A. Morphology

Dauer larvae are easily distinguished from other developmental stages. They are thin and dense due to shrinkage of the hypodermis at the dauer-specific molt (Cassada and Russell 1975; Albert and Riddle 1988). They acquire resistance to detergent treatment about 1 hour after radial shrinkage of the body (Swanson and Riddle 1981), presumably as a result of cuticle modification and the occlusion of the buccal cavity (Fig. 2) (Cassada and Russell 1975; Popham and Webster 1979; Albert and Riddle 1983, 1988).

Figure 2

External morphological differences between an L2 larva (A) and a dauer larva (B) are shown by scanning electron micrographs of the head. The field width in each micrograph (more...)

Transverse-section electron micrographs of the cuticle show a thickened outer cortex and a dauer-specific, striated inner layer (Cassada and Russell 1975; Popham and Webster 1978; Cox et al. 1981b). The dauer cuticle has lateral ridges (alae) not present on L2, L3, or L4 larvae that are visible with Nomarski optics. A detergent-soluble 37-kD hydrophobic protein exposed on the surface of the dauer larva is not found on other stages (Blaxter 1993a). Many tissues and organs exhibit dauer-specific morphology: Pharyngeal pumping is suppressed (Cassada and Russell 1975) and the isthmus and terminal bulb of the pharynx are constricted (Vowels and Thomas 1992); the lumen of the intestine is shrunken and the microvilli are condensed (Popham and Webster 1979); the excretory gland lacks secretory granules (Nelson et al. 1983); and several sensory neurons exhibit altered position or dendrite orientation (Albert and Riddle 1983).

The anterior sensory ultrastructure of the dauer larva was examined in several specimens and compared with that of the L2 larva (Albert and Riddle 1983). In some instances, comparisons were made with L3, postdauer L4, and adult stages. Whereas sensory morphology in different nondauer stages remains constant, it differs in the dauer larva, providing an example of developmental plasticity in the nervous system (Jorgensen and Rankin, this volume). Dauer-specific sensory modifications in the amphids, inner labial neurons, and the deirids may play a part in dauer-specific behavior.

The amphids are a pair of prominent chemosensory organs located on either side of the head. Each amphid consists of two support cells and 12 neurons, eight of which are exposed to the environment through a pore in the cuticle near the tip of the head (Ward et al. 1975; Ware et al. 1975; Bargmann and Mori, this volume). Dendritic processes extend anteriorly from cell bodies located near the circumpharyngeal nerve ring, and axons extend into the ring. A sheath cell forms the channel for the dendritic processes and presumably secretes the matrix of material observed in the channel. A socket cell joins the anterior end of the channel to the cuticle. Amphidial neurons AWC, AFD, ASG, and ASI and the amphidial sheath cell are altered in shape or position in the dauer stage (Albert and Riddle 1983). neurons ASG and ASI are displaced posteriorly within the dauer amphidial channel. Neuron AFD has significantly more microvillar projections in the dauer stage than in L2, L3, or postdauer L4 larvae. Wing-like processes of the two dauer AWC neurons form a much wider arc in transverse section, including extensive overlap of these cells. Such overlap does not occur in an L2. Whereas L2 larvae possess two separate bilateral amphidial sheath cells, the left and right sheath cells can be continuous in the dauer larva. The AWC neurons are involved in chemotaxis to volatile attractants, the AFD neurons are involved in thermotaxis, and ASG and ASI inhibit dauer formation (Bargmann and Mori, this volume).

Whereas the amphidial pores remain open in the dauer larva, the inner labial channels are virtually occluded (Albert and Riddle 1983). The relative positions of the dendritic tips of the two types of inner labial neurons are reversed in the dauer stage compared to the L2 and postdauer L4 stages. The inner labial neuron 1 (IL1) rather than IL2 is more anterior in each of the six sensilla, and the IL2 cilia are only one-third as long as those in the L2. Since the IL2 neuron is thought to be chemosensory, and IL1 is thought to be mechanosensory, it is possible that IL2 chemosensory input may be reduced or absent in dauer larvae. Finally, the deirid (ADE) mechanosensory dendrites exhibit a dauer-specific structure and orientation. The dendritic tip of each neuron in the dauer stage is attached to the body wall cuticle by a substructure not observed in L2 or postdauer L4 stages, and it is oriented parallel to the longitudinal axis of the body. These sensory terminals are oriented perpendicular to the cuticle in other stages.

