Dauer (German "", English "the endurance", "persistence"; "unlimited time" ) describes an alternative developmental stage of nematode worms, particularly rhabditids including Caenorhabditis elegans, whereby the larva goes into a type of stasis and can survive harsh conditions. Since the entrance of the dauer stage is dependent on environmental cues, it represents a classic and well studied example of polyphenism. The dauer state is given other names in the various types of nematodes such as ‘diapause’ or ‘hypobiosis’, 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. As Émile Maupas first proposed in 1899 and 1900, all nematodes have five stages separated by four moults. Under environmental conditions that are favorable for reproduction, C. elegans larvae develop through four stages or moults 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 levels of a dauer-inducing pheromone, a population density cue, influence this dauer decision. Dauer larvae are thus considered an alternative L3 stage larva, and this stage is sometimes preceded by L2d. L2d animals are considered pre-dauer and are characterised by delayed development and dark intestines produced by storage of fat. L2d larvae can either continue normal development or enter dauer stage depending on whether the conditions that triggered their formation persist. Dauer is not, however, a permanent condition. In fact, if the food supply and the population density become optimal for growth the dauer larvae can exit this stage and become L4s and then adults. 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 four months, much longer than their average lifespan of about three weeks during normal reproductive development. Two genes that are essential for dauer formation are daf-2 and daf-23. 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. The lifespan increase was shown to be associated with an increase in stress resistance. 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. The cuticle is thick and contains a unique striated zone in its basal area. Dauer larvae generally remain motionless, but can react to touch or vibrations. They can stand on their tails, waving their bodies in the air, and attach themselves to any passing animals, particularly insects, enabling them to travel to new food sources. For example, dauer larvae of rhabditids are often found in parallel rows under the elytra of dung beetles, which transport them to fresh supplies of dung. C. elegans strains lacking polyunsaturated fatty acids (PUFAs) undergo increased dauer arrest when grown without cholesterol. A study found endocannabinoids inhibit the dauer formation caused by PUFA deficiency or impaired cholesterol trafficking.

Parasitism in dauer larva

The dauer hypothesis

The dauer hypothesis is a theory of evolutionary parasitism, named after the alternative, “dauer” stages of nematode development. It proposes that free-living nematode lineages evolved into parasites through two major steps, phoresy, and necromeny. Models of parasitic evolution are difficult to confirm because they are difficult to test. Like other methods of studying evolution, researchers can make use of genomic data, specifically while comparing data from closely related, non-parasitic species. Parasitism is common, and it is even more common in nematodes, which have evolved into parasitism on up to eighteen separate occasions throughout their evolutionary history. This calls into question what exactly about the nematode leads to such an inclination toward parasitism.

Theory development

The hypothesis was developed from the observation that roundworms, or nematodes, undergo the same four larval stages, some species only differing by having extra components to their life cycle, leading them to an optional alternative life stage during times of high stress. In some species this alternative stage leads to dormancy, pausing organism development until conditions are more favorable, and in others that alternative stage is used for group dispersion between different habitats through carrier animals. In both of these cases, the alternative stage is called the dauer. In parasitic species of nematodes, this alternative stage is called the “infective juvenile”, and facilitates transmission not between environments, but hosts. All three of these optional stages share the common function of facilitating organism survival under states of high stress during larval stages and are similar in morphology. From this, the Dauer Hypothesis suggests that these three stages are homologous and that the parasitic “infective juvenile” life stage is derived from the ancestral, non-parasitic dauer larva.

Theory for parasitic evolution

Broadly, the Dauer hypothesis applies to all examples of parasitism in Nematoda. Four steps of an evolutionary sequence pathway to animal parasitism have been proposed. The steps are as follows: 1.) Free-living ancestors that do not associate with a larger species, 2) phoretic relationships in which nematodes superficially attach to a larger animal for dispersal, 3) necromeny, in which nematodes may feed on their dead hosts without directly contributing to the death themselves, and 4) parasitism.