Worm Breeder's Gazette 17(3): 30 (November 1, 2003)
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.
Columbia University, College of Physicians and Surgeons, New York, NY
The nervous system of C.elegans displays significant patterns of bilateral symmetry (Hobert et al., 2002). Deviations from this symmetry are usually highly stereotyped. For example, the unilateral RIS neuron is always located on the left side of the animal, but never on the right side (White et al., 1986; this observation, based on a limited number of EM-reconstructed animals, has now been confirmed with gfp reporters). One major exception to this stereotyped laterality is represented by the expression of the putative odorant receptor gene str-2, which occurs stochastically in either the left or right AWC neuron (the distribution is about 50:50; Troemel, et al., 1999). Are there other examples of stochastic laterality in the nervous system? In their EM reconstruction work, White et al. (1986) noted that the unilateral RID motor neuron (see Fig.), whose cell body is located on the dorsal midline, sends its axon along the right side of the nerve ring in one reconstructed animal, but along the left side in another reconstructed animal. Could this observation reflect a stochastic choice of the growth cone when encountering the dorsal side of the nerve ring? The sample size of two obviously did not allow one to derive such a statement, but the advent of gfp reporter technology allows us to address this question. Several previously described gfp reporter strains show RID expression, yet the presence of GFP in the axons of many other neurons precluded the tracing of the RID axon. A promoter fragment from a putative serotonin-like receptor gene that we study in the lab drives expression of gfp in RID and only one additional pair of neurons, allowing us to trace the axon of RID. Taking advantage of this, we found that in 56 out of 58 animals, the axon traveled along the left side of the nerve ring, while in 2/58 animals, the axon migrated on the right side. In another genetic background in which a gfp marker is ectopically expressed in RID (Is[unc-119::ttx-3; ttx-3::gfp]), we found a similar distribution; namely 46 out of 48 animals have their RID axon on the left side.
Another case of strongly biased, though not 100% stereotyped left/right asymmery can be found in the ventral nerve cord. The total set of D-type motorneurons sends 2 circumferential commissures around the left side of the animal; the remaining commissures are around the right side of the animal. However, this choice is not entirely stereotyped. In 25% (n=80) of wild type animals there are one or two additional commissures that run on the left side of the animal rather than on the right side.
Taken together, the nervous system of C.elegans shows several examples of non-stereotyped, biased anatomic asymmetries. It will be interesting to investigate the mechanistic basis for this left/right asymmetry.
Figure: Morphology of the RID motorneuron.
References:
Hobert et al. (2002) Nat.Rev.Neurosci.3, 629-40
Troemel et al. (1999), Cell 99, 387-98
White et al. (1986) Phil. Royal Soc. of London B. Biological Sciences 314, 1-340