2004 West Coast Worm Meeting abstract 52
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
| 1 | Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku, Yokohama 223-8522, Japan |
| 2 | Institute for Bioinformatics Research and Development (BIRD), Japan Science and Technology Agency, 3-14-1 Hiyoshi, Kohoku, Yokohama 223-8522, Japan |
Migration of male pronucleus toward the center of an egg is essential for zygote formation and is one of the earliest events in the development of multicellular organisms. For this microtubule-dependent migration, two mechanisms were previously proposed. "Pushing mechanism" uses the pushing force resulting from microtubule polymerization, and "pulling mechanism" uses the length-dependent pulling force generated by minus-end-directed motors anchored throughout the cytoplasm. However, a question, "which of the two mechanisms, the pushing or pulling mechanism, is the primary contributor to the male pronuclear migration?" remained unsolved.
To uncover an intrinsic difference between the two mechanisms, we performed computer simulations of male pronuclear migration. Simulation models were constructed based on physics and past studies of microtubule dynamics, and parameter values were set according to the past experimental studies. Simulations showed that, when the distance of pronucleus from starting point was plotted against time, the shapes of the resultant distance-time graphs were convex in the pushing mechanism and sigmoidal in the pulling mechanism. Because fluctuations of the parameter values did not affect the shapes, we concluded the shape of the distance-time graph to be intrinsic to each mechanism. To obtain the shape of distance-time graphs of in vivo migration, we objectively measured male pronuclear migration in real embryos by applying an image-processing algorithm which recognizes (pro)nucleus automatically in Nomarski DIC microscope images of an embryo. We found the shape of the distance-time graph to be sigmoidal in real embryos, which did not change upon nocodazole treatment or dysfunction of a microtubule-associated protein ZYG-9 (CeXMAP215). These objective measurements in real embryos showed that the sigmoidal shape of the distance-time graph is intrinsic to the in vivo migration, and provide the first evidence that the pulling mechanism rather than the pushing mechanism is the primary mechanism for the male pronuclear migration in C. elegans embryos. To obtain an independent evidence for the primary contribution of the pulling mechanism, we focused on the relationship between the direction of the movement of the male pronucleus and that of the growth of asters. We took advantage of a zyg-1 embryo, which has only one centrosome at one-cell stage. The male pronucleus in a zyg-1 embryo migrated toward the single aster, indicating the primal contribution of the pulling mechanism to male pronuclear migration. We concluded that the pulling mechanism is the primary mechanism for male pronuclear migration. This is the first study that presents a natural cellular process in which the length-dependent pulling force is the primary contributor to the central positioning of a (pro)nucleus.