FIGURE 3. Symmetric divisions in the developing C. elegans germ line.

a,C. elegans germline divisions during development are symmetric with respect to size and morphology of daughter cells, cleavage plane and position⁴⁶. Continued mitotic divisions rely on signalling from the stem-cell niche^{5, 46, 87}. Stem cells are orange; differentiated cells are green; the stem-cell niche is red. b, Elimination of one or more germ cells by laser ablation (marked with a cross) during early (shown) or later larval development does not affect the ability to generate pools of stem cells and differentiated cells⁵. Mitotic germ cells are therefore developmentally equivalent. c, Repositioning the niche induces germline stem cells at the new position⁵. d, Niche duplication results in duplication of the germline stem-cell pool. Niche duplication has been accomplished by alterations in either the cell-cycle machinery^{47, 48, 50} or regulators of niche specification^{49, 51}.