Worm Breeder's Gazette 9(2): 64
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.
During early embryogenesis of C. elegans the serial stemcell-like cleavages of the germline cells P0-P3 generate a number of somatic founder cells with different developmental potentials. Observations on partial embryos show that these four unequal divisions occur orthogonal to the anterior-posterior axis of the egg. In the first two of these germline divisions the somatic daughter cell comes to lie anterior to the new germline cell. In the following two, however, the somatic daughter comes to lie posterior, suggesting a reversal of polarity in the germline. By the use of a laser microbeam, egg fragments can be extruded from young embryos. These fragments can develop like partial twins. If the fragment is derived from the uncleaved zygote P0, its first division generates two P1-like cells. Further development reveals that the mirror image duplications are joined at their anterior poles. If the fragment is derived from P1 the first division generates two P2-like cells. Further development reveals that here the duplications are joined at their posterior poles. From this it is concluded that in fact a reversal of polarity takes place in the germline cell P2 as an integral part of normal development. This notion is confirmed by the finding that partial embryos derived from the posterior region of P2 were not able to undergo germline-like cleavages in contrast to those derived from P0 or P1. Thus,polarity appears not to be a static feature but a dynamic property which is (or at least can be) newly established during each cell cycle in the early germline cells. This view is in accordance with the behavior of the P-granules(Strome and Wood ,Cell 35 , 15-25 , 1983 ). What could be the developmental significance of this reversal phenomenon? It may occur in order to preserve the contiguity between the intestinal precursors (E-cells) and the germline (otherwise P3 would take the position of its somatic sister C). During gastrulation the two daughters of the primordial germcell P4 follow the E-cells in migrating into the center of the egg. During later embryogenesis the two germcells send protrusions into the intestinal cells. It has been speculated that the germcells which have to execute an extensive postembryonic program of proliferation may be nursed by the gut cells ( Sulston et al. ,Dev. Biol. 100,64-119,1983). Zur Strassen ( Zoologica 107,1-142,1959) described in detail early development of another nematode Bradynema rigidum which differs from the pattern found in C. elegans. In all early germline divisions the new germline cell lies posterior. Thus, the here discussed reversal of division polarity is obviously absent. Nevertheless, cellular positioning eventually becomes very similar to that in C. elegans because P4 performs a compensating migration around its somatic sister D and makes contact to the E-cells. The pattern observed in Bradynema appears simpler and more straightforward. It may well represent the original pattern which has been modified in C. elegans such that instead of an intercellular reversal of polarity an intracellular reversal takes place. This would abbreviate the time needed for embryogenesis and thus would be an evolutionary advantage. How could the reversal of polarity in P2 be controlled? A visible affinity between intestinal precursors and germline cells can be observed in untouched embryos of C. elegans. In the late 4-cell stage the nucleus of P2 is translocated from the posterior region of the cell to the anterior-ventral periphery. After division of EMS and P2 the nuclei of E and P3 lie eccentrically side by side separated only by the cell membrane. But if the P2 cell has been extruded in the 4-cell stage, the nucleus of E stays in the center of the cell. Subsequently the cell cycle periods in the E-cell lineage are similar (or even identical) to those of the MS lineage, in contrast to normal development where the E cells express considerably longer cell cycle periods. Also more than the normal 20 E descendants were found in two embryos which were analyzed in more detail. From all this it is concluded that a specific cell-cell interaction takes place between P2 and EMS. This interaction appears to be tightly coupled to the reversal of polarity described above. The following working model is suggested: The posterior region of the uncleaved egg exerts an attractive force ( induced by sperm entrance?). This causes the sperm pronucleus to remain there while the oocyte pronucleus migrates towards it and further causes the P-granules to be prelocalized. Also in P1 and P2 nuclei stay therefore most of the cell cycle close to the posterior periphery. After reversal of polarity the attractive force arises from the region of P2 adjacent to EMS. Both blastomeres cleave unequally into cells of different developmental potential. The nuclei of E and P3 both react visibly to the attraction with their peripheral location. Herewith the unique behavior of EMS (only somatic sister of a germline cell which does not represent a somatic founder cell, but passes through an unequal cleavage to generate two of them) finds an explanation.