Worm Breeder's Gazette 12(5): 28 (February 1, 1993)
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.
In the 4-cell stage embryo the blastomeres are named ABa, ABp, EMS, and P2 .In wild-type and in pie-1 or mex-1 mutant embryos EMS divides into MS (that produces pharyngeai cells) and E (that produces intestinai cells) (Mello et. ai., 1992 Ceil). In pie-1 mutants, P2 aiso divides into an MS-like and an E-like daughtsr. However in mex-1 mutants ABa and ABp both divide into two MS-like daughters. The skn-1 gene product is a nuclear localized protein that we believe plays an important roie in the specification of the MS fate, and also plays some role in the specification of the E fate (Bowerman et al., 1992 Ceil and Bowerman et al., unpublished results). ABa and ABp have both skn-1 protein and skn-1 (+)activity in mex-1 mutants, so why don't they produce E-like daughters? Results from B. Goldstein suggest that in the absence of interactions with P2 ,EMS may divide into two MS-like daughters (and thus not produce intestine) in experimentally manipulated wild-type embryos (Nature 1992). However although only EMS produces intestine in mex-1 mutants, P2 contacts both ABp and EMS. We asked whether cytopasmic connections between the sister cells P2 and EMS might have anything to do with this by a long-shot experiment in which we used the laser microbeam to artficially fuse ABp and P2 (and killed EMS) in the mex- 1 mutant embryos. To our amazement ABp+P2 produced a well-differentiated intestine with gut-specfic granules, esterase activity and fairly normal intestine morphology in most (14/20) experiments. But wait. Fusing ABa and ABp also resulted in intestinal development (42/67 experiments). Controls showed that this does not happen in wild-type embryos, and that the ability of the ABa+ABp to produce intestinal cells after fusion in mex-1 embryos requires skn-1 r+)activity. Furthermore, if ABa and ABp are allowed to divide and any set of their daughters or cousins ars fused, no intestinal cells result. We haven't yet figured all this out, but we are intrigued by the fact that in both wild-type and mex-1 mutant embryos, EMS divides Anterior-Posterior and ABa and ABp divids Left-Right. After fusion, ABa+ABp undergo a tetrapolar cleavage with a distinct Anterior-Posterior axis. By laser ablation experiments we have found that the intestinal cells are made only by posterior descendants of the ABa+ABp deavags (the anterior descendants produce pharynx). Therefore in mex-1 mutants ABa and ABp have a latent potential for producing intestinal cells, and their normally symmetrical pattern of development becomes asymmetrical after fusion. If cleavage plane determines whether or not a 4-cell stage blastomere produces intestine, why do cleavage-blocked embryos undergo intestinal-specific differentiation? We find that although 11/14 cleavage-blocked wild-type EMS blastomeres undergo intestine differentiation, 0/12 mex-1 mutant EMS blastomeres differentiate intestinal markers (nor do the ABa or ABp blastomeres). This might happen if the concentration of some factor required for intestinal specification was abnormally low in EMS in mex-1 (perhaps because it was also in ABa and ABp?). We hypothesize: (1) a factor required for intestinal specification is present in all blastomsres in mex-1 mutant embryos (as is the skn-1 product), (2) that this intestinal factor becomes unequally distributed along an Anterior-Posterior gradient present in all 4-cell stage blastomeres and (3) that the relative concentrations of the polarized "intestinal factor" and a second, non-polarized factor determine whether or not a blastomsre produces intestinal cells. Left-Right divisions perpendicular to this gradient (as in the ABa and ABp divisions in mex-1 )would not change ths relative concentrations of thess two factors. Is ths P2 blastomere ths source of EMS polarity, and thus required for intestinal specification? We and others have shown previously that P2 could be removed midway in the cell-cycle at the 4-cell stage without affecting intestinal specification. We have now used two micromanipulators to remove P2 from a wild-type embryo virtually as soon as it is born. As reported by E. Schierenberg (Dev. Biol. 1987) and B. Goldstein (Nature 1992) this results in E adopting an MS-like cleavage rate, however in 3/3 experiments the embryos made intestinal cells. Thus in some experimental settings P2 may be sufficient to induce EMS to produce intestine, but we have not seen any evidence that it is necessary in an otherwise intact embryo; ABa or ABp may also contribute to EMS polarity. Work continues.