Worm Breeder's Gazette 11(2): 96
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 early embryogenesis of C. elegans appears to follow an invariably rigid cell division sequence. However, we do not really know how rigid this sequence is for normal embryogenesis. We report here our observation that the cell division sequence of C. elegans early embryogenesis can vary without affecting the hatching efficiency or the fertility of hatchees. We observed embryos with the same preparation method as described by Sulston et al.(1), where we used Difco purified agar (PA) for supporting embryos instead of Difco Bacto- agar. Cover glasses were sealed with vaseline. Experiments were carried out at 25 C. When we used 5% PA, we consistantly observed that AB(16) occurred 0.5-3 minutes before P(3)(2) (division of P3 to P4+D) and that MS(8) occurred 2-4.5 minutes before D(2) (5/5) in the N2 development. These cell division sequences were reversed when 2% PA was used; that is, P3(2) occurred 0.5-1 minute before AB(16) and D occurred 2-4 minutes before MS(8) (3/4). In one of the four cases, MS(8) occurred before D2 as in the 5% PA, although P3(2) occurred before AB(16). Almost all of some 50 embryos so far examined in each condition hatched normally. We tested the fertility of the worms that showed the embryonic cell division sequence typically expected for the % agar used. All of these worms grew fertile with essentially normal brood sizes. The results show that the cell division sequence of C. elegans early embryogenesis is not absolutely rigid and allows some range of variations without affecting the normal development and the fertility. We think that the observed variations in the cell division sequence resulted from difference in the pressure applied on the embryos. The embryos on the 5% PA looked flatter than those on the 2% PA under Nomarski optics. The 5% PA must have exerted more pressure on the embryos than did the 2% PA. Then, we may expect the division sequence in the 2% PA to be closer to that under no pressure Shierenberg et al observed the embryos prepared in water under a cover glass sealed with petrolatum and plastic-foam cushions (condition without pressure) to exhibit the division sequence of P3(2) -> AB(16) and D(2)-> MS8(2), which is indeed identical to that observed with the 2% PA. ---In addition to variations in the cell division sequence, we also observed variations in the cell division axes of E cells under the two different agar conditions. Both Ea and Ep divided in parallel and dorsal-ventral in the 2% PA, whereas the division axis of Ep inclined posteriorly in the 5% PA. Neither of the division axis variations resulted in any abnormal development or infertility.