Worm Breeder's Gazette 12(3): 97 (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.

Mutations in Development of the Excretory Canal Cell

Matthew Buechner, Ed Hedgecock

Department of Biology, Johns Hopkins University, Baltimore, MD, 21218

The excretory cell, believed to regulate osmotic content, is located beneath the posterior pharyngeal bulb. The cell sends processes (canals) left and right to the lateral ridge, where they branch and extend almost the entire length of the worm. Canal outgrowth may be related to neural development, as the canals are closely associated with the CAN neural processes, and unc-5 and unc-6 mutations cause failure both of canals and of neural commissures to leave the ventral ridge.

The excretory canal cell is born shortly after gastrulation. Treatment of worms with anesthetic slightly enlarges the canal lumena and allows easy observation of the canals in early L1 animals. The canal lumena extend to V4 at hatch, and finish extension to the V6 -Tboundary by approximately 12-14 hours after hatch. It is not yet clear if the lumen is opened as the canal process extends, or if the processes are full-length at hatch, followed by later progressive opening of the lumena.

A series of mutants has been identified (primarily by the pale appearance of the worm) in which function of the excretory canals appears to be impaired. Many of these mutants have defects in canal structure. These defects fall into several classes. In some, the lumen of the canals and cell body is significantly bloated, and the canals extend only partway ( exc-1 , exc-2 )or not at all ( exc-4 ).Ultrastructural analysis reveals abnormal enlargement of the canaliculi and their failure to assemble into canals.

A second class of mutants ( rh1 O3, rh209 , rh1 O19)has canals of normal width but variable length that meander along the ventral-dorsal axis. These canals may fail to invade the hypodermal basement membrane after progressing to the lateral ridges, and therefore cannot detect the guidance cues necessary for appropriate outgrowth.

In a third class ( exc-3 , rh187 ),the canals do not meander greatly, but are simply short. These may simply indicate slow growth of the canals. In rh187 ,excess material appears at the canal tips ahead of the ends of extremely narrow lumena, and suggests that organization of the lumen and canal outgrowth are linked.

Several mutants affect other cells as well as the excretory cell. One of these, rh96 ,has canals that extend only to V2 at hatch, but continue to grow slowly. Migrations of the distal tip, Q, and HSN cells are also affected. This mutation was found to be allelic to pat-3 ;the stronger alleles are sterile ( rh151 ) or lethal ( rh54 ),and have major defects in epithelialization of the somatic gonad and muscle function as well as in canal outgrowth. This gene and its mutations have been sequenced by Sonya Gettner in Louis Reichardt's laboratory, who found the gene to be a ß1 integrin.

rh67 defines a mutation which implicates the CAN neuron in canal outgrowth. In wild-type animals, the CAN neurons migrate from the head to midbody during embryogenesis, then slowly enlarge after hatch to acquire a non-neuronal appearance. In the mutant, the CANs remain small and usually die. The canals extend to V3 at hatch, near the position of CAN, and do not extend & farther; the worm's tail is withered, as if fluid is abnormally drained from the posterior half of the animal. In contrast, mutants in which the CANs do not migrate but develop normally usually have normal length canals (Manser & Wood '90). This implies that CAN function, rather than the location of the cell body (the extent to which the neuronal processes grow out in the non-migratory CAN cells is unknown) is required for canal extension.