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(A)
Favorable environment
3 5 days
2 3 weeks
Egg
L1
L2
L3
L4
Adult
Unfavorable
environment
dauer
Several months
(B)
Neuroendocrine cells
ASI
ADF
ASG
ASJ
ASK
AWA
AWC
Others
XXX amphids
Head
Pharynx
Tail
XXX
Figure 3.42 (A) C. elegans life cycle. In favorable environments, C. elegans undergoes
reproductive development and progresses rapidly from embryo through four larval stages (L1-L4)
to reach adulthood in 3-5 days (in temperatures of 15-20°C). Adults then live another 2-3 weeks.
In unfavorable conditions, including overcrowding, limited food, and high temperature, C. elegans
undergoes development to a specialized third larval stage called the dauer diapause (L 3 d) , which
can live several months. Upon return to a favorable environment, dauer larvae recover and become
reproductive adults with normal life spans. (B) Schematic of neuroendocrine cells. Integration of
environmental cues (dauer pheromone, nutrients, and temperature) are transformed into endocrine
signals by amphid neurons (ASI, ADF, ASG, ASJ, ASK, AWA, and AWC). Serotonergic signaling
from ADF and cGMP signaling from ASJ and ASK influence the production of TGF-β from
ASI and the ILPs from ASI, ASJ, and other cells. Insulin/IGF-1 signaling and TGF-β signaling
converge on steroid hormone signaling in the XXX cells.
Source : From Fielenbach and Antebi (2008) .
DAF-2 is the worm's only insulin/IGF-1-like receptor and is similar to vertebrate
IGF receptors (Foxo). Mutations in the gene daf- 2, or the loss of function of that
gene, lead to doubling the life span of the mutant worm compared to the wild type
( Kenyon et al., 1993 ). Mutations in daf- 2 lead to the movement of DAF-16 into the
cell nuclei, where it activates a signal cascade for a longer life span and stress resist-
ance ( Bartke, 2009 ). At a molecular level, the perception of unfavorable conditions
or the simple perception of reduced food odorants activates the DAF-3/DAF-5 com-
plex that specifies the dauer program ( Fielenbach and Antebi (2008) .
It is believed that amphid sensory neurons decode the environmental information
on the availability of food resources, transforming it into instructions for adaptively
switching between the normal and dauer programs of development ( Pletcher, 2009 ).
In Drosophila as well, the insulin-like signaling pathway is fundamentally
involved in determining the fly's life span. Ablation of the neurons that secrete
insulin-like neuropeptides (Dilp2, Dilp3, and Dilp5) in the pars intermedialis of the
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