B. Behavior

On an agar surface, dauer larvae tend to lie motionless unless disturbed, perhaps to conserve energy reserves. They do, however, move rapidly in response to touch. The slender body and specialized cuticle may allow them to break the surface tension of the medium more easily than other stages. On starved plates stored for long periods, dauer larvae will accumulate in the droplets of condensation on the lids of the plates, presumably by crawling over the inside wall of the dish. Nictation is a dauer-specific behavior in which the larva mounts a projection and stands on its tail, waving its head in the air (Croll and Matthews 1977). This behavior could possibly allow the dauer larva to attach to passing soil insects in a phoretic relationship for transport to a fresh environment.

Dauer larvae respond to thermal gradients differently from other stages (Hedgecock and Russell 1975). Adults track along isothermal lines, preferring the temperature at which they were raised, but dauer larvae seek novel temperatures. They are more thermotolerant than adults, living about three times longer when exposed to 37°C (Anderson 1978). Dauer larvae respond to reduced pheromone levels and increased food by initiating recovery. Once they begin pharyngeal pumping, they become more responsive to a chemical attractant in an orientation assay (Albert and Riddle 1983).

C. Metabolism

The dauer larva exhibits a metabolism that is consistent with long-term survival in the absence of food. Dauer larvae have reduced TCA cycle activity but high phosphofructokinase activity relative to adults, indicating that dauer larvae have a greater capacity to metabolize glycogen (O'Riordan and Burnell 1989). The decreased TCA cycle activity relative to the glyoxylate cycle in dauer larvae indicates the importance of lipid storage as an energy reserve in the dauer stage (Wadsworth and Riddle 1989; O'Riordan and Burnell 1990).

After the L1 molt, C. elegans energy metabolism undergoes a major transition that is dependent on the commitment to continuous development (Wadsworth and Riddle 1989). The relative concentrations of ATP, ADP, AMP, sugar phosphates, and other metabolites result in stage-specific phosphorus nuclear magnetic resonance (NMR) spectra. These spectra are consistent with assays of isocitrate dehydrogenase and isocitrate lyase, indicating high activity of the glyoxylate pathway only during the L1 stage, whereas respiration during the L2, L3, and L4 stages occurs preferentially through the TCA cycle. Relative to the L1, L2 larvae exhibit increased isocitrate dehydrogenase activity, as well as increased concentrations of ATP and other high-energy phosphates, whereas predauer (L2d) larvae exhibit declining enzyme activities and declining levels of high-energy phosphates. Although the predominant phosphorus NMR signal in dauer larva extracts corresponds to inorganic phosphate, the higher energy state observed in growing larvae can be restored within 4 hours after wild-type dauer larvae resume feeding in bacteria. NMR analysis of living animals revealed that dauer larvae have an elevated intracellular pH relative to other stages, reflecting their unique metabolic state (Wadsworth and Riddle 1988).

On the basis of results from in vitro translation, Snutch and Baillie (1983) suggested that dauer larvae have reduced transcriptional activity relative to growing larvae. Using run-on transcription assays with isolated nuclei, Dalley and Golomb (1992) observed a depression of general RNA polymerase II transcription in dauer larvae to 11–17% of that in other stages. However, the dauer larvae could be induced for heat shock mRNAs, showing that they are competent to initiate and elongate transcripts. Interestingly, dauer larvae were found to be 15-fold enriched for Hsp90 (heat shock protein) mRNA relative to other stages. In unstressed cells, Hsp90 interacts with steroid hormone receptors, facilitating receptor activation and preventing receptors from activating transcription in the absence of hormone (Cadepond et al. 1991). Hsp90 acts as a chaperon for various protein kinases, and also complexes with other proteins (Gething and Sambrook 1992). Conceivably, Hsp90 could interact with steroid hormone receptors or other proteins to promote dauer formation. In addition, it might complex with transcriptional activators to down-regulate transcription during dauer maintenance.

Dauer larvae possess elevated activities of superoxide dismutase (Anderson 1982; Larsen 1993) and catalase (Vanfleteren and De Vreese 1995), enzymes that are involved in protection against oxidative damage. They also exhibit greater tolerance to oxygen deprivation (Anderson 1978). These traits might contribute to dauer longevity (Klass and Hirsh 1976; Kenyon, this volume).