Chapter 32-1. Drosophila Embryology
Recommended post: 【Biology】 Chapter 32. Embryology
1. Overview
2. Maternal Effect Genes: Anterior–Posterior Axis Determination
3. Maternal Effect Genes: Dorsal–Ventral Axis Determination
1. Overview
⑴ Pattern Formation: The process by which spatial components of the embryo are determined
⑵ Pattern Formation 1. Maternal Effect Genes: Also called egg polarity genes
⑶ Pattern Formation 2. Segmentation Genes
⑷ Pattern Formation 3. Homeotic Genes
2. Maternal Effect Genes: Anterior–Posterior Axis Determination
Figure 1. Anterior–Posterior Axis Determination Process
⑴ 1st. As fertilization occurs, protein translation and diffusion → regulation of gap mRNA expression.
⑵ 2nd. The oocyte moves to the posterior, and the anterior is occupied by nurse cells.
⑶ 3rd. The nucleus of the posteriorly positioned oocyte synthesizes Gurken protein from gurken mRNA.
⑷ 4th. Concurrently with ⑵, terminal follicle cells express Torpedo.
⑸ 5th. When Gurken binds Torpedo, it activates protein kinase A, determining microtubule orientation.
⑹ 6th. bicoid mRNA (= bcd mRNA) binds dynein and moves toward the anterior.
⑺ 7th. After oskar mRNA is translated, it associates with nanos mRNA (= nos mRNA).
⑻ 8th. Oskar protein holding nanos mRNA binds kinesin and moves posteriorly → nanos mRNA accumulates at the posterior.
⑼ 9th. Anterior–posterior axis expression.
Figure 2. Distribution of Anterior–Posterior Morphogens
① hunchback mRNA and caudal mRNA are evenly distributed throughout the embryo
② Bicoid distributed in the anterior inhibits translation of caudal mRNA and relatively promotes translation of hunchback mRNA
③ Nanos distributed in the posterior inhibits translation of hunchback mRNA and relatively promotes translation of caudal mRNA
○ Nanos protein reduces the poly A tail length of hunchback mRNA → mRNA lifespan↓ → translation inhibition
○ Injecting bicoid mRNA into the posterior results in head and thorax formation in the posterior
④ hunchback mRNA: Forms the tip of the head, head, and thorax
⑤ caudal mRNA: Forms the abdomen and tail
○ Note: staufen
⑽ 10th. Nucleus with gurken protein moves along microtubules and induces surrounding follicle cells into dorsal oocyte cells
3. Maternal Effect Genes: Dorsal–Ventral Axis Determination
Figure 3. Dorsal–Ventral Axis Determination Process
⑴ Premise
① Dorsal protein accumulates in the nuclei on the ventral side of the embryo, and its concentration gradually decreases from ventral to dorsal
② Dorsal protein binds to the enhancers of snail and rhomboid to induce their expression
③ Snail protein inhibits the expression of rhomboid
⑵ Result
① Areas with high dorsal protein expression express only snail
② Areas with low dorsal protein expression mainly express rhomboid
○ Note: Dorsal protein has higher affinity for the enhancer of rhomboid than for that of snail
4. Segmentation Genes
⑴ Gap Genes: Divide the embryo into anterior, middle, and posterior parts and induce the action of subsequent genes
⑵ Pair-Rule Genes: Although not visible morphologically, they divide the embryo into periodic units called parasegments
⑶ Segment Polarity Genes: Establish anterior–posterior within each segment to complete segment formation
5. Homeotic Genes
⑴ Homeotic genes
① Master regulatory genes that control other genes involved in forming body structure
② Involved in segment-specific differentiation
③ Drosophila has a single cluster.
⑵ Homeobox (HOX)
① A DNA region where transcription factors bind
② Evolutionarily highly conserved.
⑶ Example: Homeotic genes expressed in the thorax of a Drosophila embryo
Figure 4. Homeotic genes of Drosophila embryo
① In the embryo, the genes Scr, Antp, and Ubx are expressed in thoracic segments T1, T2, and T3, respectively.
② Legs form in all of T1, T2, and T3.
③ In the adult, wings and halteres form in T2 and T3, respectively.
④ If Antp is expressed in the embryo’s head, legs form on the adult head instead of antennae.
⑤ In Drosophila lacking Ubx function, wings form in T3 instead of halteres.
Input: 2019.03.06 18:52