A specialized cuticle protects dauer larvae from environmental insults, desiccation and passive loss of body metabolites. Structurally, the dauer cuticle is thicker and more highly-reinforced than cuticles of other larval stages. Dauer cuticles contain lengthwise ridges, called alae, that may promote locomotion over a variety of terrains. In addition, special coverings form over the buccal cavity and anus, occluding access to the alimentary tract. Changes in its chemical makeup cause the dauer cuticle to be resistant to detergent solubilization as well as acidic or hypertonic conditions (Cassada and Russell, 1975; Kramer, 1997).

The bodywall cuticle comprises 10.2% of the body cross-sectional area in dauers as compared to only 4.4% in the L4 and adult animals. The cuticle also comprises a greater proportion of total protein synthesized in dauers (23%) as compared to other stages (6-9% in L1, L4 and adults). The proteins that compose the dauer cuticle contain more cysteine and hydrophobic amino acid residues and are more extensively cross-linked than non-dauer cuticles (Kramer, 1997). Relatively little of the dauer cuticle (26%) is actually susceptible to sulfhydryl reducing agents compared to the adult cuticle (75%) (Cox et al., 1981).

Nematode cuticles are organized as layers of collagenous extracellular matrix arranged in outer (cortical) zones and inner (basal) zones. The cortical zone of the dauer cuticle is thickened relative to non-dauer cuticles. The dauer cuticles basal zone is striated due to convergence of longitudinally-oriented layers with circumferentially-oriented layers. Loose fibrillar structures separate the hypodermis and striated zone (DCutFIG 1) (Cox et al., 1981; Page and Johnstone, 2007).

Cuticular thickenings, called alae, run laterally along the length of the body like train tracks (DCutFIG 2&3). In cross section, the dauer alae appear as four prongs which are symmetrically oriented around a central track. Lateral alae are also present in the L1 and adult cuticles. The dauer alae are structurally distinct from L1 and adult alae and may project to relatively greater height. Dauer alae can usually be identified along the lateral edge of the midbody using light microscopy with DIC optics. For this reason, the presence of dauer alae is a commonly-used morphological marker for dauers.

2.3 Buccal Plug

In dauers, the anterior bodywall cuticle completely covers the buccal cavity to inhibit feeding. Light photomicrographs and longitudinal electron micrographs show that the plug projects across the front of the buccal orifice, at the level of hyp1 and the arcade cell epithelia (compare to InterFIG 1). Cross-sectional electron micrographs reveal three flaps of cuticle which join tightly as a three-pointed star. The buccal plug may protect the dauer from desiccation as well as maintaining the non-feeding status (DCutFIG 4&5).

In cross-section, the buccal cavity cuticle appears structurally similar in dauers and non-dauers. The cuticle lining the buccal cavity appears to be thicker than the bodywall cuticle or the pharyngeal cuticle. Reinforcing struts are can be visualized in transverse views of the buccal cuticle. These struts may be thicker in dauer cuticles than in non-dauers (DCutFIG 6). The interior of the buccal cavity is lined by three sheets of cuticle. The joints where the buccal cuticle sheets meet is reinforced in all stages, but this reinforcement appears to be more extensive in dauers than in L2 larvae (DCutFIG 7).

3.2 Pharynx lumen

Cuticle also lines the length of the pharynx lumen (see also Dauer Pharynx). The dauer pharynx is relaxed and does not pump. In general, the pharynx cuticle appears similar in dauers and non-dauers. The dauer pharynx cuticle may have greater reinforcement, as suggested by the presence of electron dense strands running circumferentially within the cuticle sheet (DCutFIG 8).

The other body orifices present in the dauer are the excretory pore and rectum. The excretory pore (EP) is lined by cuticle and serves as a portal for release of materials from the body. Pulsations of the excretory duct have been observed in dauers, suggesting that the excretory system is active (Nelson et al., 1983). The dauer EP cuticle is thickened relative to that in non-dauers, but it lacks the striated zone present in the dauer bodywall cuticle (DCutFIG 9A&B). The dauer rectum appears to remain open in SEM images of dauers (DCutFIG 9C&D). Some infective stage nematodes (equivalent to the C. elegans dauer) develop an “anal plug” sealing the rectal opening. The anal plug composition is unknown, but is thought to result from rectal gland secretions (Vincent et al., 1979). While SEM images suggests the anus is not occluded in C. elegans dauers, the internal composition of this structure has not been studied.

5 The Cuticle's Role as a Regulator of Influx and Efflux

Cuticle structure has the potential to affect body growth dynamics. Developing C. elegans larvae grow linearly during four sequential larval stages. The C. elegans bodywall cuticle is non-rigid and expands to accommodate body growth. The cuticle also allows for some diffusion of materials into and out of the body through a permeability barrier established by the hypodermis (Stawicki et al., 2011). This route for diffusion is impaired by the thick dauer bodywall cuticle. Thus, the structure of the dauer bodywall cuticle facilitates retention of internal molecules and protection from external solutes (DCutFIG 10).

In contrast to the bodywall cuticle, the cuticle lining the buccal cavity is rigid and inflexible (see Dauer Pharynx). Expansion can only occur during the molt, when a newly-secreted enlarged cuticle increases buccal cavity diameter. The rigid buccal cavity restricts the rate of food intake, through a straw-like effect (Knight et al., 2002). The dauer buccal cavity may be modestly shrunken, which could further limit entry of nutrients into the animal and maintain a non-feeding state. However, buccal closure by the bodywall cuticle would seem to obviate the need for protective mechanisms within the buccal cavity proper.

6 References

Cassada, R.C. and Russell, R.L. 1975. The dauerlarva, a post-embryonic developmental variant of the nematode Caenorhabditis elegans. Dev. Biol. 46: 326-342. Abstract

Cox, G.N., Staprans, S. and Edgar, R.S. 1981. The cuticle of Caenorhabditis elegans. II. Stage-specific changes in ultrastructure and protein composition during postembryonic development. Dev. Biol. 86: 456-470. Article

Knight, C.G., Patel, M.N., Azevedo, R.B.R. and Leroi, A.M. 2002. A novel mode of ecdysozoan growth in Caenorhabditis elegans. Evo. Dev. 4: 16-27. Abstract

Kramer, J.M. 1997. Extracellular Matrix. In C. elegans II. 2nd edition. (eds. D.L Riddle, T. Blumenthal, B.J. Meyer et al.) pp. 471-500. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY. This reference points to even more old papers on cuticle composition. Article

Nelson, F.K, Albert, P.S. and Riddle, D.L. 1983. Fine structure of the Caenorhabditis elegans secretory-excretory system. J. Ultrastruct. Res. 82: 156-71. Article

Page, A.P. and Johnstone, I.L. 2007. The cuticle (March 19, 2007). WormBook, ed. The C. elegans Research Community, WormBook, doi/10.1895/wormbook.1.138.1, http://www.wormbook.org.

Riddle, D.L. 1988. The dauer larva. In The nematode Caenorhabditis elegans (eds. D.L. Riddle, W.B. Wood) pp. 393-412. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York. Abstract

Stawicki, T.M., Zhou K., Yochem, J., Chen, L. and Jin, Y. 2011. TRPM channels modulate epileptic-like convulsions via systemic ion homeostasis. Curr. Biol. 21: 883-888. Article

Vincent, A.L., Frommes, S.P., and Lawrence, R.A. 1979. Ultrastructure of the rectum of infective-stage Wuchereria Bancrofti. J. Parasitol. 65: 246-252. Abstract