Lecture 21a Flashcards
At first, it appeared that Drosophila approached development very differently. However, what is now known from mutant screens?
The genes in Drosophila are homologs to mammalian development and related development.
What do the homologs between Drosophila and other species show?
Structures can appear to develop very differently yet use very similar genes.
What is the first stage in Drosophila embryonic development?
The establishment of the body axes.
How do the body axes become established in Drosophila during embryonic development?
The establishment of the body axes starts during oogenesis in which certain gene products (morphogens) important in early development are deposited asymmetrically within the egg.
T/F: Morphogens are deposited symmetrically within the egg.
False! Morphogens are actually deposited asymmetrically in the head.
With the asymmetric deposition of Morphogens in the Drosophila embryo, what does this establish?
The Morphogens establish the axes of polarity in the Drosophila embryo.
Where is the Bicoid mRNA concentrated?
Bicoid mRNA is concentrated in the area that will become the head.
Where is the Nanos mRNA concentrated?
Nanos mRNA is concentrated in the area that will become the tail/rear.
What happened when people mutated the Bicoid gene?
The embryo partially lost its head-tail orientation and now developed with 2 posterior ends (spiracles).
T/F: Bicoid mRNA is distributed anteriorly and the Nanos mRNA is distributed posteriorly.
True!
If a larva was defective in the Bicoid gene, how will it develop?
Develops with 2 posterior ends (spiracles).
Where is the Bicoid gene actively transcribed? After it is transcribed, where does it move to?
Actively transcribed in the nurse cells, which is outside of the oocyte.
Then, it diffuses across into the oocyte.
Where does Bicoid mRNA enter the oocyte?
Enters the anterior end of the oocyte and is trapped there.
What are 2 techniques we can use to see where gene products reside in the embryo?
1) In situ hybridization of Bicoid mRNA
2) Immunostaining of Bicoid protein (similar to Western Blot but on an embryo)
What is In situ hybridization of Bicoid mRNA?
It is essentially RNA FISH on an embryo. The DNA is not denatured so the probe only recognizes RNA, allowing us to visualize gene products (because thats what RNA is).
What is involved in Immunostaining of the Bicoid protein?
The embryo is exposed to primary antibodies recognizing Bicoid protein and then to secondary antibodies that recognize the primary antibodies and are also linked to a fluorescent molecule.
During Drosophila development, the oocyte first starts as one cell, then it becomes a Syncytial blastoderm. What does it mean?
The oocyte gets fertilized, then the nucleus divides, but NOT the actual cell. Cells will form around each nucleus.
What are Syncytial blastoderm nuclei?
A ring of nuclei that form within a cell.
In Drosophila development, the zygote goes through a series of _________ divisions, but NOT __________ divisions.
nuclear, cytoplasmic
Eventually, the Syncytial blastoderm will form what? What else clusters and forms?
Forms Blastoderm cells.
Germ cells will also cluster on the outer edges of the posterior end.
After Blastoderm cells are formed, what does the embryo undergo? What is this?
Undergoes Gastrulation, which is when the cell goes from a single cell layer to three cell layers.
Name the 3 cell layers formed in Gastrulation.
Ectoderm (outside), Mesoderm (middle), and endoderm (inside).
What is Gastrulation?
Blastoderm cells undergo a series of complicated cell movements that turn single-cell layer into 3 cell layers from invaginating cells.
Gastrulation involves a great deal of cell migration. What does this form?
Forms the embryo
What occurs after Gastrulation?
Segmentation
What is Segmentation?
Segmentation genes get expressed in specific regions of the embryo, causing the embryo to become segmented.
From anterior to posterior, the segments have been given _____.
names
T/F: The anterior and posterior halves of each segment are the same in terms of genes and gene expression.
False! The anterior half of each segment in the Drosophila embryo is different from the posterior half in terms of genes and gene expression.
In addition to segments, what else can the embryo be divided into? What are these?
Can also be divided into parasegments, which is when we have the anterior half of one segment with the posterior half of another segment.
The segments and parasegments are out of __________. This means that the anterior part of a segment coincides with the posterior region of a parasegment that came before it.
register
Scientists found genes that when mutated, screw up segmentation. Name the 3 types of segmentation genes.
1) Gap genes
2) Pair-rule genes
3) Segment-polarity genes
What are Gap Genes?
Genes in which a group of continuous segments are missing, resulting in a shorter embryo.
What are Pair-Rule Genes?
Genes in which every other segment/parasegment is missing from the larvae. The missing segments can be even or odd-paired segments missing.
What are Segment-Polarity genes?
Normally, these establish polarity within genes. However, mutations in these produce a segment that is more uniform, meaning there is less of an anterior and posterior part to each segment.
What is Metamorphosis?
The transition from segmented larva to the adult fly body plan.
In metazoa, what is the final result in development?
An adult body organized along 3 axes.
In the final stages of development, what does the Drosophila get wrapped up in?
A pupal case
During metamorphosis, what do the 19 imaginal discs in the larvae do?
They develop into adult structures.
Scientists discovered other genes that were found to be really important in development called Homeotic Genes. What do these determine?
Homeotic Genes determine the segment identity / phenotype.
Define the term Homeotic.
Refers to mutant alleles in which one body part is replaced by another.
Basically, if you have a mutant allele, it can make that segment resemble another segment.
What does Cell Fate mean?
Describes the ultimate morphological features of a cell or group of cells.
Each segment has a different fate.
T/F: In Drosophila, the cells in each body segment have their fate determined very early in embryological development, long before morphological features become apparent.
True!
How is the expression pattern of homeotic genes in Drosophila shown?
They are in 2 clusters that are physically separated from each other. Antennapedia complex and bithorax complex.
The order of gene expression anterior to posterior ____________ the order of genes on the chromosome. Each of these genes encodes for a ___________ ___________.
parallels, transcription factor
What determines the color of a segment in Drosophila homeotic genes?
The most posterior-expressed homeotic gene determines the color of the segment.
What is the expression pattern of homeotic genes determined by?
By 2 groups of chromatin-promoting proteins, Polycomb genes and Trithorax genes.
What do Polycomb genes do?
Encode heterochromatin which will repress the expression of homeotic genes in regions of the embryo where they should NOT act.
What do Trithorax genes do?
Encode euchromatin which will promote the expression of homeotic genes in regions of the embryo where they should act.
Polycomb heterochromatin limits ____ gene expression to specific sets of segments. As we go to the ___________ end of the fly, more and more homeotic genes are being expressed.
Hox, posterior
What is the homeotic gene that is being expressed in a segment?
The homeotic gene that is closest to the heterochromatin-euchromatin boundary is the one that is being expressed in a segment.
