Dauer larva

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Dauer (German for "endurance", "duration" or "permanent") describes an alternative developmental stage of nematode worms, particularly Caenorhabditis elegans, whereby the larva goes into a type of stasis and can survive harsh conditions.[1] Since the decision to enter the dauer stage is dependent on environmental cues, it represents a classic and well studied example of polyphenism [2] .[3] The Dauer state is given other names in the various types of nematodes such as ‘diapause’, ‘hyperbobis’, ‘spring rise’, but since the C. elegans nematode has become the most studied nematode, the term ‘dauer stage’ or 'dauer larvae' is becoming universally recognised when referring to this state in other free-living nematodes. The dauer stage is also considered to be equivalent to the infective stage of parasitic nematode larvae.

Under environmental conditions that are favorable for reproduction, C. elegans larvae develop through 4 stages or molts which are designated as L1, L2, L3 and L4. After L4, animals moult to the reproductive adult stage. However, when the environment is unfavorable, L1 and L2 animals have the option to divert their development from reproduction to dauer formation. Signals such as temperature, food supply, and a dauer-inducing pheromone (population density cue) influence this dauer decision. Dauer larvae are thus considered an alternative L3 stage larva, and this stage is sometimes referred to as L2d. L2d animals are also considered pre-dauer and are characterised by delayed development and dark intestines produced by storage of fat.
Dauer larvae are extensively studied by biologists because of their ability to survive harsh environments and live for extended periods of time. For example, C. elegans dauer larvae can survive up to 4 months, much longer than their average lifespan of about 3 weeks during normal reproductive development.[4] Two genes that are essential for dauer formation are daf-2 and daf-23 .[5] Dauer formation in C. elegans requires a nuclear receptor DAF-12 and a forkhead transcription factor daf-16. In favorable environments, DAF-12 is activated by a steroid hormone, called Dafachronic Acid, produced by the cytochrome p450, DAF-9. DAF-9 and DAF-12 have been implicated by Cynthia Kenyon and colleagues as being required for extended longevity seen in animals that lack germlines. Kenyon showed that, although the daf-16 gene is required for life extension in C. elegans, the life extension effect can be uncoupled from dauer growth arrest.[6] The lifespan increase was shown to be associated with an increase in stress resistance.[7]

A characteristic of the dauer stage is the pronounced alae which may be implicated in the entering (L1) and exiting (pre adult or L4 in C. elegans) of the dauer stage.

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References[edit]

  1. ^ Riddle DL, Swanson MM, Albert PS (1981). "Interacting genes in nematode dauer larva formation". Nature (journal) 290 (5808): 668–671. doi:10.1038/290668a0. PMID 7219552. 
  2. ^ Hu, Patrick J. (2007). "Dauer". WormBook. doi:10.1895/wormbook.1.144.1. ISSN 1551-8507. Retrieved 2009-11-05. 
  3. ^ Sommer, Ralf J.; Akira Ogawa (2011-09). "Hormone Signaling and Phenotypic Plasticity in Nematode Development and Evolution". Current Biology 21: R758–R766. doi:10.1016/j.cub.2011.06.034. ISSN 0960-9822. Retrieved 2011-09-28. 
  4. ^ Mayer, Melanie G.; Ralf J. Sommer (2011). "Natural variation in Pristionchus pacificus dauer formation reveals cross-preference rather than self-preference of nematode dauer pheromones". Proceedings of the Royal Society B: Biological Sciences 278 (1719): 2784–2790. doi:10.1098/rspb.2010.2760. Retrieved 2011-09-09. 
  5. ^ Gottlieb S, Ruvkun G (1994). "daf-2, daf-16 and daf-23: genetically interacting genes controlling Dauer formation in Caenorhabditis elegans". Genetics (journal) 137 (1): 107–120. PMC 1205929. PMID 8056303. 
  6. ^ Kenyon C, Chang J, Gensch E, Rudner A, Tabtiang R (1993). "A C. elegans mutant that lives twice as long as wild type". Nature (journal) 366 (6454): 461–464. doi:10.1038/366461a0. PMID 8247153. doi:10.1038/366461a0. 
  7. ^ Lithgow GJ, White TM, Melov S, Johnson TE (1995). "Thermotolerance and extended life-span conferred by single-gene mutations and induced by thermal stress" (PDF). Proceedings of the National Academy of Sciences of the United States of America 92 (16): 7540–7544. doi:10.1073/pnas.92.16.7540. PMC 41375. PMID 7638227. 

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