Worm Breeder's Gazette 10(3): 167
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 amphid cell ASH is the primary sensory cell for the avoidance response to high osmotic strength and other avoidance stimuli (WBG 10- 1, p.89 and WBG 10-2, p.41). Many mutants defective in avoiding high osmotic strength (Osm) have been identified in the past. However, in all of these mutants many different sensory neurons (including ASH) have structurally abnormal sensory endings (Perkins et al., Dev. Biol. 117: 456). Nearly all of these sensory ending mutants share three other phenotypes: they are dauer defective, chemotaxis defective, and their amphid neurons fail to fill with the dye FITC. No mutants specifically defective in the avoidance response were known. I set out to find such avoidance-specific mutants. To counterscreen against general sensory ending mutants, I took advantage of the dauer defective and FITC filling defective phenotypes shared by the sensory ending mutants. I reasoned that mutations causing defective Osm avoidance would not necessarily cause these other defects. The F2 animals from an EMS mutagenesis were screened for Osm mutants by picking animals that crossed a ring of high osmotic strength. The nineteen candidates whose progeny retested Osm were scored for filling with FITC and for dauer formation. Any mutant that was normal for either phenotype was analyzed further. Twelve of the nineteen Osm mutants proved to be normal for FITC filling or dauer formation (with one exception, see below, all twelve were normal for both). Of these twelve, half were rather weak, showing reduced reliability or slow responsiveness to the Osm ring. Only the six strong Osm mutants, plus two others that seem interesting because of their mutant pleiotropies, are discussed here. The phenotypes of each of these mutants is listed in the table, along with those of the sensory ending defective osm-3 mutant for comparison. There are some interesting general points. 1) The amphidial sensory endings in these mutants must be present and are probably normal, because it is through these endings that the neurons fill with FITC. 2) Chemotaxis, which requires amphid neurons other than ASH, is normal or mostly normal in all of these mutants except osm-7(n1515) (C. Bargmann, personal communication; Bargmann and Horvitz, WBG 10-2, p.42) . 3) The mutants can be classified into those that affect only osmotic avoidance and those that affect additional avoidance responses. We propose that the response to various avoidance stimuli involves some upstream steps that are specific to each stimulus (eg. chemoreceptors), and some downstream steps that are shared by the various avoidance responses (eg. synapse formation by ASH). 4) Many of the Osm mutations have tightly linked nervous system defective pleiotropies, supporting the notion that these mutants are affected in nervous functions. Two of the mutations deserve special comment. n1600 is unusual in a number of ways. It is the only mutation that causes a defect in osmotic, avoidance factor, and garlic avoidance. It also causes a slightly kinky jerky backward Unc phenotype. The backward Unc looks similar regardless of the stimulus that activates backward movement - avoidance response, head touch, heat, or spontaneous. These facts suggest that the defect caused by n1600 is in the activation of backward movement, rather than in the Osm sensory cells. n1600 maps on chromosome III covered by sDp3, so it's focus of action can be assessed by mosaic analysis using ncl-1. ftc-1(n1540) (FITC filling defective, pronounced fitssy) is a very weak Osm defective mutant, but has a unique FITC-filling pattern. Of the 8 classes of neurons that normally FITC fill in the wild-type, only one, ADF, fills in n1540 animals. ADF fills nearly all of the time, and to the normal staining intensity, while the other neurons ( including ASH) are invariably fully defective. This result indicates that the machinery necessary for filling with FITC is still intact, and the seven classes of FITC-filling cells other than ADF are specifically altered by n1540. Not all of the n1540 FITC-filling defective cells can be functionally defective, since chemotaxis to most compounds is normal in n1540 and requires more than ADF (C. Bargmann, personal communication). [See Figure 1]