizes MTs in a specific way with their negative ends toward anterior and positive ends (growing ends) towards the posterior
Positive end motor kinesin transports Osker (nanos is associated with this protein) protein (recruits more Par-1) to the posterior
Negative end dynein transports bicoid to the anterior of the oocyte
Maternal effect genes and A/P pattern formation
mRNAs placed in different regions of the syncytial egg cytoplasm –> they will regulate transcription and translation of zygote genes which will establish A/P and help divide embryo into segments
Specifying identity steps in flies
specification- gradients of maternal effect proteins
determination- established by formation of segments
Differentiation
All is controlled by segmentation genes
A/P pattern formation steps
Maternal affect genes to establish A/P polarity
Activate GAP genes (txn factors)
Activates Pair-Rule genes (txn factors)
Activates segment-polarity genes (Not txn factors. Are signal molecules) Are determined at their point b/c segments are established
Homeotic genes (txn factors) going to control fate of segment
bicoid is located where?
The anterior, negative side
head side
nanos is located where?
The posterior, positive side
tail side
What do the protein gradients regarding A/P pattern formation?
gradients of proteins acts a morphogen
concentration dependent
Establish by diffusion of proteins
What happens in bicoid loss of function mutants?
No head structure is formed, two sets of tails is formed
What happens in bicoid misexpression mutants?
New head regions/ new anterior regions in a concentration dependent manner
Can Bicoid and Nanos regulate other maternal mRNAs?
Yes!
caudal mRNA translation is inhibited by expression of bicoid
hunchback mRNA translation is inhibited by expression of nanos protein
Four maternal proteins of developing drosophila
Nanos- helps with embryos development. Translation inhibitor through RNA binding
Bicoid, Hunchback, and Caudal are transcription factors regulate zygotic gene transcription
Gap genes (example)
transcription factor, expression pattern determined by maternal proteins and repressive interactions with other gap gene proteins
Ex’s.
Kruppel
Mutant would miss large region of body resulting in a gap
Hunchback and Knirps
Kruppel and Giant inhibit each others expression. Makes mountains in a graph where ones peak is the others rock bottom then they swap
Love triangle of bicoid, hunchback and knirps
Caudal causes knirps
Bicoid activates both hunchback and knirps
Hunchback inhibits knirps
Knirps inhibits hunchback
Hunchback is victor of the inhibition
Pair-rule genes (transcription factors)
pair of them are expressed in non-overlapping segments
Each expressed as seven evenly spaced strips
Stripes of complementary genes are non-overlapping
What happens in Pair-rule gene loss of function mutants?
Mutant lacks portions of each segment
How do Cis elements affect pair-rule expression?
They control pair-rule expression
Non coding sequences that control when/where/how much of a gene is transcribed
Enhancer and repressor sites for eve stripe 2?
Bicoid; Caudal; Giant; Hunchback; Kruppel; Knirps; Tailless
Gap genes are turning on/off pair-rule genes in a very precise way which is how we get pair-rule pattern(s)
The segment polarity genes
engrailed is expressed in posterior compartment of each segment
Wingless (Wnt) is in anterior compartment of each segment
Pair-rule –> segment polarity
Wingless is expressed in the intervening spaces during lack of Eve and Ftz
Engrailed is in cells with high concentrations of Eve or Ftz
Segment polarity genes
segment polarity genes are signaling centers that create gradients of morphogen
Ex. wg and Hh
Hedgehog and wnt expression feedback loop and its positives
We get a loop which is beneficial because they are not expressed forever then they fade away
wg. is expressed which causes wnt paracrine factor which binds to frizzled receptor on neighboring cell
The wnt factor allows Beta-catenin to not be degraded so it can go to the nucleus and changes gene expression which activates engrailed expression
This txn factor activates hedgehog expression which leaves the cell and goes to the first cell in the loop which has hedgehog receptors.
The hedgehog receptor takes the factor and causes wg expression which causes wnt expression and the cycle repeats
Homeotic selector genes
aka Hox genes!
Genes specify structures on segments
Cluster of genes that function like a single unit
Location/position on the chromosome is linked to location of expression in the animal
Homeotic mutant examples
- Ubx mutant allows the Antp to expand in its domain to get another set of wings in the place of Ubx place
Fly gets two second thoracic segments instead of one - Gof mutant allows the Antp domain to grow where the antennas would be
The result is a pair of legs where the antennas would be
Paralogues
Same gene in organism
Why are there four “Hox” complexes in mice
Results from gene duplication during mouse evolution
What is the Hox hypothesis?
Means combinations of these gene’s expressions specify fate of regions along the A/P axis
Evidence for the Hox Code hypothesis?
Comparative anatomy- Compare types of vertebrate cross vertebrate species. Differences in Hox expression boundaries correlate with differences in skeletal structure
Knockout of KO expression. Complete knockout of Hox 10 (a,c,d) resulted in the expansion of the thoracic region and the sacral as well. Lumbar converted to thoracic vertebrate. Hox 10 –> Hox 9 fate
Complete knockout of Hox 11 (a,c,d) resulted in sacral vertebrae are lost and are lumbar vertebrae. Extension of Hox 10 domain.
