Worm Breeder's Gazette 11(3): 73
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.
Although the C. elegans larval and adult body plans are generally bilaterally symmetric, they also exhibit several handed asymmetries at gross anatomical as well as cellular levels. Moreover, the embryonic cell lineage and the embryo itself are markedly asymmetric from the 6- cell stage onward. Bilateral symmetry appears to be superimposed on the asymmetric lineage during embryogenesis by lineally nonhomologous cells adopting analogous fates at equivalent positions on the two sides of the animal, as pointed out by Sulston et al. (Dev. Biol. 100:64, 1983). These observations raise questions regarding first, how the polarity (handedness) of the left/right embryonic axis is established, and second, how bilateral symmetry is achieved by contra- laterally analogous programming of nonhomologous cells. Handedness appears to be established during the transverse cleavage of the two AB cells in the 4-cell embryo when their spindles, initially parallel to the left-right axis, become skewed in a clockwise direction as viewed from the dorsal side, so that the resulting daughter cells on the left are anterior to those on the right. The handedness of the gonad in adult hermaphrodites can be scored efficiently under the dissecting microscope after rolling animals through a quarter turn with a pick. Among F3 broods from EMS- mutagenized N2, I have found animals with reversed gonad handedness at an overall frequency of about 0.2%. Some of these are also reversed for other asymmetries such as the placement of the excretory cell nucleus and the handedness of the ventral nerve cord, while others are not. Most showed no increase in frequency of reversed gonads among their progeny. So far 200 F1 clones screened have yielded two strains that produce 10%-15% reversed-gonad animals, but no mutants with higher penetrance effects. I am attempting to test the likelihood, pointed out by zur Strassen on the basis of experiments with Ascaris almost a century ago, that reversed animals can arise from reversed 6- cell embryos via a mirror-image cell lineage.