Worm Breeder's Gazette 8(1): 20

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.

b274: A New Maternal Effect Mutation that May Affect Determination Events in Early Embryos

K. Kemphues, J. Priess, N. Wolf, S. Strome, D. Hirsch

We have isolated a maternal effect lethal mutation that appears to 
affect determination events in early embryogenesis.  The mutation, 
b274 V (rol-4 b274 unc-61), is non-conditional, 100% penetrant, and 
not rescuable by wild type sperm.  The mutant exhibits the following 
1) The first cleavage spindle, and consequently, the furrow tend to 
be displaced anteriorly.  Measurements of the cross sectional area of 
AB and P1 cells at the end of the first division reveal that first 
divisions in b274 embryos yield AB cells whose volume is 53%+/-3% of 
the total embryonic volume.  Wild type divisions give AB cells whose 
volume is 57%+/-2% of the total.  The variability in the position of 
the cleavage plane in mutant embryos (from even cleavage to normal 
asymmetry) is important in light of the following observation.
2) The second, third, and fourth cleavages occur synchronously with 
a normal AB cleavage rhythm, irrespective of the relative volumes of 
the first two blastomeres.  This observation provides evidence that 
the differences in cell cycle rhythms in the blast cells are 
established by a mechanism that is not strictly dependent upon nuclear 
to cytoplasmic ratio.
3) In normal embryogenesis, 671 nuclei are generated and 558 remain 
after programmed cell deaths.  Terminal stage b274 embryos have more 
than 600 nuclei on the average.  Some embryos have more than 700 
nuclei, indicating that failure to undergo cell death is not the sole 
basis for the extra cell divisions.  It seems likely that control of 
cell number is regulated at least in part by controlling cell cycle 
rate and that this aspect of the phenotype may be a direct consequence 
of increased cell cycle rate in the blast cells.
4) Terminal stage embryos show no evidence of morphogenesis, but 
some cell differentiation takes place.  Cells with neuronal and 
hyperdermal characteristics are present and cell deaths have been 
observed.  Indirect immunofluorescence staining of fixed terminal 
stage embryos using anti-paramyosin antibodies reveals that only 50% 
of the embryos differentiate muscle cells, and among these the number 
of muscle cells is variable.  Gut-specific granules, however never 
5) Germ-cell-specific P granules, which are distributed normally in 
b274 gonads and oocytes, fail to segregate to P-cells during the early 
cleavages.  P granules do not appear to be specifically segregated to 
the posterior cytoplasm prior to first cleavage as in normal embryos, 
nor are they localized at the two cell stage.  By the four cell stage, 
however, granules are seen in some embryos to be localized around the 
nucleus in all four cells.  We have not yet determined the ultimate 
fate of the granules in the b274 embryos.
Our present working hypothesis based on these phenotypes is that 
b274 affects a component of the system that is required for proper 
segregation of cytoplasmic determinants during the early cleavages.  
We also hypothesize that this system is identical to, or shares a 
common component with, the system responsible for the asymmetric 
placement of the first mitotic spindle.