Worm Breeder's Gazette 13(4): 49 (October 1, 1994)

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.

Low Temperature Affects Embryonic Handedness Reversal.

Bill Wood, Amy Florance, Dominique Bergmann

Department of MCD Biology, University of Colorado, Boulder

  C. elegans embryos become left-right asymmetric at the 6-cell stage with a
handedness which is essentially invariant among N2 animals reared at 20260, and which
persists into adulthood, determining the asymmetric placement of the gonad, intestine,
the coelomocytes, and several other cells. Reversal of embryonic handedness at the
6-cell stage by micromanipulation results in development of healthy adults with all
left-right asymmetries reversed(1). Spontaneous reversal occurs at a frequency of a
few percent among animals developing embryos treated with chitinase at the 2-cell
stage to remove the egg shell(2). We have now found that when N2 are reared at 10260,
the frequency of animals with reversed gonad handedness as adults increases to above
0.5%. Those examined by Nomarski microscopy exhibited handedness reversal of the
ventral nerve cord (cell bodies on the left) and reversed placement of the coelomocytes as
well as the intestine and gonad, suggesting that these animals developed from reversed
embryos. To determine the cold-sensitive period for handedness reversal, young
gravid adults reared at 20260 were placed at 10260 for 2 hours, then transferred to new 20260
plates and subsequently transferred to fresh 20260 plates at 2-hr intervals. Eggs laid
during the cold pulse (interval 1; exposed to 10260 after fertilization), the first 2 hr after
return to 20260 (interval 2; exposed to 10260 either just before or just after fertilization), and
each of the four subsequent 2-hr intervals at 20260 (3-6, exposed to 10260 before fertilization)
were allowed to hatch and develop at 20260 into adults, which were then scored for gonad
handedness. Among animals developing from embryos exposed to 10260 after
fertilization (interval 1), no reversals were observed (N=1,956), whereas among animals
developing from oocytes and sperm exposed to 10260 before fertilization (intervals 3-6), the
frequency of reversal was 0.21% + 0.09% (N=9,223). Surprisingly, although left-right
asymmetry first appears at the 6-cell stage, and the embryo's left and right are highly
unlikely to be specified until the 4-cell stage,(1) the cold-sensitive period for reversal is
before fertilization. This result suggests that the effect is on the gametes. We can
imagine two possible explanations: 1) Production of some eggshell precursor
component, synthesized during oogenesis, could be cold sensitive, such that embryos
with "soft" eggshells are produced at 10260, leading to a low frequency of reversal as seen
in chitinased embryos(2). 2) Some handed structure in a gamete, such as the sperm
centriole for example, may normally dictate the skewing of the ABa and ABp spindles
that leads to handed asymmetry in the embryo;l when this structure is exposed to 10260
its subsequent ability to dictate handedness could be impaired, leading to a low
frequency of reversal. We are currently carrying out experiments with chilled males
to determine whether egg or sperm is the cold-sensitive gamete, as well as examining
effects of other treatments on handedness reversal. Finally, we are continuing small
scale screens for maternal-effect mutations that result in a high frequency of
embryonic reversal. None have been found, but so far we have screened only about 1000
EMS-mutagenized genomes.
 (l) Wood, W. B. (1991) Nature 349: 536-538.
 (2) Wood, W. B. and Kershaw, D. (1991) In Biological Asymmetry and Handedness, CIBA
Foundation Symposium 162: 143-164.