Lecture 2 Notes (Axes, Segmentation and Patterning in Drosophila) PDF

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Lecture 2 Notes (Axes, Segmentation and Patterning in Drosophila Part 1)...

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Developmental Biology Lecture 2 Notes: Axes, Segmentation and Patterning in Drosophila Development of Axes 

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Drosophila are one of the most important model organisms in developmental biology because they are easily accessible and have a very fast life cycle (i.e. can view developmental process quickly and see the effects of altered genes). Pattern formation is the process by which cellular activity is organised in space and time so that a wellordered structure develops within the embryo. It is of fundamental importance in the early embryo and in the formation of organs. For example, in the developing arm, pattern formation enables cells to ‘know’ whether to make an upper arm or fingers, and where the muscles should form. There is no universal strategy for patterning; it is achieved through various cellular/molecular mechanisms in different organisms and different stages of development. Pattern formation initially involves laying down the overall body plan – defining the main body axes of the embryo that run from the AP (head to tail) and DV (back to underside). A striking feature of these axes is that they are almost always at right angles to each other so they can be thought of as establishing a system of coordinates on which any position or dimension in the body can be specified. The simplest non-radial pattern is an asymmetry. All animals (except sponges, cnidaria and ctenophores) are triploblastic (made of 3 germ layers). In vertebrates, the outer neuroectoderm will later develop into the skin, and the inner circle of neuroectoderm will develop into the central nervous system. The ‘hole’ in the centre of the embryo develops into the gut/GI tract and the remaining mesoderm develops into the bones (spine), muscles and circulatory system. In insects, a simpler developmental process occurs, and the neural cord is at the ventral (belly) side of the embryo. The ectoderm becomes the epidermis, respiratory and nervous system. The mesoderm becomes the circulatory system and muscles. The endoderm becomes the midgut. These germ layers were first observed in echinoderms (sea urchins). The formation of the ectoderm, endoderm and mesoderm via gastrulation is the most important morphogenic process. Establishment of longitudinal axis and of germ layers are relatively independent in Drosophila but they will be related in vertebrates. Early development landmarks include: (a) Cleavage/nuclear divisions (short, synchronous cell divisions, no G1 or G2 so no increase in mass, in Drosophila nuclei divide in shared cytoplasm)

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(b) Blastulation (formation of a ‘ball’ of cells in one layer or leaving a central cavity, in Drosophila this is a yolk in the centre rather than being hollow) (c) Gastrulation (dramatic cell movements resulting in the formation of a hollow tube running A-P and of the three germ layers, body plan set up) (d) Neurulation (initiation of the CNS from the ectoderm) (e) Organogenesis (formation/differentiation of individual organs) Drosophila development from fertilised egg to a larvae it takes less than 24 hours. Embryogenesis begins with the formation of a ‘syncytium’ (within 0-4 hours). A syncytium is essentially a giant cytoplasm with many nuclei and no membrane. A zygote has one nucleus, but an embryo has around 5,000. Synchronous nuclear divisions begin (i.e. without cytokinesis, no separate cells forming). Nuclei then migrate to the periphery of the syncytium and become surrounded by individual cell membranes (i.e. via invagination of the outer membrane surrounding the nucleus). No transcription occurs (mRNAs are present but predate fertilisation). The body of an insect is based off of 3 main regions: the head, thorax and abdomen. Embryogenesis before 2 hours is based off of maternal (i.e. mRNA left in the embryo from the mother) transcripts (i.e. no gene transcription occurs in the nuclei because they are proliferating). After this point, development is based off of zygotic gene transcripts. After around 3 hours, transcription of genes begins. Individual cells now change shape or move (patterning). The formation of layers and external shapes (externally visible segments) begins, which are visible (through an electron microscope) on the exterior of the embryo. The body of the embryo folds over itself (germ line extension), lead by the movement of pole cells, from the ventral to dorsal side. Next, small indentations, which act as air ducts (later spiracles) form, to allow the flow of air. Finally, the embryo retracts (germ line retraction) and moves back. Through the identification and study of mutant genes in Drosophila, we can study the process of embryo development. However, usually a mutant gene in an embryo leads to the death of the embryo (i.e. they are lethal embryo mutations). Through the use of heterozygous carriers of these mutated genes in Drosophila (Ed Lewis et. al) we can identify the genes involved in establishing the AP axis.

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A mutation in which the embryo seems to be missing the anterior section (gut/head terminus and thorax) is known as bicoid. When the embryo is missing the posterior end (i.e. abdomen, gut and rear terminus) it is identified as nanos. Torso embryos have normal midsections but the termini are mutated. These mutations have strictly additive effects on the phenotype. Genes specify which proteins are made in which cells and when during development. The proteins they encode are the agents which directly determine cell behaviour, including which cells are expressed. The AP axis is caused by gradients in the products of two genes. Initially, maternal bicoid gene mRNA is heavily concentrated at the anterior end of the embryo. Its product, bicoid protein is able to move/diffuse through the large cytoplasm and forms a diffusion gradient (from the anterior to posterior end). The bicoid protein is a ‘master switch’ transcription factor (i.e. it binds DNA and activates other genes) which acts similarly to a morphogen (i.e. signalling position along the body). The nanos gene, plays a similar role at the posterior end. Nanos mRNA appears ‘immobilised’ at the posterior end of the embryo where maternal mRNA expression occurs. However, nanos protein creates a concentration gradient (from the posterior to anterior end of the embryo, acts as a morphogen). The nanos protein acts indirectly, preventing translation of another factor (hunchback) at the posterior end. The posterior end is also crucial to isolate the future germ cells (gametes)....