Worm Breeder's Gazette 9(3): 83
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.
Early embryogenesis of C. cterized by the presence of a germline and its stepwise separation from the soma. Germline and soma exhibit different cleavage patterns.Typical for the germline is a series of stemcell-like,unequal cleavages.Typical for members of somatic lineages are their equal and synchronous clevages. Using a laser microbeam various experiments were carried out to investigate the basis for the two different types of cleavage. Germline-like cleavage potential is localized in the posterior region of the zygote. Various portions of cytoplasm were removed from the uncleaved zygote with or without nucleus.The cleavage patterns of partial embryos indicate that the potential for germline-like cleavage is localized in the posterior region of the uncleaved egg. Germline- specific 'P-granules' are obviously not involved in the control of cleavage pattern. If some cytoplasm is removed from the posterior pole of the egg prior to formation of pronuclei(when P-granules are still equally distributed over the egg;Strome & Wood,Cell 35,15-25, 1983)the male pronucleus and centrosomes can form within the extruded fragment. This fragment can pass through the complete series of unequal cleavages like an intact zygote.Thus, the major part of the P-granules( if not all) and in addition the presence of an oocyte nucleus are dispensable for normal early cleavage pattern.However,neither in this experiment nor in any of those described below,fragments have been found to develop into worms.But tissue-specific differentiation was observed. Cleavage Pattern appears to be correlated to orientation of cleavage-spindle. To test whether the typical antero-posterior orientation of the cleavage spindle(and resulting transverse cleavage membrane) is a prerequisite for for unequal cleavage,embryos were manipulated such that a cell which normally divides longitudinally was forced to perform a transverse cleavage instead. The partial removal of AB was found to cause a more or less transverse cleavage of EMS( instead of a-p).As a result the behaviour of EMS was variably altered. In several cases no visible separation into MS and E could be detected and no gut-specific autofluorescence developed. Cytoplasmic control of cleavage pattern The zygote nucleus was extruded from the anterior pole together with some cytoplasm.The extruded fragment could only perform AB-like equal cleavages.If after several divisions one nucleus was allowed to slip back into the posterior,anucleate remainder of the zygote,germline-like cleavages took place. Thus, a nucleus(or centriole) are not imprinted to follow a once started cleavage pattern and an anucleate,posterior fragment can preserve its potential to promote germline-like cleavage. Early determination of cleavage pattern The zygote nucleus was extruded with some cytoplasm as described in the previous experiment. After one or several cleavages the remainder of the zygote was shunted into one ABlike cell. Neither the cell which had received posterior cytoplasm nor its descendants were observed to perform germline-like cleavage.Other experiments during which cytoplasm was transferred from a germline cell to a somatic cell and vice versa never resulted in a switch of the basic cleavage pattern in the recipient cell.The inability of posterior cytoplasm to promote germline-like cleavage is probably not due to a dilution effect because the fusion of AB(ca. 60% of total cytoplasm) to P1(ca. 40 %) does not abolish germlinelike cleavage.It rather seems that the cleavage potential of a cell was ( irreversibly?) fixed before cytoplasmic transfer took place. Somatic quality is dominant over germline quality A centrosome was removed from the anterior pole of the zygote.If this was done after pronuclear fusion,cell division was delayed by one cycle until a new centrosome had been synthesized.The first cleavage was unequal.But further development was different from normal in such that the uncleaved egg showed certain characteristics of P rather than P . If the centrosome was removed prior to pronuclear fusion, the egg did not perform any unequal cleavages a passed through equal and synchronous cleavages. From this the following is concluded: a. the potential of the germline to express its typical cleavage pattern is a very sensitive property and can be easily destroyed if separation from the soma does not take place on time.b. the germline is protected from losing its cleavage potential if the cell cycle has passed a certain threshold line,even if no immediate cell division takes place.