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.

Localization and Segregation of Lineage-Specific Cleavage Potential in Embryos of C. elegans

E. Schierenberg

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.