1. Hormone Signaling and Phenotypic Plasticity in Nematode Development and Evolution 
Ralf J. Sommer, Akira Ogawa 
Current Biology 
Volume 21, Issue 18, Pages R758-R766 (September 2011)
DOI: /j.cub
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2. Figure 1
Life cycle of C. elegans as an example of phenotypic plasticity in life-history adaptations.
C. elegans has a simple life cycle that can be completed in three days in the laboratory with a sufficient food supply and a temperature of 20°C. The self-fertilizing hermaphrodite lays eggs, which hatch into an L1 larva. The four larval stages are separated by molts. Larval growth is primarily due to the enormous increase in the reproductive organ, the gonad. Adults can lay more than 200 eggs and can live for several weeks. When conditions become harsh, animals will redirect their development and form an arrested dauer larva, an alternative L3 stage.
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3. Figure 2
Morphological similarities between dauer larvae of free-living nematodes and infective juveniles of parasitic nematodes.
The dauer larva of P. pacificus (top) and the infective juvenile of the parasitic species S. papilossus (bottom) show striking similarities in morphology.
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4. Figure 3
Small molecules and their role in the regulation of C. elegans dauer formation.
The control of dauer formation in C. elegans involves two classes of small chemicals. (A) Structures of ascarosides that function as dauer pheromone in C. elegans. (B) Structures of two bile acid-like steroids, Δ-4 and Δ-7 dafachronic acid, that function as conserved hormones in dauer regulation in C. elegans and P. pacificus, as well as in the control of infective juvenile formation in various parasites.
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5. Figure 4 Model of signaling pathways controlling the dauer stage.
A network of signaling processes regulates dauer formation in C. elegans with small molecules playing important roles. The ‘dauer pheromone’ is a blend of glycosides (ascarosides) indicating the population density of worms. Downstream of insulin and TGF-beta signaling, sterol-derived dafachronic acid hormones interact with the nuclear hormone receptor DAF-12 to determine non-dauer development. After depletion of the hormone, DAF-12 acts as a switch and regulates dauer entry.
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6. Figure 5
The Strongyloides life cycle as an example of phenotypic plasticity in nematode parasitism.
Members of the parasitic genus Strongyloides have alternative direct or indirect life cycles. This unique situation represents an example for phenotypic plasticity between free-living and parasitic life styles in nematodes.
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7. Figure 6
Mouth dimorphism in Pristionchus pacificus as an example of phenotypic plasticity.
The buccal cavity of P. pacificus and related nematodes can occur in two forms. The eurystomatous form (right) has a claw-like denticle and an extra ventral denticle (arrow), the latter being absent in stenostomatous (left) animals.
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