Worm Breeder's Gazette 11(5): 89
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 mutant in C. elegans which we call 'skinhead' ( skn-1). The progeny of homozygous skn-1 mutants lack two major body organs, the pharynx and the intestine. Instead, these mutant embryos have extra hypodermal cells abnormally located inside their bodies. I have shown that the EMS blastomere, which in a wild type embryo produces the posterior pharynx and the intestine, is transformed in skn-1 mutant embryos such that it instead makes hypodermal and muscle cells. Although skn-1 embryos lack a pharynx and an intestine, they show extensive differentiation of other cell types, including anterior sensory neurons, suggesting that the defect is specific to the pharyngeal and intestinal lineages. By genetic analysis, I have shown that the mutation is recessive and that the skn-1 gene product is maternally contributed to the oocyte. Analysis of skn-1 mutants. skn-1 embryos appear to have additional hypodermal cells by several criteria. (i) Using Nomarski optics, one can see what appear by morphological criteria to be hypodermal cells on successive focal planes throughout the bodies of these mutant embryos. This is in contrast to wild type embryos in which hypodermal cells are seen only on the surface of the embryo. (ii) Joel Rothman found that antibodies that recognize the Drosophila protein engrailed recognize hypodermal cell nuclei on the surface of C. elegans embryos (WBG 10:3). We see additional nuclei staining inside the bodies of skn-1 embryos using these antibodies. (iii) Another characteristic of skn-1 embryos is the presence of internal cavities which are surrounded by the putative internal hypodermal cells. After fixing embryos and sectioning them for transmission electron microscopy, we have been able to confirm that these internal cavities are lined by cuticle. Most significantly, the cuticle lining the cavities includes a striated layer, which is normally seen only in the cuticle that surrounds the hypodermal cells on the exterior of wild type animals. When I use a laser beam to kill all the blastomeres except for EMS in a wild type 4-cell embryo, EMS produces a posterior pharynx and an intestine, as it would in an intact animal. When I ablate all the blastomeres except EMS in a skn-1 embryo, EMS produces instead a clump of hypodermal cells (a cell type never produced by a wild type EMS) and muscle cells. Therefore, by the 4-cell stage of embryogenesis, the lineage that normally produces the posterior pharynx and the intestine has been transformed into a new lineage in skn-1 mutants. A Model. One way to interpret the phenotype of skn-1 mutants is to propose that the wild type skn-1 gene product normally programs EMS to make a pharynx and an intestine. Strikingly, a reduction of function mutation in this gene causes EMS to adopt a fate very much like that of its sister cell in the 4-cell embryo, which normally makes almost exclusively hypodermal and muscle cells. Experimental and genetic analyses have suggested that cell determination in C. elegans partly involves the asymmetric segregation of maternal factors (often called determinants) to specific embryonic blastomeres. Perhaps the skn-1 gene encodes such a maternally provided, segregated determinant. skn-1 maps near unc-5 on LGIV. I am currently working to identify deficiencies that include skn-1 and to isolate additional alleles of the gene.