Patterning invertebrate embryos Flashcards
How do you identify genes involved in the developmental process?
use of forward genetics
mutation > gene > protein > function
use of mutagenesis to create random mutations in the genome
inbreed the mutant gemone
use phenotype analysis to find the function of the gene
clone the gene to find molcule identity
analyse the function of the gene product
what are the cleavage divisions of drosophila (30, 70, 90, 2hrs, 3hrs
30 - fusion of sperm and egg nuclei
70 - nuclear division creating the synctium
90 - nuclei migrate to the periphery of cytoplasm
2 hrs - synctial blastoderm forms
3hrs - cellular blastoderm forms
how do you recognise the anterior end of a drosophila egg?
micropyle - a nipple shaped sturcture in the tough external coat surrounding the egg
where do sperm enter the drosphila egg?
the micropyle
what occurs after fertilisation and fusion of nuclei in drosophila?
a series of mitotic divisions
no cleavage of cytoplasm and no formation of cell membrane to separate nuclei
what forms after 12 nuclear division in drosophila development?
formation of the synctium, approx 6000 nuclei present in the cytoplasm
what does formation of the synctium in drosophila allow?
allows large molecules to to diffuse between nuclei during the first 3hrs of development
what happens at synctial stage of drosphila?
small no of nuclei move to the posterior end and become surrounded by the cell membrane to form pole cells
What do pole cells in drosophila do?
pole cells give rise to germ cells from which gametes develop
what happens after 9 nuclear divisions in drosophila?
nuclei move to the periphery to form the synctial blastoderm
what is the synctial blastoderm in drosophila development?
a superficial layer of nuclei and cytoplasm which surrounds a central mass of yolky cytoplasm, membranes grow in from the surface to enclose the nuclei and form cells
what does the blastoderm in drosophila give rise to?
somatic cells of the embryo
what are the three germ layers?
ectoderm, endoderm, mesoderm
where is the mesoderm located in drosophila?
in the ventral region
what is the future midgut derived from in drosophila?
2 regions of the prospective endoderm
1 at anterior and one at posterior end of the embryo
when do drosophila endodermal and mesodermal tissues move to their future positions?
during gastrulation, this leaves the ectoderm as the outer layer
when does gastrulation start in drosophila?
3hrs after fertilisation
how is the the future mesoderm of drosophila formed?
how is it internalised?
what does it eventually give rise to?
at ventral region it invaginates to form a furrow along the ventral midline
internalised by formation of a mesoderm tube where the cells separate from the surface layer of the tube and migrate under the ectoderm to internal locations
later gives rise to muscle and other connective tissues
what does each germ layer become?
a unit of lineage, once cells have been specified to one fate they all of its descendants will adopt this fate
where in insects does the main nerve chord lie?
ventrally
in drosohila after the invagination of the mesoderm, what is formed?
the nervous system
how is the nervous system of drosophila formed?
ectodermal cells in the ventral region leave the surface individually and form a layer of prospective neural cells between the mesoderm and the outer ectoderm
how and when is the gut formed in drosophila?
at the same time as nervous system formation , 2 tubular invaginations develop at the future posterior and anterior midgut. they grow inward toward each other and fuse to form the endoderm of the midgut.
the ectoderm is dragged inward behinds the tubular invaginations to form the foregut and hindgut
what does the outer ectoderm develop into in drosophila?
the epidermis, ectoderm continues to divide during gastrulation but the mesoderm does not
when does germ band extension occur in drosophila?
during gastrulation, the central body of the blastoderm undergoes extension along the antero-posterior axis
what happens to mesoderm cells and the epidermis after gastrulation is complete in drosophila?
mesoderm begin to divide again
epidermis 2 times more and then secrete a thin cuticle of protein and polysaccharide chitin
what does germ band extension in drosophila drive?
drives posterior regions round the posterior end and onto the dorsal side
what happens to the germband after embryonic development is completed?
it retracts
when are the first external signs of segmentation in drosophila seen?
as germ band extension occurs
what mark parasegments externally in drosophila?
evenly spaced grooves
how many parasegments are there in drosophila and what do they show?
14, 3 to mouth parts of the head, 3 to the thoracic region and 8 to the abdomen
what is the telson?
the posterior end of larva
how many segments are there between the head and telson?
