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Comments on von Elsner, Sophie et al. (2009) International Worm Meeting "Three cib-genes, three functions, one phenotype News in germ line development." (0)
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von Elsner, Sophie, Wiekenberg, Anne, Schmidt, Henning, & Schnabel, Ralf (2009). Three cib-genes, three functions, one phenotype News in germ line development presented in International Worm Meeting. Unpublished information; cite only with author permission.
In C. elegans, the embryonic germ line is defined by the first four rounds of division. In contrast to the soma, the germ line cells (P-cells) divide asymmetrically in a stem-cell-like manner. To understand the mechanisms underlying this asymmetry, we analysed mutants in three genes with major defects in the embryonic germ line. In cib-1 (Schnabel and Schnabel, 1990), cib-2 and cib-3 (changed identity of blastomeres) the germ line cell P1 to P3 divide, after a delay up to one cell cycle, symmetrically. The initial defect in cib-2 is that the mitotic chromosomes do not condense. As a consequence, in cib-2, DNA is shredded during mitosis. Therefore the delay occurs (as a secondary defect) in the activation of the cell cycle checkpoint. We confirmed, that the activation of the cell cycle control depends on the damaged DNA by laser irradiation of one pronucleus in the wildtype embryo. This phenocopies the cib-2 mutant. Since cib-1 codes for the only thymidylate synthase (Winska et al., 2005) in the worm, the P-cell problem caused by mutations in both genes are probably due to DNA integrity. The germ line phenotype suggests maybe the existence of a germ line specific DNA integrity checkpoint. CIB-3 is essential for the organisation of the cytoplasmic distribution of germ line specific factors. Nevertheless, CIB-2 and CIB-3 have identical interaction partners. This indicates a linkage between DNA integrity and the organisation of the cytoplasmic polarity. Thus, the system may serve to survey integrity of the worm population, by eliminating embryos with defect germ line DNA.