Worm Breeder's Gazette 10(1): 133

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.

Developmental Consequences of Brief Microfilament Disruption in the First Cell Cycle of C. elegans

D. Hill and S. Strome

We are continuing the studies that we reported at the worm meeting 
by analyzing the fates of embryos that have been pulsed with 
cytochalasin D (CD) in the first cell cycle.  We previously showed 
that if we pulse embryos with CD during pronuclear migration and the 
later stages of pseudocleavage we disrupt many of the aspects of 
asymmetry that we monitor: pseudocleavage, the posterior meeting of 
the pronuclei, segregation of germ-line specific P granules, posterior 
positioning of the mitotic spindle, and the generation of a large AB 
cell and a smaller P cell.  Disruption of microfilaments before or 
after this 'critical interval' seems to have no effect on the 
formation of a normal two-cell.
To analyze the developmental consequences of brief microfilament 
disruption during the 1-cell stage, we've been assaying  1) the early 
cleavage patterns and cell cycle rates of pulsed embryos, 2) the 
internal migration of the germ-line cells, Z2 and Z3, as an indication 
of gastrulation, 3) the appearance of three differentiation markers: 
gut granules, paramyosin and a seam cell antigen, and 4) morphogenesis.

We have found that a CD pulse before or after the critical time 
interval described above does not affect any of the aspects of 
development and differentiation we assay; pulsed embryos develop as 
far as control embryos (to late comma stage), correctly express 
differentiation markers and twitch.
In contrast, the symmetric or variably asymmetric 2-cell embryos 
formed after a pulse during the critical interval divide into 
multicellular masses of cells that do not express differentiation 
markers of undergo morphogenesis.  Preliminary analysis of the early 
cleavage patterns of these embryos suggests an interesting hypothesis: 
the disruption of actin filaments during the critical time interval 
leads to the missegregation of multiple factors that control spindle 
orientation, spindle-position, and cell cycle rates.  The embryos 
display several intriguing cleavage patterns.  We've seen what appear 
to be 'reverse polarity' embryos, whose anterior cell divides like a P 
cell and posterior cell divides like an AB cell, although the timing 
of cell divisions is not always normal.  We've seen what appear to be 
'mirror-image duplication' embryos in which both cells divide like 
mirror image P cells.  And we've seen embryos that divide evenly and 
synchronously, similar to AB cells.  We do not know yet whether the 
cells in these embryos have characteristic 'identities' or are 
'composite' blastomeres with characteristics of multiple lineages.  To 
discriminate between these possibilities, we are currently analyzing 
the early divisions and developmental potential of many more such 
pulsed embryos.  Stay tuned.