Drosophila and their body plan (L6 &7) Flashcards
Explain the Drosophila life cycle
Life cycle is about 10 days at 25 degrees and over 3 weeks at 18 degrees. They tend to live about 60 days. Drosophila basically shed their skin twice as they go between different instar stages (When they’re larvae). It takes about 14 hours to go from an embryo to 1st instar (they hatch)
What is the history of drosophila being used in science?
1910- Morgan discovers a white-eyed fly- discovers the ‘white’ gene. Used this to discover hereditariness and how genes are passed to generations. 1913- sturtevant constructs first genetic map, genes are arranged in linear order. 1914/16- Bridges shows that chromosomes must contain genes. 1927- Muller shows that X-rays cause mutations and chromosomal rearrangements
1979/80 - Nusslein-Volhard and Wischaus identify genes involved in the development and patterning of larval cuticle.
What technical and methodilogical advances occured in the 1980s-2000s?
Transgenics - P-element transformation Promoter trapping - Enhancer trap Gene misexpression Clonal mutant analysis RNAi in vivo and ex vivo. Omic technologies All the advancement lead to the huge breakthrough of the Drosophila genome being sequenced in 2000. Because of this, you could now also see how other types of flies and insects were related (see genes that were conserved and therefore their importance) Showed untranslated exons include regulatory regions
Explain drosophila courtship
! female and 1 male can lead to 200 offspring in less than 2 weeks. Their courtship is very complex. It is a genetically encoded behaviour. When pupae were isolated and grown int he dark they still knew how to do it. Their courtship includes tapping wing vibration and licking.
Explain how gametes are made in drosophila
in the testes, there are many types of cells used to make sperm. Hub cells secrete factors e.g. unpaired (a ligand) which is required to maintain stem cells and decide fates via the JAK/STAT pathway. Stem cells are localised adjacent to the hub. Cells further away differentiate into sperm-making cells. Females only need to mate once because they can store the sperm in the seminal receptacle. Eggs are fertilised then drilled into food.
Explain how the egg develops and fertilises
To begin with, the egg contains stem cells which are continually maintained. It undergoes 4 incomplete cytoblast mitotic divisions. One of the division is chosen as the future oocyte and is left with a diploid nucleus, and undergoes meiotic recombination before it divides again. It then undergoes activation. The other cells become nurse cells and they replicate their genome (endorepilation) so they can make all the stuff they egg needs and act as support cells. The egg is activated via Meiosis I and II and then the sperm fuses with the female pronucleus. It is like a production line. The nurse cells can be stained with DAPI to see bc they have ALOT of DNA in them. The nurse cells contain polytene chromosomes (all sister chromatids joined together). The fatter areas are where DNA is being actively replicated (puffs) can be seen when you stain for polymerase II.
What are the maternal contributions to the embryo?
Proteins and RNA made in the nurse cells. Transferred into developing oocyte. The nurse cells dump cytoplasm into the germ cell via ring canals.
Explain the subcellular localisation of maternal factors
Microtubule transport. Minus and plus-end motors. GLue anchors them in position
E.g. bicoid.
How is the egg protected?
Chorion (eggshell, secreted by follicle cells) protects the egg. The vitelline membrane is on the inner surface of the shell. It stops it from drying out.
What is the segmentation of Drosophila like?
Drosophila has a segmented pattern which is mimicked in larvae. The adults have abdominal and thoracic segments. Each T segment has 1 pair of legs. The larvae have a pattern of denticles and naked cuticle which they use to crawl around. You can start to see structures when they are pupae. The folds are the first manifestations of segments
Explain how segmentation genes and their mutations were discovered
In 1980s, the first segmentation genes were discovered via genetic screens. The work won a Nobel prize in 1995. In 190s the Nusslein and Weischaus guys did the mutagenesis screen. to identify genes involved in segment patterning. They found 580 mutations causing embryonic phenotypes, they ran complementation tests and found 139 groups. Means they found each gene about 4 times so its v unlikely they missed any (saturated system). the genes were named after the mutant phenotype. E.g. mutations in gap genes (like knirps) leave big gaps in the segmentation. Mutations in pair ruled genes (like paried) cause the pairs of segments to be skipped - e.g. no odd or even ones. Segment polarity genes (like gooseberry) cause the fine tweaking of the segments to go wrong e.g. no naked cuticle.
What is bicoid and what is its function?
Bicoid is a DNA binding transcriptional activator. Maternally loaded into the developing oocyte. Acts as a morphogen. We know this because if you KO bicoid, you get loss of anterior head structures, then if you ectopically transplant it you get a partial rescue of head structures. If you transplant some cytoplasm containing bicoid to the middle of the embryo you get a mirror image of thoracic segments and head structures in the middle. If you alter the gradient of bicoid you can change where the segments are, but you still get the same number of segments. Bicoid is a maternal gene.
How do Gaps genes get stripped lol
the level of TF also causes an on and off the expression of Gap genes which is how you get their stripped expression. Combinations of repressors and activators used to turn on even skipped paired rule genes. Giant and Kruppel are repressors. Dependent on the interaction of positively and negatively acting transcriptional regulators (many of which are gap genes). So where giant or kruppel aren’t present, you get an even stripe. Bicoid causes expression of hunchback - this causes Kruppel expression in the middle of the embryo (too high Hb or too low Hb you get repression of it)
How do segment polarity genes cause the naked cuticle/ denticle belts
Each segment contains a denticle belt and a naked cuticle. Parasegments (the expression of genes) and segments (the physical indentation) are basically the same thing but were discovered at different times, so they don’t exactly line up with each other. Hh and Wg feedback onto each other to define their borders. Hh increases expression of Wg and Wg increases its own expression as well as Hh’s. Hh maintains Wg which suppresses denticle development. The gradient is asymmetric because HSPGs and receptors which soak up the secreted factor in one direction. the cells become hairs without signalling because that’s their default fate when they have no signals.
What are Hox genes?
Genes that provide the who am I for segments. Expression of homeotic genes along the A/P body axis occurs in the same order as the genes within the genome. Controlled by a combination of gap and pair-rule genes. Homeobox-containing DNA binding. Mutations in antennapedia lead to the fly having legs on its head.
