Worm Breeder's Gazette 14(4): 30 (October 1, 1996)
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.
Dept. of Biochemistry, UC San Francisco, San Francisco CA 94143-0554Shortly after hatching the Q neuroblasts begin their migrations. QL and its descendants migrate posteriorly, while QR and its descendants migrate anteriorly. Hedgecock et al found that hch-1 mutants have delayed hatching as well as Q cell migration defects (Development 100:365-382) and Hishida et al have reported that hch-1 encodes a Zn++ protease (WBG 14(2):47). There are two models to explain the Hch-1 phenotype: 1) the hch-1 protease could be acting both to allow escape from the eggshell and to allow the Q cells to migrate correctly on their substrate or 2) hch-1 could be just required for timely hatching, and the Q defect could be an indirect result of the delay in hatching. If the Q phenotype is indirect, then artificially liberating hch-1 animals before they have had time to stew in their own juices should restore normal Q migration. To generate a roughly staged population of eggs, we allowed N2 and hch-1(e1907) adults to lay eggs for 1 hour on seeded plates. After 9.5 hr (20 deg. C), ~5% of the N2 eggs had hatched and most N2 and hch-1 eggs had reached the pretzel stage. We washed these eggs off the plate with egg salt solution, treated them with 0.4% bleach for 1 min, rinsed with egg salts, added 20 mg/ml chitinase + 20 mg/ml chymotrypsin for 3 min, and then rinsed 3 times with egg salts (a slight modification of Lois Edgar's technique in the Methods Book). These liberated hatchlings were allowed to mature overnight on seeded plates. In untreated hch-1 animals, 51% of the QL descendants incorrectly migrated anteriorly (n=54), but in the artificially hatched hch-1 mutants, only 8% of QL descendants migrated anteriorly (n=53). Artificial hatching did not affect the Q migrations in N2 (n=26). Thus it seems likely that the Hch-1 Q phenotype is a secondary defect resulting from the delay in hatching.
The results of the converse experiment, in which hatching is artificially delayed in wild-type worms, have been stewing in our lab for several years. We found that placing N2 eggs in 0.4M NaCl for 12 hr blocked hatching in ~80% of eggs. When these delayed eggs were allowed to hatch and mature on seeded plates, 14% of the QL descendants migrated in the wrong direction, toward the anterior (n=21). The QR descendants were also mispositioned, with 26% remaining in the posterior (n=23). [hch-1(e1907) does not have a QR migration defect at 20 deg. C (n=50). e1907 may cause QR defects at 25 deg. C, but these animals are too sick to score reliably.]
Anecdotal observations suggest that starvation in early L1 can cause Q migration defects, but it remains possible that something else about the environment within the unopened egg shell (not just the lack of food) contributes to the Q cell phenotype in hatching-delayed animals.