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Lecure 5 - role of hormones .docx

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University of Toronto Scarborough
Biological Sciences
Ian Brown

Lecture 5 – role of hormones Metamorphosis (slide 2) Some animals undergo a dramatic developmental change in morphology when the juvenile form transforms to the adult form • We will examine metamorphic change in insects and amphibians • Hormones play key roles in triggering metaphoric change • Metamorphosis consists of loss of certain structures in the juvenile form and differentiation and growth of new structures in the adult form Metamorphosis in insects (slide 3) Top row shows a caterpillar  pupa  butterfly The dragonfly’s life stages are different because they inhabit different environments (juvenile in water while the adult is on land) Types of insect metamorphosis (slide 4) 1. Ametabolous (without metamorphosis)  Life cycle: egg  immature form  adult  Adults differ from immature forms in size and maturity of reproductive organs  Young and adult forms live in same environment, eat same food and have same life style Lecture 5 – role of hormones Types of insect Metamorphosis (slide 5) 2. Parametabolous (gradual metamorphosis)  Life cycle: egg  nymphal instar    adult  Adults differ from immature forms in size and in possession of fully developed reproductive organs, but also adults have wings.  Wings develop externally and gradually with each molt: wingless  wing bud  wings  Young and adult forms live in same environment, eat same food and have same life style Types of insect metamorphosis (slide 6) 3. Hemimetabolous (incomplete metamorphosis)  Life cycle: egg  naiad instar    adult  Same as parametabolous, but young and adult forms do not live in same environment. Naiads are ‘water nymphs’  Eggs laid underwater  naiad hatches  undergoes successive molts  adult lives outside of water  Juvenile and adult forms eat different food, live in different environments Lecture 5 – role of hormones Type of Insect Metamorphosis (slide 7) 4. Holometabolous (complete metamorphosis)  Life cycle: egg  larval instar stages  pupa  adult  Larvae (function: eat and grow) very different from adult (function: reproduction)  Pupal stage is transition stage: do not eat; great metabolic changes * Has a new stage  pupa (doesn’t eat and change more) Type of Changes Stages Example Metamorphosis Ametabolous Size & reproductive organs Egg  Lice immature form  adult Paramerabolous Size, reproductive organs & Egg  Nymph Grasshopper wings  adult Hemimetabolous Size, reproductive organs, Egg  naiad Dragonfly wings & habitat change (food (water nymphs) change) instar  adult Holometabolous Complete change (additional Egg  larval  Butterfly stage = pupa) pupa  adult Rhodnius prolixus (Slide 8) Rhodnius prolixus: a blood-sucking parametabolous insect that has five instars before adult stage * Each one shows a stage of instar (they are all in order) This insect has 5 stages before it becomes an adult Note difference in size between instar stages, especially the head * Hormones play a huge role in metamorphic change Lecture 5 – role of hormones Decapitation experiment (slide 9) • Experiment is preformed by Wigglesworth (1934) • Method: decapitated first instar Rhodnius larva and fused the body with head of fifth instar larva st * First the 1 instar is taken and the head is pulled off of the body then the body of the 1 instar is fused with the head of the 5 instar using st wax. The 1 instar shows changes similar to those in the 5 instar This indicates that the 5 instar head was transferring a molecule to the body • Result: first instar larva developed the cuticle, body structure, and genitalia of the adult • Conclusion: molecules in blood (we now know as hormones) are responsible for the induction of metamorphosis Experiments by Wigglesworth (slide 10) Corpora allota are two tiny glands behind the insect brain Experiment 2: remove corpora allota from 3 instar larva (*normally the 3 instar rd th should have went on to the 4 instar) Result: Larva matured into adult Experiment 3: Transplant corpora allota from 4 instar larva into 5 instar larva Result: 5 instar larva molted into extremely large 6 instar larva Conclusion: Corpora allota induces molting into the next instar stage Hormones involved in insect metamorphosis (slide 11) Prothoracicotropic Hormone (PTTH) • Secreted by the neurosecretory cells in the brain in response to neural, hormonal or environmental factors • Initiates molting, but is also essential for pupa formation • Acts on prothoracic glands causing ecdysone secretion Hormones involved in insect metamorphosis (slide 12) Ecdysone • Steroid hormone secreted by prothoracic glands in ‘waves/pulses’ • Prothoracic glands degenerate in the adult • Induces either molting or pupation depending on levels of juvenile hormone Lecture 5 – role of hormones Hormones involved in insect metamorphosis (slide 13) Juvenile Hormone • Secreted by corpora allata during larval molts (inactive during metamorphic molt into final adult form) Prevents final stage of metamorphosis • High JH = molting • Low JH = pupation/maturation • As long as JH is present, ecdysone will result in a new larval instar Effects of ecdysone depends on levels of JH (Slide 14) During molting the levels of juvenile hormone are high Hormonal control of insect metamorphosis (slide15) *PTTH is released from the neurosecretory cells, which induces the prothoracic glands to release the hormone ecdysone ,which acts on the different stages of insect metamorphosis. Juvenile Hormones are produced by the corpus allatum. Increased JH from the larvae to the later larvae Increased JH from the later larvae stages to the pupa stage Decreased JH pupa to the butterfly stage. Lecture 5 – role of hormones Amphibian metamorphosis (slide 16) • Amphibians undergo metamorphic change from tadpole to the adult stage • Many organ systems of the tadpole undergo change during metamorphosis * Egg  tadpole  tadpole with (hind legs)  tadpole with front and hind limbs  frog with shrunken tail  frog (breaths through lungs) * Note that the body size increases throughout the changes * At first they live in the water and are vegetarians and then they become semi carnivorous (insects)
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