Worm Breeder's Gazette 9(3): 90
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.
dth-1 ('deep-throat' or 'demi-throat', e2072, e2141, e2142, e2144) is a maternal-effect lethal mutant that makes only the posterior half of the pharynx as an embryo (and supernumerary hypodermal cells- see Abstracts of 1985 C. and H.Schnabel and R. Schnabel, WBG 9-2, p. 78). In trying to interpret the mutant phenotype, I have examined whether pharyngeal development normally is 'cell- autonomous' or involves cellular interactions. Both the AB and Pl blastomeres of a normal two-cell embryo produce descendants that become pharyngeal muscles; AB produces most of the anterior half of the pharynx, and Pl produces most of the posterior half. I have removed either the AB or the Pl blastomeres with a microneedle and stained the resulting partial embryo with an antibody that recognizes pharyngeal muscles. Pl-derived partial embryos contain pharyngeal muscles (14/14), and some of these embryos develop a fairly well- formed posterior pharynx (including the grinder). However AB-derived partial embryos do not make any pharyngeal muscles (0/25), suggesting an interaction with Pl-derived cells is necessary for descendants of AB to become these muscles. Because all of the AB-derived pharyngeal muscles originate from just one of the AB daughters, ABa, I was interested in whether the other daughter, ABp, could produce pharyngeal muscles if it occupied the position of the ABa blastomere. In normal development, the AB blastomere initially divides transversely, then skews toward the anterior-posterior axis of the egg. This results in an anterior daughter, ABa, and a posterior daughter, ABp. It turns out to be relatively simple to reverse the direction of skewing in a dividing AB blastomere with a blunt-ended micropippete needle, such that the daughter that normally becomes ABp instead becomes ABa. These embryos form an apparently normal pharynx, hatch, and develop into fertile adults. This result explicitly rules out the possibilites that the nuclei, peripheral cytoplasm or membranes, of the AB daughters are qualitatively different when these cells are born. Instead, it appears likely that the different fates of ABa and ABp descendants normally arise from cellular interactions in the embryo. Therefore my current favorite model for the dth-1 gene product is that it plays some role in the interaction between AB- and Pl-derived cells (rather than being a pharyngeal 'determinant' partitioned into ABa, as I speculated at the last worm meeting.) Andrew Fire and I are now trying to clone dth-1 with the goal of determining the nature and embryonic localization of the gene product. dth-1 maps near lin-12 and within the Tc1 polymorphisms eP6 and eP7, an area now covered entirely by a contig (Greenwald et al., WBG 9-2). We have isolated DNA from the majority of the cosmids in the region, and are attempting to rescue the mutant using the general protocols for transient expression (no luck yet) and stable integration previously described by Andy (EMBO, Oct. issue).