Worm Breeder's Gazette 13(5): 59 (February 1, 1995)
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.
Institute of Molecular Biology, University of Oregon, Eugene OR 97403-1229 Prior to the 28-cell stage of the C. elegans embryo, differences in the lineages of several AB daughters with equivalent potential are generated by two inductive interactions (Hutter and Schnabel 1994; Mango et al., 1994; Mello et al., 1994; Moskowitz et al., 1994). The P2 blastomere affects the lineage pattern of the ABp blastomere, breaking the equivalence of ABa and ABp and leading to the production of ABp specific cell types such as intestinal/rectal valve cells. Later, at the 12-cell stage, the MS blastomere induces lineage differences in the ABara and ABalp blastomeres, resulting in, for example, the production of pharyngeal cells. Although the two signals occur at different times, both inductive signals require the same transmembrane protein, GLP-1, to alter the AB lineage. Two simple models can be considered: first, the response to the signal may be specified by qualitative differences in the ligands used by the P2 and MS blastomeres; second, a time-dependent cell-intrinsic change may take place within the AB daughters from the four-cell stage to the 12- cell stage such that inductive cues received at these two different times are "interpreted" differently. By removing the eggshell of embryos and manipulating early blastomeres (Goldstein, 1992), we are examining where the specificity lies regarding the response of the AB blastomeres to the inductive cues emanating from the MS and P2 blastomeres. First, we reproduced both interactions. In 15/21 cases, we have been able to place a P2 blastomere in contact with ABa and ABp blastomeres and obtained production of intestinal valve cells, as assayed by staining with monoclonal antibody (MAb) J126. AB blastomeres alone did not produce J126 positive cells, in 9/9 cases. By combining laser ablation with physical manipulations, we also have reproduced the MS inductive event; in 4/4 cases, cells staining positive with MAb 3NB12, a marker of pharyngeal muscle cells, have been obtained when we placed the MS and E blastomeres in contact with the 8 grand- daughter cells of ABa and ABp and, after waiting one cell division, ablated the MS and E blastomeres (eliminating MS derived pharyngeal cells). To test the models outlined above, we performed "heterochronic" blastomere recombination experiments; we placed a P2 blastomere next to the 8 grand daughter cells of ABa and ABp obtained from a separate, older, embryo. Using double labeling, 6/9 cases produced 3NB12 positive cells and 0/9 produced J126 positive cells. Thus, contact by the two daughters of AB with a P2 blastomere induces the production of intestinal valve cells while contact between the 8 grand-daughters of ABa and ABp with a P2 blastomere induces the production of pharyngeal muscle cells. This result indicates that the response by the AB daughters to inductive cues depends not on the nature of the signal, but on time-dependent molecular changes within the AB descendants. By recombining blastomeres from different genetic backgrounds, we also are addressing which blastomeres require the function of two previously identified maternal effect loci to allow these inductions to occur. First, the apx-1 gene is required for the P2 signal and has been proposed to encode a ligand for the GLP-1 protein (Mello et al., 1994). Consistent with this idea, we have found that apx-1 function is required in the P2 blastomere both for induction of intestinal valve cells from the two daughters of AB and for heterochronic induction of pharyngeal cells from the eight daughters of AB. Second, the maternal gene skn-1 encodes a transciption factor that is required to specify the identity of the 4-cell stage blastomere EMS (Bowerman et al., 1992). In addition, ABa fails to produce pharyngeal cells in skn-1 mutants, either because skn-1 is required in ABa descendants or because skn-1 is required for MS to induce ABa descendants. By mosaic analysis using isolated blastomeres from wild- type and skn-1 mutant embryos, we have found that skn-1 function is required in MS, but not in the daughters of AB, for the MS-mediated induction of pharyngeal cells from the AB daughters. This result suggests that SKN-1 may control the expression of the signaling molecule(s) responsible for this induction. By continuing these blastomere manipulation experiments, we hope to define more precisely how embryonic inductions distinguish the fates of initially equivalent blastomeres.