Centrosome reduction and monopolar spindle formation in haploid mouse embryos. (A, C, and D) Immunostaining of α-tubulin and histone H2B (A and C), or γ-tubulin (D) in B6D2F1 mouse parthenogenetic embryos at E4.5. Mitotic cells are shown in C and D. (B) Frequencies of morphological abnormalities in A. Values represent means ± SE of four independent experiments. At least 109 embryos were analyzed per condition. (E) Spindle polarities and numbers of γ-tubulin foci in D. At least 93 cells from 71 embryos from 11 independent experiments were analyzed per condition. (F) A model for the ploidy–centrosome link. During mitosis, astral MTs (lines) develop in a ploidy-linked manner. Inadequate or excess amounts of asters lead to deceleration or acceleration of centriole disengagement (and subsequent duplication), respectively, in haploid or tetraploid cells, respectively. As a result, the efficiency of the entire centriole duplication cycle scales proportionally to ploidy level, which drives uncoupling of centriole duplication and DNA replication in non-diploid states.