Download Metamorphosis

Metamorphosis in
Insects and Amphibians
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
Metamorphosis in Insects
The 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
Temperature,
Light, Stress,
etc.
Brain
Prothoracicotropic
hormone (PTTH)
Corpus Allatum
Prothoracic Gland
Juvenile
Hormone (JH)
Ecdysteroid
Larva
Pupa
Adult
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.
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
USP
Binding 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 PremRNA Creates Several Forms of the EcReceptor Allowing Cell Type Specific Ecdysone
Response
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 ecdysisspecific 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)
Regulation of JH Levels
Amount of Hormone
Juvenile Hormone
Ecdysone
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)
Metamorphosis in Action: Remember Imaginal Discs?
Adult Insect: - JH
Immature 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
Metamorphosis
Thyroid
Thyroid Hormones
(T3 and T4)
Transcriptional
Activation
HIGH
LOW
Number of Receptors
in Affected Tissue
Amount of Hormone
LOW
HIGH
T3
RXR
Binding Site
Early
Response:
Transcription
Factors

Late Response:
Transcription
initiated by
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