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LECTURE 5: Role of Hormones in Development.doc

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University of Toronto Scarborough
Biological Sciences
Ian Brown

LECTURE 5: ROLE OF HORMONES IN DEVELOPMENT Slide 3 • A butterfly starts off as an egg. Its juvenile stage is caterpillar which then changes into a pupa. Inside the pupa, the caterpillar goes through metamorphosis and changes into butterfly (adult stage) • The juvenile form of dragonfly lives under water. It grows and crawls out of water and eventually changes into a dragonfly Slide 5 • In this type, the animal must go through a series of instar forms • Wings develop externally and gradually with each instar Slide 6 • In this type, the insect in its early stages, lives under water and in its adult stage, lives outside of water. • MAKE THIS CORRECTION ON SLIDE: Hemimetabolous is NOT the same as parametabolous Slide 7 • In this type, the insect goes through a new stage of development = pupa stage • Juvenile form is larvae • In pupa, the caterpillar no longer moves around, lives in a cocoon, and the juvenile form changes into an adult form Slide 8 • What factors cause molts up to the adults form? Ans: Hormones • One of the early experiments showed off that something circulating in the blood role. These experiments used an insect called Rhodnius prolixus who has 5x instar stages before it changes into an adult • These insects go through parametabolous - meaning the adult looks similar to juvenile form but bigger (especially the head, in this case) and has wings • They noticed that something in blood was causing this Slide 9 • This was a very early study that concluded that there must be some molecules in blood that move from 5th instar to 1st and some factor moving from 1st to 5th instar, causing some evidence of development of adult structures Slide 10 • He then started looking at the structure of insect brains and the glands around the brain. He located 2 glands, just behind the brain, called corpora allota • In his experiment 2, he surgically removed the corpora allota and as a result, the 3rd instar developed into adult form, instead of 4th instar. • The result of the 3rd experiment was that the instar went into an extra molt, instead of the adult form • Conclusion: these glands are giving off factors that induce molting and without these factors, the larva just develops into an adult stage. Slide 11 • Such early experiments led to analysis of hormones that may be contributing to insect metamorphosis • First one of these hormones is PTTH • The sensory system in the insect may sense environmental change and trigger secretion of PTTH which then acts on prothoracic glands Slide 12 • 2nd hormone for insect metamorphosis • Prothoracic glands degenerate in adults because we don't need ecdysone then, since metamorphosis has been complete. Slide 13 • 3rd or last hormone for insect metamorphosis • Levels of JH control what ecdysone will do: induce pupation or molting? Slide 15 • Summarizes importance of hormones in metamorphosis • MAKE THIS CORRECTION ON THE SLIDE: The red arrow should be coming from neurosecretory cells • The steps in the diagram: 1) Environmental change induces release of PTTH by neurosecretory cells 2) Released PTTH affects prothoracic glands and induces them to make ecdysone 3) Endysone is produced and goes into the bloodstream and acts on 3 stages of development. How it triggers the next change depends on the level of JH that is present in stages of development. 4) JH is produced as pulse levels (hi or low) throughout development. High levels are produced in early stages which makes ecdysone lead to molting of a new larval instar 5) At the pupa stage, glands are producing at low levels of JH. So, in this environment of low JH, ecdysone cause maturation (pupa to adult) • So, it's the hormones interacting in specific ways that results in metamorphosis of insects Slide 17 • Amphibian metamorphosis is both internal and external • The frog goes through a metamorphosis to anticipate a change in it’s environment • first external change in frogs is the emergence of hind legs • By "eye migrate rostrally and dorsally," he's referring to the placement of eyes (thus, an external change) • In the last stage of frog metamorphosis, the tail is lost. It does not fall off, but is gradually decreases in size and is resorbed by the body. This is called tissue resorption. Slide 18 • From the tadpole in bottom left corner to the frog on the top right corner A. A tadpole with a tail for swimming B. Tadpole increases in size C. Rear legs start to develop and an overall increase in size D. Rear legs developed and an overall increase in size E. Front legs start to develop F. Front legs developed G. Tail shortens and legs are functionally developed H. Tail shortens and the frog forms a well-developed structure I. Tail shortens more J. No Tail Slide 19 • Tail and gills undergo tissue resorption when a hormone induces different hydrolases t
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