Cell lineages and developmental defects of temperature-sensitive embryonic arrest mutants in Caenorhabditis elegans

The cellular phenotypes of 11 temperature-sensitive mutants in nine genes ( emb-1-emb-9) arresting embryogenesis in Caenorhabditis elegans are described, including early embryonic cell lineages and developmental defects and the terminal phenotypes at the stage of arrest at the nonpermissive temperature (25°C), as well as residual phenotypes at the permissive temperature (16°C). By Nomarski microscopy of living embryos, the behavior of individual cells in mutant embryos is compared to that of the same cells in wild-type embryos. All mutants but one ( emb-9) have visible defects before the 50-cell stage, even if they arrest much later. Cell proliferation continues in the absence of normal morphogenesis. The terminal phenotype of the late-arresting mutants looks grossly abnormal. The overall rate of cell division is faster than in the wild-type in two mutants, emb-3 and emb-5(hc67), and slower in five mutants, emb-2, emb-4, emb-5(hc61), emb-6, and emb-7. In five of these mutants, emb-3, emb-4, emb-5 (both alleles), and emb-7, the rate and the sequence of divisions of specific cell lines is altered. For all mutants with timing defects, maternal gene expression is at least sufficient. Gastrulation is abnormal in the two emb-5 mutants. A premature division of the intestine precursor cells occurs in close temporal coincidence with the defective execution stage (determined by temperature-shift experiments). The order of migration and division of the intestine precursor cells is reversed in gastrulation, and the cell pattern of the intestine primordium is abnormal. It seems possible that the timing error is responsible for the pattern defect. The cleavage behavior of emb-3 eggs indicates that germ line determinants are prelocalized in the egg of C. elegans.