3 thoracic and 8 abdomen
what are dendrites?
smooth outgrowth from the epidermis showing the segments
what happens after the third instar (shed of cuticle) in larva?
they become pupa, the metamorphosis into fly form occues
how are suitable informative developmental mutations found?
v rare
induce random muts into a large no of individuals
and screen systematically for developmental defects
how do maternal effect mutations effect embryos?
the mutations are present in the mother but do not affect her appearance of physiology, they have profound effects on development of progeny independent to the father
what does maternal factors in drosophila do?
guide earliest stages of development
50 maternal genes involved in setting up 2 axes and basic molecular frame work
3 classes of maternal genes that specifies regional differences in drosophila eggs
ones that affect only anterior
ones that affect only posterior
ones that effect both terminal regions
what does bicoid mutation in the anterior end of drosophila result in?
reduction of head, thoracic features and in some cases replacement with posterior end
what does nanos mutation in the posterior end of drosophila result in?
loss of the abdominal region which leads to a smaller than normal larva
where is biciod mRNA located?
anteriorly
what do female flies that dont express bicoid produce?
embryos with no proper head or thorax
what do nanos and caudal control?
posterior patterning of flies, nanos located at posterior end and translated after fertilisation, this suppresses the hunchback gene
what creates positional information?
opposing gradients of maternal gene products
what determines the dorsal ventral axis?
toll receptor activation
what creates the intracellular gradient from ventral to dorsal in flies?
dorsal protein entering the nuclei of the synctial blastoderm
what divides the dorsal ventral axis into 4 distinct regions?
dorsal transcription factor gradient
what does the ventral ectoderm become?
neuroectoderm which gives rise to the nervous system and ventral epidermis
how is the dorsal gradient acheived?
differing binding affinity
most ventral end has the highest intracellular conc of dorsal
what activates dorsal to enter the nucleus and trigger transcription?
toll signalling
what controls segmentation in drosophila?
a hierarchy of gene expression
gap genes are the first to be expressed where? and what do they code for?
expressed along antero-posterior axis and code for transcription factors
what is gap gene expression triggered by?
gradient of bicoid proteins
how do gap genes act?
in combination with other transcription factors, short lived proteins only extend slightly beyond region of expression in which overlaps sharpen the domains of diff regions
how do bicoid genes target hunchback expression?
in a concentration dependent manner
bicoid induces expression of hunchbacck over most of the anterior half of the embryo
hunchback is only switched on when bicoid is of a certain treshold
hunchback expression extends posteriorly as so does bicoid protein above treshhold conc
when is Kruppel expressed?
above a certain threshold (low conc) of hunchback
what acts as a morphagen to other gap genes?
hunchback
how are gap genes expressed?
in transverse stripes which depend on control genes being sensitive to diff concs of hunchback
where does the gradient of hunchback locate Kruppel?
in the centre of the embryo - when bicoid isnt present, hunchback conc is low so kruppel is seen anteriorly, when hunchback conc is high, kruppel is shown at the posterior end of the embryo
how are larval segments made up?
posterior region of one parasegment and anterior of the next, initially parasegments are similar to one another but soon have their own identity
how is the expression of even skipped stripe 2 defined?
pair rule genes are expressed in stripes
mutations effect alternate segments
striped pattern is present before cells are formed
how are the 14 parasegments created?
- production of local combination gap gene transcription factors
- activation of each pair rule gene in 7 transverse stripes along antero posterior axis
- pair rule gene expression defines 14 parasegments each pair rule gene is expressed in alternate parasegments
segments have their own polarity
what happens in the absence of hedgehog?
in the absence of hedgehog, membrane protein patched inhibits membrane protein smoothened
so transcription factor Cubitus interruptus is held in the cytoplasm in 2 protein complexes
1. associated with smoothened 2. associated with protein suppressor of fused
Ci in smoothened is phosphorylated by several protein kinases which leads to the cleavage of Ci and formation of truncated protein CiRep
CiRep enters the nucleus as a repressor of the hedgehog target gene
what happens in the presence of hedgehog?
it binds to patched and lifts the inhibition of smoothened to block production of CiRep
Ci is release from both complexes in the cytoplasm to then enter the nucleus and act as a gene activator
the genes activated include - wingless, decapentapleigic and engrailed
wingless and hedgehog define compartment boundary
what do Homeobox genes provide?
identity of body segments (hox genes)
what make sup the hom-c complex?
hox genes organised into 2 gene clusters of gene complexes
what does the homeobox DNA motif encode?
part of the DNA binding sites of hox genes
