1. Metamorphosis in Insects and Amphibians

2. ENDOCRINE- describing or relating to any gland or other group of cells that synthesizes hormones and secretes them directly into the blood, lymph, or other intercellular fluid
Endocrine cells release protein and non-protein hormones
Synthesis of hormones is orchestrated by the CNS
Hormones effects are tissue dependent

3. Metamorphosis in Insects
The transformation of an immature insect from a larva to a pupa to an adult

4. Evolution of Metamorphosis
Complete metamorphosis occurs only in higher insects; it evolved only once
Early expression of juvenile hormone suppresses the development of adult characteristics
Larval and adult forms can occupy different environments and consume different food sources
Lack of competition between larva and adult allows species success and diversification
Metamorphosis can serve as a model for understanding how shifts in protein production can create different body forms

6. Hormonal Control of Insect Metamorphosis
Temperature, Light, Stress, etc.
Brain
Prothoracicotropic hormone (PTTH)
Corpus Allatum
Prothoracic Gland
Juvenile Hormone (JH)
Ecdysteroid
Pupa
Adult
Larva

7. Control of Metamorphosis by Internal and External Factors
Temperature, Light, Stress, etc.
Brain
Temperature (day degrees)
Critical size matched (availability of food)
Light (photoperiod)
Chemicals
Amount of moisture
Stress: mutagens, predators, etc.

8. Ecdysone: “Molting Hormone”
Steroid hormone produced by prothoracic gland (lipid soluble, passes through cell membrane to the nucleus)
Activates early response genes (TFs) and then late response genes (may cause differentiation,cell proliferation and migration, structural changes, apoptosis)
Primes insect to respond to second hormone, EH
EcR Ec
Early Response: Transcription Factors
Late Response: Transcription initiated by Transcription Factors
USP
Binding Site

9. Chromosome Puffing in Flies
Observed in giant salivary gland chromosomes (no cell division after replication)
Can be inhibited by actinomycin
Puffing is where transcription is occurring.
Ecdysone can be detected by fluorescent antibodies localized to the puffing
Early puffs and late puffs seen in larva to pupa and pupa to adult molt

10. Alternative Splicing of Ecdysone Receptor Pre-mRNA Creates Several Forms of the Ec-Receptor Allowing Cell Type Specific Ecdysone Response

11. Eclosion Hormone
Peptide hormone (water soluble and binds cell receptors)
Released by tracheal endocrine cells in response to EH
Binds cell receptor and results in increase in levels of cGMP, a secondary messenger
Increases cuticle extensibility and prompts ecdysis-specific behavior

12. Juvenile Hormone Major endocrine factor controlling metamorphosis
Released from corpus allata (paired endocrine glands)
Methylated to make active form
JH can pass through plasma membrane and influence gene expression (like a steroid hormone)
Can also act as a peptide hormone by activating second messengers (cAMP, etc.)
Maintains larval features by repressing adult genes
Modifies effects of ecdysone (prevents changes in gene expression)

13. Regulation of JH Levels
Juvenile Hormone
Ecdysone
Amount of Hormone
Larva Pupa Adult
Low = larva stage; Medium JH levels = pupa stage; No JH = adult stage
Rate of release limited by synthesis
Amounts of JH also regulated by protein degradation and methyltransferase levels (can be protected by JH binding proteins, degraded by JH esterase)

14. Metamorphosis in Action: Remember Imaginal Discs?
Adult Insect: - JH
Immature Insect: + JH

15. Hox Gene Expression Determines Leg Segments

16. Which Stage would You Target?

17. Insect control by targeting metamorphosis
Juvenile hormone mimic: Keep insects in larval stage
-- Effective control for insects such as mosquitoes
Juvenile hormone antagonist: Cause death of larva or early metamorphosis
-- Effective control for crop pests such as hornworm
Genes for juvenile hormone binding hormone and JH esterase have been identified

18. Frogs: Tadpole to Adult

19. Hormonal Control of Frog Metamorphosis
Secretions of two hormones, thyroxine (T4) and triiodothyronine (T3) cause metamorphic changes
Hormones have different effects depending on location in body
Timing of changes regulated by tissue dependent hormone sensitivity
Thyroid receptor is transcriptional repressor until thyroid hormone binds causing it to become a transcriptional activator
Positive feedback loop is established between thyroid hormone and pituitary gland allowing incremental increases in hormone concentration

20. Metamorphosis Pituitary Thyroid Thyroid Hormones (T3 and T4)
Transcriptional Activation
Thyroid Hormones (T3 and T4)

21.  HIGH LOW LOW HIGH T3 Early Response: Transcription Factors
Number of Receptors
in Affected Tissue
Amount of Hormone
LOW
HIGH TR T3
Early Response: Transcription Factors
Late Response: Transcription initiated by Transcription Factors 
RXR
Binding Site

22. TH does not determine the developmental program, but initiates it
Changing the location of tissue or organ does not alter its response to TH
Transplant eye to tail region
Differentiates & grows into eye in response to TH while tail regresses
Transplant tail to trunk
Tail regresses while limb grows

23. Species Diversity: Variation in Metamorphosis