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.

Cuticle chirality and body handedness

Dominique Bergmann1, Jennifer R. Crew2, James M. Kramer2, Bill Wood1

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can alter the pitches of the cuticle helices (Kramer, unpublished) to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.