Worm Breeder's Gazette 15(2): 32 (February 1, 1998)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
|1||Dept. of MCD Biology, Univ. of Colorado, Boulder, CO 80309|
|2||Dept. of Cell and Molecular Biology, Northwestern Univ. Medical School, Chicago, IL 60611|
During development, coordinating the body axes is as critical as establishing them. We have looked at the relationship between two handed asymmetries in C. elegans, the embryonically generated dextrality of the left/right (L/R) axis, and the chirality of the helically organized adult cuticle. C. elegans adults exhibit an invariant handedness of their anatomical L/R asymmetry in the placement of the gonad, intestine, coelomocytes and components of the nervous system. The fibers in the secreted cuticle also exhibit an invariant chirality in their direction of coiling around the body. The basal layer of the C. elegans adult cuticle consists of an amorphous sublayer and two fibrillar sublayers that spiral in opposite directions around the animal, each offset 65 degrees from the long axis of the body. The outer sublayer normally exhibits a left-handed, and the inner, a right-handed, screw sense. The fiber layers of the cuticle may distribute the forces of movement and internal pressure in complementary and opposite directions via their orientations. Dextral anatomical handedness is determined early in embryogenesis, at the 4-6-cell stage. The L/R cleavages of ABa and ABp are normally skewed so that the left daughters are displaced anteriorly relative to their right sisters. If instead, the right ABx daughters are displaced anteriorly by micro-manipulation or as a consequence of the spn-1(it143) mutation (1997 Intl. C. elegans Mtg. Abstr. p. 653) the result is a sinistral worm with all embryonic and adult L/R asymmetries reversed. It is unknown how the chirality of cuticle layers in the wild-type worm is determined. However, mutations in cuticle collagens can alter the pitches of the cuticle helices (Kramer, unpublished) to impart chirality to the entire structure, resulting in either a left-handed or right-handed overall twist to the cuticle and a corresponding LRol or RRol phenotype. If there is a handedness to the hypodermis, then the orientation of cuticle fiber layers might be influenced by a reversal of anatomical handedness, which could lead to reversal of the relative orientations of the fiber layers. If so, then reversal of the body axis in a Roller mutant might reverse the direction of body twist and observed rolling direction. We tested the second possibility by constructing strains carrying spn-1(it143ts) mutation, which at 25 C results in about 40% sinistral animals among surviving progeny, and a second mutation conferring an RRol or an LRol phenotype, chosen to include several different loci and types of rol alleles. Genotypes of the strains tested [all homozygous for spn-1(it143)] were RRol: sqt-1(e1350)/+ and rol-9(sc148); LRol: rol-4(b238), sqt-1(sc13) and rol-8(sc15). In none of these combinations did animals scored as sinistral by gonad morphology change their direction of rolling, indicating that the chirality of the cuticle twist in these mutants is independent of anatomical handedness. To test whether handedness affects orientation of the fiber layers, we examined them directly in dextral and sinistral singly mutant spn-1 adult animals using transmission electron microscopy. We determined the handedness of each animal prior to fixation, embedding and examining low-angle longitudinal sections, which capture both fiber layers and allow their relative orientations to be determined. We could find no difference in the orientation of the fiber layers between dextral and sinistral animals. We conclude that there is no dependency of cuticle chirality on the underlying anatomical handedness of the worm. It remains unclear how the orientation of the cuticle fibers secreted by the hypodermis is determined.