Worm Breeder's Gazette 2(1): 16
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.
Zyg mutants fail to hatch at restrictive temperature. The end of their temperature sensitive periods can be defined relative to hatching. Late L4's and gravid adults are allowed to lay eggs overnight at permissive temperature. By the next day a heterogeneous population of eggs at all stages of embryogenesis is obtained. Only newly hatched L's are removed from the plate which is then shifted to 25 C. In subsequent 1-2 hour intervals newly hatched L's are removed and counted. Hatching rates of 50 to 200 L's per hour are obtained for plates containing 40-50 gravid adults. This rate is dependent, however, on individual mutant egg production. The removal of L's continues until no further hatching is observed. The end of the tcrit is defined to be when the hatching rate is reduced 50%. This method allows us to measure any tcrits that end during gonadogenesis and before egg laying since no adults need be removed. The tcrits in hours before hatching are tabulated below. The maternal versus the non-maternal mode of inheritance for the listed zyg mutants was determined earlier (Boulder Worm Group, 1976). Most of the true maternals (M,M) exhibited tcrits at 11-17 hours before hatching, while the non-maternals (N,N) exhibited tcrits at later times in embryogenesis. Even for mutants that were rescued by wild type sperm (M,N), if the rescue was by a cytoplasmic factor, the tcrit was earlier in embryogenesis compared to the rescue by a zygotic function of normal sperm. To determine more exactly the embryonic stage at the end of the tcrit, individual zygotes were released from gravid adults by dissection and then transferred to plates at 25 C (Hirsh and Vanderslice, 1976). Using a stereomicroscope at 50X magnification, one to sixteen cell stages can be readily distinguished prior to their transfer to restrictive temperatures. For some of the mutants (tsB1 and tsB10) all their 1-16 cell stage embryos hatched. This means that these embryos were past their tcrits, or, their tcrits were before the one cell stage. If the gravid tsB10 mutant for example was initially raised to 25 C for one hour before dissection, 50% of the two cell embryos died. The tcrits for tsB1 and tsB10 were just before and after fertilization respectively, since it takes about two hours for the fertilized egg in C. elegans to reach the two cell stage (Hirsh et al., 1976). Other mutants exhibited temperature sensitive periods during the one to sixteen cell stages. For example, tsB211 failed to hatch if the 1- 4 cell stage embryos were raised to the restrictive temperature while six cell stage embryos or later ones did hatch. Lastly, many of the mutant embryos failed to hatch if any of their 1-16 cell stage embryos were raised to the restrictive temperature, i.e., the end of their tcrit was at a later stage but 6- 9 hours before hatching. As noted above, all the true maternals exhibited early tcrits that were around the time of fertilization and before the 16 blastomere stage. The non-maternals or those that were dependent on zygotic expression for normal embryogenesis exhibited tcrits beyond the 16 cell stage. In conclusion, when a mutation alters a zygotic function necessary for normal embryogenesis, its function is required later in embryogenesis compared to those mutants that require a cytoplasmic factor. [See Figure 1]