Lecture notes from the BIOL2018 adaptive physiology lecture insect metamorphosis. Covering: Insecta, general endocrine model of metamorphosis, Biosynthesis, Ecdysteroids and Juvenile Hormones.
Insect Metamorphosis
Insecta
Approx. 26 orders, most species diversity, oldest fossil 400mya (Devonian)
All terrestrial habitats (even beetles in moss beds in Antarctica); some freshwater,
few marine, and lots of flying insects
Reasons for insect diversity
Evolution of wing/flight – escape predators – dispersal to new habitats
Diversification of mouth parts for feeding on plants
Plants and insects’ adaptive radiation – parallel 100mya until 50mya – pollinators of
angiosperms
General Endocrine Model of Metamorphosis
PTTH from brain is released by the CC
PTTH stimulates the PG to produce E
E is released periodically during molting
JH from the CA determines the type of molt
PTTH (Prothoacicotropic hormone)
PTTH acts on the prothoracic glands (PGs) to regulate the synthesis of ecdysteroids
Release – environmental stimuli such as photoperiod, temperature – nervous stimuli
such as stretch receptors – Via haemolymph in Bombyx, Manduca – Direct PG
innervation in Drosophila
Mode of action – Via Receptor Tyrosine Kinase
Daily Ecdysome rythms in Rhodnius
Lateral ventral clock neurons (LNv) signal to PTTH-producing neurons.
Light and PTTH signals entrain the PG cells for ecdysone secretion, resulting in
rhythmic release of the hormone.
The ecdysone receptor, EcR, anticipates these daily rhythms by undergoing cyclic
nucleocytoplasmic oscillations in target cells.
PG: prothoracic gland, PTTH: prothoracicotropic hormone.
Control of E biosynthesis in Drosophila PG
Multiple signaling cascades that regulate ecdysone biosynthesis in the prothoracic
gland.
Inactivation of these pathways could compromise ecdysone production and results
in developmental defects.
Arrows represent positive regulations, and cross lines indicate inhibitory effects.
P, phosphorylation; NO, nitric oxide radical.
Cholesterol is converted into the prohormone ecdysone via a series of reactions that
occur in the ER, cytosol and mitochondria, represented here by stars, and exported
possibly by secretory vesicles into hemolymph.
The early gene broad (isoform BR-Z4) positively regulates the expression of Npc1a,
which encodes a cholesterol transporter mediating the trafficking of cholesterol out
of late endosomes into the ER, a critical step toward the synthesis of ecdysone.
LDLR, low-density lipoprotein receptor; MT, mitochondria; ER, endoplasmic
reticulum; N, nucleus; PG, prothoracic gland.
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