[
Worm Breeder's Gazette,
1986]
The nervous system of Ascaris contains 298 neurons each of which is an identified cell whose morphological features are reproducible from animal to animal. For some years, we have been analyzing the pattern of motor neuron commissures in Ascaris. (Commissures are single processes which exit from the ventral nerve cord and run around the body, either to the right or to the left, to the dorsal cord). Although in general the commissure of each motor neuron is reproducibly to the right or to the left, we have found rare individuals in which the handedness of specific commissures is reversed. These exceptions fall into two classes. The first class includes animals in which a single commissure or commissure pair is reversed (Table 1). Most of these involve the reversal of normally righthanded VI and DE1 commissures to produce single lefthanded VI commissures or lefthanded VI/DE1 commissure pairs. Righthanded DE1 commissures are almost never observed to reverse alone, although 4 examples of normally lefthanded DE1 commissures being reflected to the right have been observed. Single reversals are seen most frequently in VI/DE1 pairs adjacent to two lefthanded VI/DE1 commissure pairs which occur normally in the head and in the tail (Table 2); perhaps they reflect the spread of a local 'reversing' factor present at these sites. Selective reversal of righthanded VI commissures and lefthanded DE1 commissures suggests that the factor's influence may be stronger on VI than on DE1 motor neurons. The second class consists of variant animals in which entire sets of motor neurons have reversed commissures. Initially, we found 2 Ab stained preparations in which all DE2, DE3 and DI commissures were reversed whereas all DE1 and VI commissures have their normal handedness. Subsequent screening of 1639 animals with darkfield optics (which allows commissures to be viewed in live animals) has revealed 2 additional animals with identical patterns of commissure reversals as well as 6 other animals in which the commissures of all motor neurons are reversed. Since in C. elegans, neurons analogous to the DE2, DE3 and DI neurons are generated during embryogenesis whereas the DE1 and VI neurons are from post-embryonic lineages, we refer to these two types of variant animals as MIR-EMB and MIR-ALL respectively. Neither MIR-EMB nor MIR-ALL animals have any apparent behavioral alteration. Analysis of other neurons which have asymmetric shapes has shown that two other embryonic neurons in the head (analogous to RID and SABD) have mirror image morphologies in both MIR-EMB and MIR-ALL animals and that cells analogous to the (postembryonic) AVF cells are normal in MIREMB animals, but mirror images in MIR-ALL variants. Neurons with asymmetric commissures in the tail are also inverted in MIR-ALL animals (MIR-EMB animals have not yet been examined. In contrast to these alterations in the shapes of neurons, we find that the location of asymmetrically positioned neuronal cell bodies ( Ascaris analogues of the PDEL/PDER, PVDL/PVDR, PVM/AVM and AQR/PQR neurons) are not reversed in either of the variants. (The nucleus of the excretory duct, on the other hand, is on the left in normal and MIR-EMB animals, but on the right in MIR-ALL animals.) Thus it appears that MIR animals are not complete mirror images of the normal animals, but rather that they are variants with mirror image neuron morphologies. The existence of two classes of MIR variants suggests that there are two separate mechanisms for determining the handedness of neurons: one operating during the growth of embryonic neurons, the other affecting the post-embryonic neurons, possibly reflecting maternal vs. zygotic expression of a common gene. Confirmation of this possibility awaits genetic experiments in Ascaris or the isolation of comparable variants in C. elegans.[See Figure 1] [See Figure 2] [See Figure 3]
[
Worm Breeder's Gazette,
1997]
In order to identify genes that are differentially expressed as a consequence of stress due to paraquat, we used the differential display technique (1) to compare mRNA expression patterns in Caenorhabditis elegans. A C. elegans mixed-stage worm population, as well as a homogeneous larval population were treated with 100 mM paraquat, parallel to controls. Over 50 mRNA species that are potentially up-regulated in response to paraquat were identified. Sixteen of these candidates were re-amplified, ligated into plasmid vectors and the nucleotide sequences were determined. The induction of four of these expressed sequence tags (ESTs) designated L1, M47, M96 and M132 were confirmed in two independent stress / differential display experiments, as well as by northern blot analysis with RNA from stressed and unstressed worms. The nucleotide sequences of the independently isolated L1 and M47 ESTs were found to be identical. Their corresponding mRNA level increased more than 40-fold in the larval stage, and to a lesser extent in the mixed-stage worm population, in response to paraquat. Induction levels of 3 - 5 fold were observed for the M132 and M96 ESTs, in both larval and mixed populations. All of the isolated ESTs showed homology to portions of the C. elegans cosmids. The L1/M47 EST is derived from a gene encoding one of the putative C. elegans glutathione S-transferases. The paraquat-inducible M132 EST was also identified, and encodes a putative C2H2 - type zinc finger protein, possessing an N-terminal leucine zipper. However, the M96 EST appears to be a novel gene whose product has not yet been identified. Searches of other eukaryotic nucleotide sequence databases did not reveal any significant homologies to known sequences from any organism. Since paraquat is known to generate superoxide radicals in vivo, and the cellular superoxide dismutase (SOD) enzyme complex is responsible for the quenching of the deleterious effects of these radicals, the response of the C. elegans superoxide dismutases (2,3,4) to paraquat was also investigated in this study. Northern blot experiments demonstrated that mRNA steady state levels of the C. elegans manganese type and the copper/zinc type superoxide dismutases increased two-fold in response to paraquat, in the larval population. In contrast, mixed-stage populations did not show any apparent increase in the levels of these SOD mRNAs in response to paraquat. References: 1. Liang, P., Pardee, A.B. (1992). Science 257: 967 - 971 2. Giglio, M-P., Hunter, T., Bannister, J.V., Bannister, W.H., Hunter, G.J. (1994). Biochem. Mol. Biol. Int. 33 (1): 37-40 3. Giglio, A. M., Hunter, T., Bannister, J.V., Bannister, W.H., Hunter, G.J. (1994) . Biochem. Mol. Biol. Int. 33 (1): 37-40 4. Larsen, P.L. (1993). Proc. Natl. Acad. Sci. USA, 90: 8905-8909