Worm Breeder's Gazette 16(1): 50 (October 1, 1999)

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.

The homologous chemosensory neurons ASER and ASEL are functionally distinct in chemotaxis.

Jon Pierce-Shimomura, Shawn Lockery

Institute of Neuroscience, 1254 University of Oregon, Eugene, OR 97405-1254

The chemosensory neuron class ASE, comprising ASEL and ASER, is essential for normal chemotaxis to several soluble attractants¹. Though anatomically homologous, ASEL and ASER are derived from different lineages in development. Moreover, three different putative receptor guanylyl cyclase genes are differentially expressed in these two neurons². Preliminary ablation studies suggest that chemotaxis to a range of soluble compounds is more sensitive to killing ASER than ASEL¹. We have examined the question of a functional difference between ASEL and ASER in more detail by studying the effects of unilateral ablations on chemotaxis to a single soluble attractant (NH₄Cl).

Ablations were done in L1 worms in which GFP was expressed specifically in either ASER or ASEL (gifts from D. Garbers & O. Hobert). Ablated worms were grown to adulthood, then assayed for chemotaxis ability for 20 min in a gaussian gradient of NH₄Cl and, as a control, in the volatile attractant diacetyl, which is sensed by the chemosensory neuron class AWA, but not ASE³. Chemotaxis behavior was recording using an automated tracking system the reported the animal's position and direction of movement with respect to the gradient peak at 1-s intervals. After each assay the success of the ablation was confirmed by checking for the presence or absence of GFP label. We found that 17 of 21 ASER- animals failed to reach the peak of the NH₄Cl gradient, whereas 18 of 18 ASEL- animals were able to reach the peak. Chemotaxis to the volatile attractant diacetyl, sensed by the chemosensory neuron class AWA, was unaffected, since 8 of 8 ASER- animals and 7 of 7 ASEL- animals reached the peak of a diacetyl gradient. Sham operated worms, anesthetized and recovered but not ablated, all reached the peak (ASER+ n = 14, ASEL+ n = 14).

To quantify chemotaxis performance further, we computed for each worm a chemotaxis index (I_ctx) equal to the time average of the NH₄Cl concentration normalized to the concentration at the gradient peak⁴. Statistical comparisons between the four groups (ASER-, ASEL-, ASER+, and ASEL+) showed that average I_ctx was significantly reduced in the ASER- group relative to the other three groups, which were statistically indistinguishable (see Table 1).

We have shown previously that bursts of sharp turns, called pirouettes, play a fundamental role in C. elegans⁴. Wild-type worms are, on average, oriented down the gradient immediately before pirouettes and oriented up the gradient immediately after pirouettes. In the ASER- group, however there was no consistent orientation before or after pirouettes. In the ASEL- group, by contrast, pirouettes were oriented normally. Taken together, these results indicate that ASER and ASEL differ in the extent to which they are responsible for chemotaxis to NH₄Cl in our assays. Whether the behavioral differences shown here are reflected at the physiological level remains to be determined.

References:

1. Bargmann & Horvitz (1991) Neuron 7:729-42
2. Yu et al. (1997) PNAS 95:3384-3387
3. Bargmann, Hartwieg & Horvitz (1993) Cell 74:515-527
4. Pierce-Shimomura, Morse & Lockery (in press) J Neurosci