Metamorphosis in Insects and Amphibians
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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
nt o
f Hor
mon
e
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
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