Fig. 4. Developmental consequences of the reduction in APC/C activity in the early embryo. DAPI and tubulin localization during the pronuclear stage and two- to four-cell stage in wild-type embryos (A-E) and mat-1 2PB class (F-J) and 1PB class embryos (K-O). (A-C) Wild-type pronuclear stage embryo in which the sperm pronucleus (A; right white arrowhead) is anchored in the peripheral cortex by the sperm asters (B, white arrow). The female pronucleus (A; left white arowhead) is positioned more centrally. The centrosomes of the sperm aster lie on opposite sides of the sperm pronucleus (B, other centrosome is below the focal plane) and each centrosome has multiple microtubules emanating from the centrosome to the cortex (C). (D) Dividing wild-type two-cell embryos have two polar bodies (black arrowheads; one polar body is outside the focal plane), a larger blastomere (left) and a smaller blastomere (right). The second mitotic division is asynchronous and the individual spindles set up perpendicular to each other (E). (F-J) In the less severely affected 2PB class embryos (F,I; black arrowheads indicate position of the two polar bodies), the relative position of the female pronucleus is normal (F; left white arrowhead). In addition, the sperm asters and microtubules extend normally from each centrosome to the cortex (G,H). (I,J) Although the relative blastomere size and cleavage orientation of the two blastomeres are similar to wild type, the mutant blastomeres tend to divide more synchronously. (K-O) Under semi-permissive temperature conditions when MI predominates (K,N; black arrowhead indicates single polar body), embryos exit meiosis but zygotic development is severely compromised. The pronuclear stage of this 1PB class is characterized by abnormalities in the relative positions of the pronuclei (K) and the maturation of the sperm asters (L,M). The first division in this 1PB class is symmetric (N). In the second division, the blastomeres divide synchronously with both spindles perpendicular to the long axis of the embryo (N,O). C, H and M are enlargements (4x) of the sperm asters in B, G and L. The carets in E, J and O indicate the orientation of the mitotic spindle of the right blastomere of two- to four-cell embryos. The average embryo is 50 µm in length.