Metamorphosis in Insects and Amphibians

Endocrine cells release protein and non-protein hormones

Synthesis of hormones is orchestrated by the CNS

Hormones effects are tissue dependent

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

Metamorphosis in InsectsThe transformation of an immature insect from a larva to a pupa to an adult

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

Hormonal Control of Insect Metamorphosis

BrainTemperature, Light, Stress, etc.

Prothoracic Gland

Ecdysteroid

Corpus Allatum

Prothoracicotropic hormone (PTTH)

Juvenile Hormone (JH)

LarvaPupa Adult

Control of Metamorphosis by Internal and External Factors

Temperature (day degrees)

Critical size matched (availability of food)

Light (photoperiod)

Chemicals

Amount of moisture

Stress: mutagens, predators, etc.

BrainTemperature, Light, Stress, etc.

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

Ec

EcR

USPBinding Site

Early Response:

Transcription Factors

Late Response: Transcription initiated by

Transcription Factors

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

Alternative Splicing of Ecdysone Receptor Pre-mRNA Creates Several Forms of the Ec-

Receptor Allowing Cell Type Specific Ecdysone Response

Eclosion HormonePeptide 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

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)

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)

Regulation of JH LevelsA

mou

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f Hor

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Larva Pupa Adult

Juvenile Hormone

Ecdysone

Metamorphosis in Action: Remember Imaginal Discs?

Immature Insect: + JH

Adult Insect: - JH

Hox Gene Expression Determines Leg Segments

Which Stage would You Target?

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

Frogs: Tadpole to Adult

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

Pituitary

Thyroid

Thyroid Hormones (T3 and T4)

Transcriptional Activation

Metamorphosis

T3

TR

RXRBinding Site

Late Response: Transcription initiated by

Transcription Factors

Number of Receptors in Affected Tissue Amount of Hormone

HIGH

LOW HIGH

LOW

Early Response:

Transcription Factors

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

Species Diversity: Variation in Metamorphosis