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.
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.