Fig. 5. Diversity of parental requirement for centrosome formation in the zygote. In all cases displayed, PCM components (shaded disk) are contributed from maternal stores. In C. elegans and X. laevis, the sperm contributes a pair of centrioles (black rectangles), which recruits PCM components from maternal stores to reconstitute one centrosome in the zygote (Karsenti et al., 1984; Wolf et al., 1978). In D. melanogaster and humans, the sperm contributes a single centriole (reviewed by Callaini et al., 1999; Manandhar et al., 2000). This is due to the lack of centrosome duplication prior to meiosis II in insects and to the disappearance of one centriole during sperm maturation in primates. In both cases, the single centriole must duplicate twice prior to mitosis to give rise to four centrioles, two in each centrosome. In mice and in many parthenogenetic species, no centrioles are present in the embryo at fertilization. This results from the disappearance of both centrioles during sperm maturation in the mouse (Manandhar et al., 1999) and from the absence of a male gamete altogether in parthenogenetic species (reviewed by Callaini et al., 1999). In both the mouse and parthenogenetic species, the non-centrosomal pathway of spindle assembly presumably ensures bipolarity during mitosis (reviewed by Karsenti and Vernos, 2001). In the clam S. solidissima, a pair of centrioles is also contributed maternally, but it is silenced in the zygote and does not nucleate microtubules or duplicate (Wu and Palazzo, 1999).