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=== Post-embryonic development === [[File:C_elegans_developmental_stages.svg|thumb|300px|right| Anatomy and scale of ''C. elegans'' developmental stages]] [[File:C_elegans_life_cycle.svg|thumb|300px|right| Life cycle and developmental stages of ''C. elegans'']] Under environmental conditions favourable for [[reproduction]], hatched [[larvae]] develop through four larval stages - L1, L2, L3, and L4 - in just 3 days at 20 Β°C. When conditions are stressed, as in food insufficiency, excessive population density or high temperature, ''C. elegans'' can enter an alternative third larval stage, L2d, called the [[dauer larva|dauer]] stage (''Dauer'' is German for permanent). A specific dauer pheromone regulates entry into the dauer state. This pheromone is composed of similar derivatives of the 3,6-dideoxy sugar, [[ascarylose]]. Ascarosides, named after the ascarylose base, are involved in many sex-specific and social behaviors.<ref>{{Cite journal|last1=Ludewig|first1=Andreas H.|last2=Schroeder|first2=Frank C.|date=2013-01-18|title=Ascaroside signaling in C. elegans|journal=WormBook|pages=1β22|doi=10.1895/wormbook.1.155.1|issn=1551-8507|pmc=3758900|pmid=23355522}}</ref> In this way, they constitute a chemical language that ''C. elegans'' uses to modulate various phenotypes. Dauer larvae are stress-resistant; they are thin and their mouths are sealed with a characteristic dauer cuticle and cannot take in food. They can remain in this stage for a few months.<ref name="Introduction to C. Elegans">{{cite web |title=Introduction to ''C. Elegans'' |url=http://avery.rutgers.edu/WSSP/StudentScholars/project/introduction/worms.html |work=C. Elegans as a model organism |publisher=Rutgers University |url-status=dead |archive-url=https://web.archive.org/web/20020818213550/http://avery.rutgers.edu/WSSP/StudentScholars/project/introduction/worms.html |archive-date=2002-08-18 |access-date=August 15, 2014}}</ref><ref>{{Cite web|url=http://www.wormatlas.org/hermaphrodite/introduction/mainframe.htm|title = Handbook - Introduction}}</ref> The stage ends when conditions improve favour further growth of the larva, now moulting into the L4 stage, even though the gonad development is arrested at the L2 stage.<ref>{{Cite web|url=http://www.wormbook.org/chapters/www_dauer/dauer.html|title=Dauer|website=www.wormbook.org|access-date=2018-09-27}}</ref> Each stage transition is punctuated by a molt of the worm's transparent cuticle. Transitions through these stages are controlled by genes of the heterochronic pathway, an evolutionarily conserved set of regulatory factors.<ref name="pmid20232378">{{cite journal | vauthors = Resnick TD, McCulloch KA, Rougvie AE | title = miRNAs give worms the time of their lives: small RNAs and temporal control in ''Caenorhabditis elegans'' | journal = Developmental Dynamics | volume = 239 | issue = 5 | pages = 1477β89 | date = May 2010 | pmid = 20232378 | pmc = 4698981 | doi = 10.1002/dvdy.22260 }}</ref> Many heterochronic genes code for [[microRNA]]s, which repress the expression of heterochronic [[transcription factor]]s and other heterochronic miRNAs.<ref name="pmid23962842">{{Cite book | vauthors = Rougvie AE, Moss EG | title = Developmental transitions in ''C. elegans'' larval stages | chapter = Developmental Transitions in C. Elegans Larval Stages | journal = Current Topics in Developmental Biology | series = Developmental Timing | volume = 105 | pages = 153β80 | year = 2013 | publisher = Academic Press | pmid = 23962842 | doi = 10.1016/B978-0-12-396968-2.00006-3 | url = https://pubmed.ncbi.nlm.nih.gov/23962842 | isbn = 9780123969682 }}</ref> miRNAs were originally discovered in ''C. elegans.''