Worm Breeder's Gazette 12(3): 105 (June 15, 1992)

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.

Mutants That Fail to Avoid Touch to the Nose

Joshua M. Kaplan, Erika Hartwieg, Bob Horvitz

HHMI, Dept. Biology, MIT, Cambridge, MA. 02139

C. elegans responds to touch to the nose with either of two alternative responses. Animals either undergo backward locomotion, which is called the backing reflex, a they withdraw their head from the stimulus, which is called the head withdrawal reflect. Distinct classes of mechanosensory neurons mediate these two behaviors. ASH, FLP, and OLQ neurons mediate the backing reflex, whereas IL1 and OLQ neurons mediate the head-withdrawal reflex. We have conducted genetic screens for mutants that fail to undergo either of these responses. Mutants that have severe ultrastructural defects in ciliated sensory endings, the so-called dye-filling mutants, fail to respond to any of the stimuli known to be sensed by ciliated sensory neurons, i.e. touch to the nose, chemical attractants, and chemical repellants. To avoid isolating dye-filling defective mutants, we have studied only those mutants that stain normally with the fluorescent dye DiO. In this manner we isolated 11 touch-avoidance defective mutants from 13,000 mutagenized F2 animals. These 11 mutants correspond to 9 genes.

Mutations in two genes ( n2474 IIIC and n2147 )cause defects in many ciliated sensory neurons. These mutants fail to respond to any of the stimuli known to be sensed by ciliated sensory neurons. Thus, the behavior of these mutants is similar to that of the dye-filling defective mutants; however, n2474 IIIC and n2147 animals stain normally with DiO. These results suggest that n2474 and n2147 are required in many ciliated neurons for sensory function, but probably do not encode essential structural components of the sensory endings.

A mutation in one gene, osm-9 ( n2473 ),causes defects in both touch avoidance and osmotic avoidance. Since the ASH neuron mediates both touch avoidance (the backing reflex) and osmotic avoidance, we expected to isolate a class of mutants that were defective for both of these behaviors. We isolated one such mutant in our screen, which was a new allele of the osm-9 gene. Mutations in the osm-9 gene were previously shown by Jim Thomas to be defective for osmotic avoidance but normal for chemotaxis toward chemical attractants. Although both sensory functions of the ASH neuron are defective in osm-9 animals, the morphology of DiO stained ASH neurons in these animals appears normal. The osm-9 gene is probably required for the function or development of additional classes of ciliated mechanosensory neurons, because osm-9 ( n2473 )animals are defective for both the head-withdrawal and backing reflexes.

Mutations in four genes ( n2461 111, n2463 I, n2465 ,and n2472 )cause defects only in touch avoidance. These mutants all respond normally to chemical repellants. Thus, these four genes are required for the the mechanosensory function (the backing reflex) but not the chemosensory function (osmotic avoidance) of the ASH neuron. These results suggest that these genes are required for mechanotransduction per se . Since these mutants are defective for both the head-withdrawal and the backing reflexes, these genes are probably required for the mechanosensory function of the ASH, FLP, OLQ, and IL1 neurons. By contrast, these mutants respond well to touch to the body, which indicates that these genes are not required for the function of the microtubule touch cells.

Mutations in two genes, dig-1 ( n2467 )and n2469 ,cause ultrastructural defects in ciliated sensory endings. These mutants were isolated because they have a defective head-withdrawal reflex but a normal backing reflex and a normal osmotic avoidance response. Previously isolated alleles of the dig-1 gene ( n1321 and n1480 )have similar effects. To determine if these behavioral defects were caused by ultrastructural defects in ciliated sensory endings, we examined the sensory anatomy of these mutants by electron microscopy of serial sections. The dig-1 ( n2467 )animals have defects in several tissues. OLQ and CEP sensory endings are variably missing, the former explaining the head withdrawal defect. AFD processes fail to adhere to the adjacent amphid sheath membrane. The AWC sensory ending is fragmented. The cuticle in the buccal cavity fails to adhere to the pharyngeal epithelial cells. Also, as previously described (Thomas, Stern, and Horvitz, Cell 62: 1041-10521990), the hermaphrodite gonad fails to adhere to the ventral hypodermis and is consequently easily displaced. These results suggest that the dig-1 gene is required for adhesion in several tissues. The n2469 animals also have ultrastructural defects: their amphid socket channels are filled with a darkly staining diffuse matrix, and their amphid cilia are variably shortened. These results suggest that the n2469 gene is required for amphid socket function.

Since most of the genes that we have identified thus far are represented by single alleles, we expect to identify other genes in further screens of this sort.

Literature Cited:

Thomas, Stern, and Horvitz, Cell 62: 1041-10521990