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.
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 abnormalities. 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 appear. 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.