Fig. 4. C. elegans centrioles and PCM. SAS-4 or TAC-1 staining in sperm (A,E), oocyte (B,F) and one-cell stage telophase embryo (C and D, G and H). SAS-4 and TAC-1 are shown in red, DNA in blue and microtubules in green (shown only in D,H). The outline of sperm cells and oocytes is indicated. Bars, 10 µm. (A) In sperm, the centrosome is reduced to a pair of centrioles during sperm maturation (A, arrow). Owing to their small size, the two units of the pair cannot be distinguished. Although -tubulin has been reported to localize to sperm centrioles (Kirkham et al., 2003), our staining conditions did not allow us to detect a focus of TAC-1 nor other PCM components in mature sperm. Regardless, PCM material potentially provided by the sperm would probably be negligible in comparison to the contribution of the oocyte. (B,F) In the oocyte, centrioles are lacking, but PCM and centriolar components are present diffusely in the cytoplasm. Note the somatic sheath cell nucleus on the top-left in B. (C,D,G,H) After fertilization, a centrosome is reconstituted from a paternally contributed pair of centrioles and maternally contributed PCM components. This reconstituted centrosome then enters the canonical duplication cycle. Splitting of centrioles occurs already during mitosis in embryonic systems where cells oscillate between M and S phase (Callaini and Riparbelli, 1990; Kirkham et al., 2003; Leidel and Gönczy, 2003). Therefore, centrioles of the posterior (right-most centrosome) are sufficiently distant from one another in telophase embryos to be recognized separately (C, right arrow and inset). Note that the two centrioles of the anterior centrosome are not yet distinguishable as individual units at this stage (C, left arrow) (see also Leidel and Gönczy, 2003; O'Connell et al., 2001).