Caenorhabditis is a genus ofnematodes which live in bacteria-rich environments likecompost piles, decaying dead animals and rotting fruit. The name comes from Greek: caeno- (καινός (caenos) = new, recent); rhabditis = rod-like (ῥάβδος (rhabdos) = rod, wand).
The genusCaenorhabditis contains the notedmodel organismCaenorhabditis elegans and several other species for which agenome sequence is either available or currently being determined. The two most-studied species in this genus (C. elegans andC. briggsae) are bothandrodioecious (they have male andhermaphrodite sexes) whereas most other species aregonochoristic (they have male and female sexes).[2]
C. elegans is the type species of the genus.[3] In 1900,Maupas initially named the speciesRhabditis elegans, Osche placed it in thesubgenusCaenorhabditis in 1952, and in 1955, Dougherty raisedCaenorhabditis to the status ofgenus.[4]
Caenorhabditis occupy various nutrient and bacteria rich environments. They do not form self-sustaining populations in soil, as it lacks enough organic matter. Juvenile worms and alsodauer larvae can be transported by invertebrates includingmillipedes,insects,isopods, andgastropods. Some species also appear to be associated with vertebrates includingzebu cattle, although the nature of this association is not clear. The species can be classified as 'phoretic' or 'necromenic' based on their relationships to their invertebrate hosts. A phoretic worm rides on the host until it finds a favorable environment, and then leaves. A necromenic worm waits for the host to die, and lives on the bacteria which thrive in the dead animal. Many species are capable of both phoretic and necromenic lifestyles.[5]
There are about 50 known species in this genus, some of them not yet formally described and named,[6] in spite of 15 of the species being named in one article 2014.[7] Based onITS2 sequence comparison, these can be grouped like this:[7]
'Elegans' supergroup
Caenorhabditis inopinata - Prior to 2017 referred to asC. sp. 34. A gonochoristic (male-female) species was isolated from figs and fig wasps. Its genome is being sequenced at theUniversity of Miyazaki[8]
Caenorhabditis briggsae - genome sequence finished 2003 at Washington University in St. Louis.[11]C. briggsae is the second-best studied species in the genus. WhileC. briggsae are also mostly XX protandrous hermaphrodites, they are not the closest relatives toC. elegans, and the hermaphroditic reproductive strategy of these species, as well asC. tropicalis, is an example ofconvergent evolution. The evolutionary distance betweenC. briggsae andC. elegans is similar to that of humans and mice.C. nigoni is the closest relative ofC. briggsae, and the two species can occasionally produce somewhat fertile hybrids.[12]
Caenorhabditis remanei - genome sequenced byWashU GSC.[13] More closely related toC. briggsae thanC. elegans,C. remanei is agonochoristic (male-female obligate) species in theElegans group. In the past, there was some confusion about placement of strains betweenC. remanei,C. vulgaris (now seen as a subspecies ofC. remanei) andC. brenneri"".[14]
Caenorhabditis brenneri - (prior to 2007 referred to asC. sp 4,C. sp CB5161, andC. sp PB2801) - genome sequenced by WashU GSC.[15] This gonochoristic species is found in theElegans group, closer toC. briggsae thanC. elegans.[16]
Caenorhabditis tropicalis - Prior to 2014 referred to asC. sp. 11[7] Similar toC. elegans andC. briggsae,C. tropicalis populations are made up of XX protandrous hermaphrodites and X0 males. These three species arenot each other's closest relatives
Caenorhabditis japonica - genome being sequenced by WashU GSC.[17] This gonochoristic species is found in theJaponica group, the sister clade to theElegans group. In the wild, this species is found non-parasitically associated with the burrower bugsParastrachia japonensis and may be able to enter the dauer stage regardless of food and crowding conditions.[18][19]
Caenorhabditis afra - (also referred to asC. sp. 7,C. sp. JU1199 andC. sp. JU1286). This gonochoristic (male-female) species was isolated by Matthias Herrmann in Begoro, Ghana, Africa in 2007.[20][7] Its genome is being sequenced at WashU.[21]
'Drosophilae' supergroup : group of species generally found on rotten fruits and transported byDrosophila flies
Caenorhabditis angaria - (prior to 2011 referred to asC. sp. 2,C. sp. 3, andC. sp. PS1010)[22] - genome sequenced at the California Institute of Technology in 2010.[23] This gonochoristic species, found in theAngaria group of theDrosophilae super-group, has distinct morphology and behavior compared toC. elegans. Notably,C. angaria males exhibit a spiral mating behavior. Its divergence fromC. elegans is similar to the distance between humans and fish.C. castelli is its closest relative, and the two species can produce F1 hybrids.[24]
TheCaenorhabditis species group with the 'Protorhabditis' group, containing species in the generaProtorhabditis,Diploscapter andProdontorhabditis, on the one hand, and withOscheius species, on the other hand, to form the 'Eurhabditis' group ofRhabditidae genera.[25]
Members ofCaenorhabditis exclusively share 39conserved signature indels that are found in the conserved regions of various proteins, such as the Rab44 protein and apoly ADP-ribose glycohydrolase protein (PARG-1), and are specifically located on surface-exposed loops.[26] These molecular markers help distinguish this genus from all other species, and their presence on surface-exposed loops suggest implications in protein-protein or protein-ligand interactions.[26]
^Ellsworth C. Dougherty (1955) The Genera and Species of the Subfamily Rhabditinae Micoletzky, 1922 (Nematoda): a Nomenclatorial Analysis—including an Addendum on the Composition of the Family Rhabditidae Örley, 1880. Journal of Helminthology 29(3): 105-152doi:10.1017/S0022149X00024317
^Sudhaus, Walter; Kiontke, Karin; Giblin-Davis, Robin M. (2011). "Description of Caenorhabditis angaria n. sp. (Nematoda: Rhabditidae), an associate of sugarcane and palm weevils (Coleoptera: Curculionidae)".Nematology.13 (1):61–78.doi:10.1163/138855410X500334.
^The phylogenetic relationships of Caenorhabditis and other rhabditids. Karin Kiontke and David H. A. Fitch, Wormbook, 2005,doi:10.1895/wormbook.1.11.1
