The gene
cut-1 (
c47g2.1) was shown several years ago to code for a specific component of the cuticle of dauers. CUT-1 is present in the dauer cuticle in a highly cross linked, insoluble form and is the first, molecularly identified, component of cuticlin, the insoluble residue of the nematode cuticle. Immuno-EM has shown that CUT-1 is secreted by the seam cells during dauer formation. It is found in the dauer cuticle in ribbons running underneath the alae on the two sides of the animal. Using reporters,
cut-1 appears to be expressed only during dauer formation and only by the seam cells. If
cut-1 (RNAi) worms are induced to become dauers they produce larvae that under the stereoscope appear to not undergo radial shrinking and which look somewhat dumpish. At high magnification they appear to have a larger diameter and to completely lack the alae. EM confirms that the alae are missing and that the section of their circumference corresponding to where the alae usually form is wider. No other phenotype is shown by
cut-1(RNAi) worms. An hypothesis to correlate these data and the biochemical characteristics of CUT-1-like proteins to the processes of radial shrinking and dauer alae formation is as follows. In C. elegans the dorsal and ventral halves of the cuticle are joined together by the cuticle secreted by the seam cells along the left and right sides of the animal. During dauer cuticle formation, after the most external layers have been secreted, CUT-1 is deposited by the seam cells in the fibrous layer, underneath of where the alae will eventually form. Upon secretion, CUT-1 assembles non covalently (e.g. via hydrophobic interactions) with itself and with other cuticle components. Later, a cross-linking reaction, involving CUT-1 as a substrate, occurs and generates the force to pull together, along the lateral lines, the internal layers of the two halves (dorsal and ventral) of the cuticle. This process is possibly the basis for the radial shrinking observed in dauers. The shape of the alae would result from a sort of wrinkling of the more external layers of the cuticle on the more internal ones. In the absence of CUT-1 (e.g. RNAi worms) the cross-linking does not occurr because a necessary substrate is missing. The two halves of the cuticle are not pulled together, the diameter of the worm remains larger, no wrinkling occurs and no ala forms. The scenario just depicted allows many predictions. One of them regards the fact that alae are present in other stages beside the dauer larvae, namely they are present in L1 and adults. Since CUT-1 is not expressed at these stages one predicts that other CUT-1-like proteins, possibly closely resembling CUT-1 might be involved in formation of the alae at these other stages. We used RNA interference to identify genes that might serve a
cut-1 role in the formation of the alae of L1s and adults. Interference with
cut-3 (
f22b5.3) results in L1 larvae that appear dumpish at the stereoscope, and that at high magnification or with EM appear to completely lack alae. Dauer and adult alae are normal. L1s and dauers of worms interfered with
cut-5 (
r07e3.3) have alae that present many abnormalities. They are interrupted or are shallower at many points, they often split with the formation of two branches. As in the case of
cut-1 and
cut-3 the alae of the adults in
cut-5(RNAi) animals appear perfectly normal. It is possible that CUT-5 is one of the components on which cross-linking of CUT-3 or CUT-1 occurs; however, rather than being essential for alae formation it appears to influence their shape. So far we have been unable to find a
cut-1-like gene, if it exists, responsible for the formation of the alae in adults. The differences between the shapes of L1, dauer and adult alae suggests that the latter may be formed by a different mechanism.