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 Pupa Adult Larva

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 EcR Ec Early Response: Transcription Factors Late Response: Transcription initiated by Transcription Factors USP Binding Site

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 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

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 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)

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

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

 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

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