<ref name="pmid8252621">{{cite journal | vauthors = Lee RC, Feinbaum RL, Ambros V | title = The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14 | journal = Cell | volume = 75 | issue = 5 | pages = 843β54 | date = December 1993 | pmid = 8252621 | doi = 10.1016/0092-8674(93)90529-y | doi-access = free }}</ref> Important developmental events controlled by heterochronic genes include the division and eventual [[syncytium|syncitial]] fusion of the hypodermic seam cells, and their subsequent secretion of the alae in young adults. It is believed that the heterochronic pathway represents an evolutionarily conserved predecessor to [[circadian rhythm|circadian clocks]].<ref name="pmid15691769">{{cite journal | vauthors = Banerjee D, Kwok A, Lin SY, Slack FJ | title = Developmental timing in ''C. elegans'' is regulated by kin-20 and tim-1, homologs of core circadian clock genes | journal = Developmental Cell | volume = 8 | issue = 2 | pages = 287β95 | date = February 2005 | pmid = 15691769 | doi = 10.1016/j.devcel.2004.12.006 | doi-access = free }}</ref> Some nematodes have a fixed, genetically determined number of cells, a phenomenon known as [[eutely]]. The adult ''C. elegans'' hermaphrodite has 959 somatic cells and the male has 1033 cells,<ref>{{Cite journal|last1=Sulston|first1=J.E.|last2=Horvitz|first2=H.R.|date=March 1977|title=Post-embryonic cell lineages of the nematode, Caenorhabditis elegans|url=https://linkinghub.elsevier.com/retrieve/pii/0012160677901580|journal=Developmental Biology|language=en|volume=56|issue=1|pages=110β156|doi=10.1016/0012-1606(77)90158-0|pmid=838129}}</ref><ref>{{Cite journal|last1=Sulston|first1=J.E.|last2=Schierenberg|first2=E.|last3=White|first3=J.G.|last4=Thomson|first4=J.N.|date=November 1983|title=The embryonic cell lineage of the nematode Caenorhabditis elegans|url=https://linkinghub.elsevier.com/retrieve/pii/0012160683902014|journal=Developmental Biology|language=en|volume=100|issue=1|pages=64β119|doi=10.1016/0012-1606(83)90201-4|pmid=6684600}}</ref><ref name="Sammut 385β390">{{Cite journal|last1=Sammut|first1=Michele|last2=Cook|first2=Steven J.|last3=Nguyen|first3=Ken C. Q.|last4=Felton|first4=Terry|last5=Hall|first5=David H.|last6=Emmons|first6=Scott W.|last7=Poole|first7=Richard J.|last8=Barrios|first8=Arantza|date=October 2015|title=Glia-derived neurons are required for sex-specific learning in C. elegans|journal=Nature|language=en|volume=526|issue=7573|pages=385β390|doi=10.1038/nature15700|issn=0028-0836|pmc=4650210|pmid=26469050|bibcode=2015Natur.526..385S}}</ref> although it has been suggested that the number of their intestinal cells can increase by one to three in response to gut microbes experienced by mothers.<ref>{{Cite journal|last1=Ohno|first1=Hayao|last2=Bao|first2=Zhirong|date=2020-11-14|title=Small RNAs couple embryonic developmental programs to gut microbes|journal=bioRxiv|url=http://biorxiv.org/lookup/doi/10.1101/2020.11.13.381830|language=en|doi=10.1101/2020.11.13.381830|s2cid=227060212}}</ref> Much of the literature describes the cell number in males as 1031, but the discovery of a pair of left and right MCM neurons increased the number by two in 2015.<ref name="Sammut 385β390"/> The number of cells does not change after cell division ceases at the end of the larval period, and subsequent growth is due solely to an increase in the size of individual cells.<ref>{{cite book |title=Invertebrate Zoology |edition=7th |last1=Ruppert |first1=Edward E. |last2=Fox | first2=Richard S. |last3=Barnes |first3=Robert D. | name-list-style = vanc |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=753 }}</ref>
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