Changes in microtubule structures during the first cell cycle of physiologically polyspermic newt eggs

Abstract

The unfertilized egg of the newt, Cynops pyrrhogaster, has a second meiotic spindle at the animal pole and numerous cortical cytasters. After physiologically polyspermic fertilization, all sperm nuclei incorporated into the egg develop sperm asters, and the cortical cytasters change into bundles of cortical microtubules. The size of the sperm asters in the animal hemisphere is ∼5.6‐fold larger than that in the vegetal hemisphere. Only one sperm nucleus moves toward the center of the animal hemisphere to form a zygote nucleus with the egg nucleus. This movement is inhibited by nocodazole, but not by cytochalasin B. The centrosome in the zygote nucleus divides into two parts to form a bipolar spindle for the first cleavage synchronously with the nuclear cycle, but centrosomes of accessory sperm nuclei in the vegetal hemisphere remained to form monopolar interphase asters and subsequently degenerate around the first cleavage stage. The size of sperm asters in monospermically fertilized Xenopus eggs was ∼37‐fold larger than those in Cynops eggs. Since sperm asters that formed in polyspermically fertilized Xenopus eggs exclude each other, the formation of a zygote nucleus is inhibited. Cynops sperm nuclei form larger asters in Xenopus eggs, whereas Xenopus sperm nuclei form smaller asters in Cynops eggs compared with those in homologous eggs. Since there was no significant difference in the concentration of monomeric tubulin between those eggs, the size of sperm asters is probably regulated by a component(s) in egg cytoplasm. Smaller asters in physiologically polyspermic newt eggs might be useful for selecting only one sperm nucleus to move toward the egg nucleus. Mol. Reprod. Dev. 47:210–221, 1997. © 1997 Wiley‐Liss, Inc.

Localization of γ‐tubulin and cyclin B during early cleavage in physiologically polyspermic newt eggs

“…Because γ‐tubulin has been shown to be important for MTOC activity in the amphibian egg cytoplasm (Stearns et al . 1991), this is consistent with our previous observation that the sperm nuclei in the animal hemisphere are associated with larger sperm asters than those in the vegetal hemisphere (Iwao et al . 1997).…”

“…2J), but their centrosome underwent degeneration. These results indicate that a greater amount of γ‐tubulin is localized in the principal sperm nucleus due to the higher concentration of protein in the animal hemisphere, which probably causes the formation of a larger sperm aster associated with the principal sperm nucleus (Iwao et al . 1997).…”

Germ plasm in Eleutherodactylus coqui, a direct developing frog with large eggs

“…Nonetheless, specification of the dorsal axis in urodeles is likely caused by a cortical rotation. A grey crescent, which forms due to the rotation, appears after fertilization in several species [56-58], and cortical arrays of parallel microtubules have been detected in C. pyrrhogaster [59]. These observations raise the question as to whether RNA of orthologues of wnt11 , fatvg , and trim 36 are localized in urodele oocytes or whether urodeles use a different mechanism to initiate development of the dorsal axis.…”

Independent Spatial and Temporal Functions of Human Sperm Centrosomes After Dispermic Microinjection Into Bovine Oocytes

“…During amphibian fertilization, the distribution of tubulin and functional proteins necessary for sperm aster formation in the ovum cytoplasm is not uniform (Nakamura et al, 2005a). A sperm that is in a suitable position can organize an aster and fuse its pronucleus with the female pronucleus, but sperm that are not at the appropriate position disappear in the egg cytoplasm (Iwao et al, 1997). In contrast, during porcine polyspermic fertilization, multiple male pronuclei are separated by microtubule domains, and if 2 male pronuclei are distant from one another, pronuclear apposition and fusion do not occur.…”