Worm Breeder's Gazette 10(2): 52
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 interested in identifying and characterizing genes that are required zygotically for normal embryonic development of C. elegans. As a rapid screen to locate such genes and get a preliminary idea of the consequences of their loss of function, we are looking at the phenotypes of embryos homozygous for known deficiencies on each of the linkage groups. We have previously reported observations made on embryos deficient for the X chromosome (1). The earliest defect displayed by a deficiency embryo should generally be characteristic of the null phenotype of the earliest required deleted gene (2). We report here analysis of another set of such deficiencies. One- and two-cell embryos were dissected out of hermaphrodites heterozygous for a deficiency and mounted on slides. Embryos were observed until after gastrulation by Nomarski microscopy and time-lapse videorecording, incubated overnight at 20 C, and then scored for hatching or embryonic lethality. Lineages were subsequently followed on the videorecording through gastrulation for a minimum of two of the embryos that subsequently failed to hatch from each of two slides, and the terminal phenotypes of all embryos that failed to hatch were noted. Additionally, the percentage of embryos dying on slides was compared to the percentage of embryos dying on plates, and was found to be approximately 25% in all cases, indicating that there was little or no non-specific death resulting from the mounting procedure. The analysis was carried out with five deficiencies that uncover loci in the cluster of LGII: mnDf96, mnDf98, homozygous for mnDf98 or mnDf77 hatch and arrest as L2 or L1 larvae, respectively. Analysis of early cleavages for these embryos and anatomy of the resulting arrested larvae are still in progress. (Six of the 25 embryos from mnDf98/+ hermaphrodites showed an apparent pattern abnormality between the 8- cell and 30-cell stages, but it is not yet established whether this feature is correlated with L2 arrest.) Embryos homozygous for mnDf96, re hatching. The early cleavages and gastrulation of these embryos appeared to be normal. Their terminal phenotypes were as follows. Embryos homozygous for mnDf96 displayed twitching and gut granules; two of the six embryos that arrested began morphogenesis and progressed to the comma stage. This range of terminal phenotypes is similar to that resulting from let-22( mn22), which defines a zygotic lethal locus uncovered by mnDf96. Such a result is consistent with the observations of Wieschaus and coworkers that the earliest observable defect in a fly homozygous for a given deficiency is similar to that resulting from a mutation in the earliest acting gene in the region uncovered by the deficiency (2). Embryos homozygous for mnDf88 arrested before the onset of morphogenesis and displayed gut granules. Lethal alleles defining four loci that map to the region uncovered by this deficiency are all described as resulting in larval lethality (3). Three possible explanations for the observed embryonic lethality of the deficiency are: 1) it reveals the null phenotype for one of the let's, which was previously identified only by a hypomorphic point mutation; 2) defects resulting from mutations in the four genes are additive, such that deleting all of them results in embryonic lethality, or 3) the deficiency reveals a new gene essential for embryogenesis. Embryos homozygous for mnDf66 arrested at approximately 200 cells and displayed gut granules. In approximately half the embryos, the 8 E cells formed a circle, rather than the normal linear array. However, there was no correlation between the abnormal E-cell configuration and embryonic death, although the rounded E cell morphology did appear to be specific to progeny of the mnDf66 heterozygote used. 1. See Abstracts. (1987) C. elegans meeting. 2. Wieschaus, E., C. Nusslein-Volhard, and G. Jurgens. (1984) Mutations affecting the pattern of the larval cuticle in Drosophila melanogaster. III. Zygotic loci on the X-chromosome and fourth chromosome. Roux's Arch. Dev. Biol. 193: 296-307. 3. Sigurdson, D. C., G. J. Spanier, and R. K. Herman. (1984) Caenorhabditis cy mapping. Genetics 10 & 331- 345.